Slide 1

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 2

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 3

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 4

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 5

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 6

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 7

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 8

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 9

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 10

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 11

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 12

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 13

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 14

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 15

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 16

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 17

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 18

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 19

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 20

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 21

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 22

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 23

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 24

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 25

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 26

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 27

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 28

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 29

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 30

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 31

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 32

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 33

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 34

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment


Slide 35

AUTOMATION & ROBOTICS

LECTURE#06
POSITION SENSORS
(DISPLACEMENT) TRANSDUCERS
By: Engr. Irfan Ahmed Halepoto
Assistant Professor

POSITION (DISPLACEMENT) SENSORS
• A position sensor is any device that permits position
measurement.
• It can either be an absolute position sensor or a relative one
(displacement sensor).
– Gear Mechanism
• Position sensors can be either linear , rotary, or angular .
– Revolution per minutes.

Rotary Sensors

linear position sensors

Position Sensor Types
•
•
•
•
•
•
•
•
•

Linear variable differential transformer (LVDT)
Encoders
Potentiometer
Capacitive transducer
Eddy-current sensor
Hall effect sensor
Proximity sensor (optical)
Inductive Non-Contact Position Sensors
Piezoelectric transducer

LINEAR VARIABLE DIFFERENTIAL TRANSFORMER (LVDT)
• LVDT is a non contact transducer based on
Electromechanical device that measures Displacement.
• LVDT are responsible for electrical output, proportional to
displacement of a movable core.
– Movements are usually measured in range of ±12 inches
(Rotary , Linear)
– Some LVDT’s have capabilities to measure up to ±20
inches (Range: 0.01-20 inches).
• LVDT Nonlinearity is in range of 0.10%-0.25%.

LVDT Transducer
• LVDT transducer comprises a coil
former on to which three coils are
wounded.
• One primary and two secondary
coils.
• Primary transformer coils wound on
non-magnetic cylindrical-coil forms.
• Two secondary transformer coils are
wound on top of the primary.
• LVDT consists of a ferromagnetic
core which moves inside the coil
form.
– Transfer of current b/w primary
and secondaries of the LVDT
displacement
transducer
is
controlled by the position of a
magnetic core called as armature
(core).

LVDT Internal Parts
Ferrite core

Epoxy encapsulation

Primary coil
Secondary coil
Bore shaft

Magnetic
shielding
Stainless steel end
caps
Secondary coil

High density glass filled coil forms

Signal conditioning circuitry

LVDT Transducer Construction
• Primary coil typically is excited at 58V AC b/w 60 Hz and 20 kHz,
– causing a voltage to be induced in
each secondary proportional to its
mutual inductance with the
primary.
• Two secondary windings are located
on either side of the primary winding
• The A.C in the primary winding
creates an axial (Lines) magnetic flux
field that is concentrated in the core.
• This flux is coupled to the secondary
windings through the core.
• Core causes the magnetic field
generated by the primary to be
coupled to the secondaries.

LVDT Operation
• When the core is centered b/w the
two secondary windings the voltage
induced in each is identical.
– Voltage Va induced in secondary
a and voltage Vb induced in
secondary b will be in phases
(both voltages cancel), output
(VaVb) will be zero.
• Core position where the output
voltage is zero is referred to as the
null position
• As core move to the left, first
secondary is more strongly coupled
to the primary than the second
secondary.
– Voltage of the first secondary
causes an output voltage which
is in phase with the primary
voltage.

LVDT Operation ………….
• Likewise, when core moves to the
right, second secondary is more
strongly coupled to the primary than the
first secondary.
– Voltage of the second secondary
causes an output voltage to be outof-phase with the primary voltage.
• LVDT signal conditioners generate a
sine wave for the primary and
synchronously
demodulate
the
secondary output signal, so that the DC
voltage that results is proportional to
core displacement.
• Sign of the DC voltage indicates whether
the displacement is to the left or right.
• In one direction, output signal will be in
phase with the excitation and 180° out of
phase in the other direction

LVDT Transducer-Operation

LVDT Applications
•
•
•
•
•
•
•
•
•

Automation Machinery
Civil/Structural Engineering
Power Generation
Manufacturing
Metal Stamping/Forming
Pulp (soft tissue) and Paper
Industrial Valves
R & D and Tests
Automotive Racing

LVDT Application Circuits
• Signal conditioning associated with differential transformers
includes filtering and amplification
Amplification

Filtration

Why use a LVDT?
• Friction – Free Operation
– No mechanical contact between core and coil (usually)
• Infinite Mechanical Life
• Infinite Resolution
– Electromagnetic coupling
• Limited only by electrical noise
• Low risk of damage
– Most LVDT’s have open bore holes
• Null Point Repeatability
– Zero displacement can be measured
• Single Axis Sensitivity
– Effects of other axes are not felt on the axis of interest
• Environmentally Robust
– Stable/Strong sensors – good for structural engineering
tests!

