Transcript SENSOR - Gadjah Mada University

INSTRUMENTATION
CHARACTERISTIC
WHAT IS INSTRUMENTATION
• is a collection of Instruments and their
application for the purpose of Observation,
Measurement and Control. Reference: ISA
std. S 51.1
• The key word is
– Observation or measurement
– control
Instrumentation
Cold water in
process
steam in
hot water out
3-15psi
Set point
TT
I/P
4-20 mA
TIC
Instrumentation
Process Control
Priyatmadi 2008
4-20 mA
MEASUREMENT
INSTRUMENTATION MODEL
What is sensor
• Def. 1. (Oxford dictionary)
– A device giving a signal for the detection or measurement
of a physical property to which it responds.
• Def. 2.
– A sensor is a device that receives a signal or stimulus and
response with an electrical signal.
Electrical
mechanical
Magnetic
Chemical
Optical
Radiation
Thermal
• Passive and active sensors
– Passive sensors are sensors which do not
provide energy to sense, they just absorb the
energy form the measurand and convert it to
electrical signal, e.g. pressure gauge,
thermocouple
– Active sensors are sensors which provide
energy in measurement process , e.g. radar
Sensor properties
Measurements
Heisenberg (1927): ”The momentum and position of a particle can not both
be precisely determined at the same time.”
Measuring activity disturbs the physical process (loading effect), produce
error
Measurement error:
That is the difference between the measured value and the true value.
error = measured value - true value
Deterministic errors:
They are repeated at every measurement, e