ENCODERS
• An encoder is a device, circuit, transducer, software program,
algorithm that converts information from one format or code to
another, for the purposes of standardization, speed, space
accommodation, security etc.
• In Automation Industry, Encoders are sensors that generate
digital signals in response to movement.
– Rotary (Shaft) encoders: respond to rotation,
– Linear encoders: respond to motion in a line.
• Rotary encoder (shaft encoder) is an electro-mechanical device
that converts the angular position or motion of a shaft to an
analog or digital code.
• Linear encoder is a rotary device that outputs digital pulses in
response to incremental angular motion.
– When used in conjunction with mechanical conversion
devices, such as rack-and-pinions, measuring wheels, or
spindles, shaft encoders can also be used to measure linear
movement, speed, and position.

Encoder Applications
• Encoders have many uses in positioning applications.
– For example, a rotary encoder attached to a DC motor
can be used to keep track of the number of revolutions
the motor has rotated from its initial position.
• One of the simplest applications of rotary encoders is the
mechanical computer mouse.
– A mechanical mouse has 2 rotary encoders: One for X
position and one for Y position.
– As the mouse moves, each encoder outputs square
wave pulses.
– The number of pulses indicate how far the mouse has
moved in X or Y direction.
• Encoders are also used in Computer Numerical Control (
CNC ) systems to accurately position the X-Y table.

Encoders Sensing Technology
• Encoders can use either optical or magnetic sensing technology.
• Optical sensing: provides high resolutions, high operating speeds,
reliability, long life operation in most industrial environments.
– Typical incremental scale periods vary from hundreds down to
a few micrometres.
– Light sources used include infrared LEDs, visible LEDs,
miniature light-bulbs and laser diodes.
• Magnetic sensing: often used in such rugged applications as steel
and paper mills, provides good resolution, high operating speeds,
and maximum resistance to dust, moisture, and thermal and
mechanical shock.
– Magnetic linear encoders employ either active (magnetized)
or passive (variable reluctance) scales and position may be
sensed using sense-coils, Hall Effect or magnetoresistive
readheads.

Optical Encoders
• Optical encoders use a glass disk with a pattern of lines
deposited on it.
– a metal or plastic disk with slots (in a rotary encoder), or a
glass or metal strip (in a linear encoder).
• Light from an LED shines through the disk or strip onto one or
more photo detectors, which produce the encoder’s output.
• An incremental encoder has one or more of these tracks, while an
absolute encoder has as many tracks as it has output bits.

Optical Encoders
• Optical encoder's disc is made of
glass or plastic with transparent and
obscure areas.
• A light source and photo detector
array reads the optical pattern that
results from the disc's position at
any one time.
• This code can be read by a
controlling device, such as a
microprocessor or microcontroller to
determine the angle of the shaft.
• The absolute analog type produces
a unique dual analog code that can
be translated into an absolute angle
of the shaft (by using a special
algorithm).

Magnetic Encoders
• Magnetic sensing technology is very resistant to dust,
grease, moisture, and other contaminants common in
industrial environments, and to shock and vibration.

Magnetic Encoders Types
• There are several types of magnetic sensors.
• Variable reluctance sensors: detect changes in the magnetic field
caused by the presence or movement of a ferromagnetic object.
– Variable-reluctance rotary sensor (magnetic pickup) consists of a
coil wound around a permanent magnet, generates a voltage
pulse when a gear tooth moves past it.
• Another type of sensor uses a permanent magnet and a magneto
resistive device to produce a change in either voltage or electrical
resistance in the presence of ferromagnetic material, which can be in
the form of a gear tooth (in a rotary encoder) or a metal band with
slots (in a linear encoder).
– This type of sensor will work down to zero speed, and is available
in both rotary and linear forms.
• Another type of magnetic sensor uses a magneto resistive device to
detect the presence or absence of magnetized “stripes,” either on the
rim of a drum or on a nonmagnetic strip.

Encoders Classification
• Encoders are available with a choice of outputs.
– Incremental (relative-linear) & Absolute (Rotary)
• Incremental encoders: generate a series of pulses as they
move.
– These pulses can be used to measure speed, or be fed
to a counter to keep track of position.
– Output of incremental encoders provides information
about the motion of the shaft which is further processed
elsewhere into information such as speed, distance,
RPM and position.
• Absolute encoders: generate multi-bit digital words that
indicate actual position directly.
– Output of absolute encoders indicates the current
position of the shaft, making them angle transducers.

Rotary Encoder….inside
• Typical rotary encoder looks like a
potentiometer, it has infinite rotation.
• You can turn it in either direction without
hitting an end point.
• Inside, the encoder, there is a slotted
disc and 2 optical interrupters.
• Two opto interrupters are mounted on
the slotted disk.
• As the slotted disk turns, the light beam
between
the
LED
and
the
phototransistor of the opto interrupters
are connected and disconnected.
• This results in a pulse outputs from
each of the opto interrupters:

rotary encoder:

slotted disc:

Absolute Encoders
• Absolute encoder uses multiple groups of segments that
form concentric circles on the encoder wheel.
• The concentric circles start in the middle of the encoder
wheel and as the rings go out toward the outside of the ring
they each have double the number of segments than the
previous inner ring.
• The first ring, which is the innermost ring, has one
transparent and one opaque segment.
• The second ring out from the middle has two transparent
and two opaque segments, and the third ring has four of
each segment.
• If the encoder has 10 rings, its outermost ring will have 512
segments, and if it has 16 rings it will have 32,767
segments.

Absolute Encoders
• Since each ring of the absolute encoder has double the
number of segments of the prior ring, the values form
numbers for a binary counting system.
• Since the absolute encoder produces only one distinct
number or bit pattern for each position within its range.
– it knows where it is at every point between the two ends
of its travel,
– and it does not need to be homed to the machine each
time its power is turned off and on (like in Incremental
Encoder).

Rotary binary encoding-Example
• An example of a binary code, in an extremely simplified encoder with
only three contacts, is shown below.
• In general, where there are n contacts, the number of distinct positions
of the shaft is 2n. In this example, n is 3, so there are 2³ or 8 positions
Sector

Contact 1

Contact 2

Contact 3

Angle

1

off

off

off

0° to 45°

2

off

off

ON

45° to 90°

3

off

ON

off

90° to 135°

4

off

ON

ON

135° to 180°

5

ON

off

off

180° to 225°

6

ON

off

ON

225° to 270°

7

ON

ON

off

270° to 315°

8

ON

ON

ON

315° to 360

Rotary encoder for anglemeasuring devices marked in
3-bit binary.

Rotary binary encoding……issue
• In Rotary binary encoding, contacts produce a standard binary count as
the disc rotates.
• However, this has the drawback that if the disc stops between two
adjacent sectors, or the contacts are not perfectly aligned, so switches
can have a different moment, thus it become impossible to determine
the angle of the shaft.
– If contact 1 switches first, followed by contact 3 and then contact 2,
for example, the sequence of codes will be :
– off-on-on (starting position) on-on-on (first, contact 1 switches on)
on-on-off (next, contact 3 switches off) on-off-off (finally, contact 2
switches off)
• This will produce sectors sequence as 4, 8, 7 and then 5 from table
• In addition contracts can have different sequence and momentum, this
behavior can be undesirable and could cause the system to fail.
– i.e.: if the encoder were used in a robot arm, controller would think
that the arm was in the wrong position, and try to correct the error by
turning it through 180°, perhaps causing damage to the arm.

Linear Encoder
• A linear encoder is a sensor that is based on pair of
different scales to encodes position.
• The sensor reads the scale in order to convert the
encoded position into an analog or digital signal, which
can then be decoded into position motion controller.
• Motion can be determined by change in position over
time.
• Linear encoder technologies include optical, magnetic,
inductive, capacitive and eddy current.
• Linear encoders are used in metrology instruments,
motion systems and high precision machining tools
ranging from digital calipers to coordinate measuring
machines.

INCREMENTAL ENCODERS
• Incremental encoders are used to track motion and can be
used to determine position and velocity.
• Incremental encoders provide a specific number of equally
spaced pulses per revolution (PPR) or per inch or millimeter
of linear motion.
• This can be either linear or rotary motion.
– A single channel output is used for applications where
sensing the direction of movement is not important.
– Where direction sensing is required, quadrature output is
used, with two channels 90 electrical degrees out of
phase; circuitry determines direction of movement based
on the phase relationship between them.
• With Incremental Encoders as direction can be determined,
very accurate measurements can be made.

Incremental Encoder Classification
• They can be either mechanical or optical.
– Mechanical type requires debouncing and is typically
used as digital potentiometers on equipment including
consumer devices.
• Most modern home and car stereos use mechanical
rotary encoders for volume control.
• Mechanical type Incremental encoders are limited up to
10,000 counts per revolution.
– Optical type is used when higher RPMs are encountered
or a higher degree of precision is required.

Incremental rotary encoder---output
• An incremental encoder’s output
indicates motion.
• Incremental
rotary
encoder
provides cyclical outputs, when the
encoder is rotated.
• They employ two outputs called A &
B, known as quadrature outputs, as
they are 90 degrees out of phase.
• These signals are decoded to
produce a count-up pulse or countdown pulse.
• Since two sets of pulses are out of
phase from each other, it is
possible to determine which
direction the shaft is rotating by the
amount of phase shift b/w the first
set and second set of pulses.

Incremental encoder---output
• Incremental
encoder’s
output
indicates motion.
• To determine position, its pulses
must be accumulated by a counter.
• Counting is subject to loss during a
power interruption by electrical
transients.
• When starting up, the equipment
must be driven to a home position to
initialize the position counters.
– Some incremental encoders also
produce another signal known as
the “Z channel, this signal,
produced once per revolution of a
shaft encoder.
•

Incremental Encoders…..issue
• One of the major drawbacks of the incremental encoder is that
the number of pulses that are counted are stored in a buffer or
external counter.
• If power loss occurs, the count will be lost.
• This means that if a machine with an encoder has its electricity
turned off each night or for maintenance, the encoder will not
know its exact position when power is restored.
• The encoder must use a home-detection switch to indicate the
correct machine position.
• The incremental encoder uses a homing routine that forces the
motor to move until a home limit switch is activated.
• When the home limit switch is activated, the buffer or counter is
zeroed and the system knows where it is relative to fixed
positional points.
• The absolute encoder is designed to correct this problem.
• Absolute encoder is designed in such a way that the machine will
always know its location.

Incremental v/s absolute encoder …construction
Traditional absolute encoders
• Traditional absolute encoders have multiple code rings with various
binary weightings which provide a data word representing the absolute
position of the encoder within one revolution.
• This type of encoder is often referred to as a parallel absolute encoder.
• The distinguishing feature of the absolute encoder is that it reports the
absolute position of the encoder to the electronics immediately upon
power-up with no need for indexing.
Traditional incremental encoders
• A traditional incremental encoder works differently by providing an A and
a B pulse output that provide no usable count information in their own
right.
• Rather, the counting is done in the external electronics.
• The point where the counting begins depends on the counter in the
external electronics and not on the position of the encoder.
• To provide useful position information, the encoder position must be
referenced to the device to which it is attached, generally using an index
pulse.
• The distinguishing feature of the incremental encoder is that it reports an
incremental change in position of the encoder to the counting electronics

Incremental vs. Absolute encoders…operation
• The difference between incremental and absolute encoders is
analogous to the difference between a stop watch and a clock.
• A stop watch measures the incremental time that elapses
between its start and stop, much as an incremental encoder
will provide a known number of pulses relative to an amount of
movement.
• If you knew the actual time when you started the watch, you
can tell what time it is later by adding the elapsed time value
from the stop watch.
• For position control, adding incremental pulses to a known
starting position will measure the current position.
• When an absolute encoder is used, the actual position will
constantly be transmitted, just as a clock will tell you the
current time

Encoders Applications
• Encoders can be used in a wide variety of applications. They act as
feedback transducers for motor-speed control, as sensors for
measuring, cutting and positioning, and as input for speed and rate
controls. Some examples are listed below
• Door control devices
• Assembly machines
• Robotics
• Labeling machines
• Lens grinding machines
• x/y indication
• Plotters
• Analysis devices
• Testing machines
• Drilling machines
• Ultrasonic welding
• Mixing machines
• Converting machinery
• Medical equipment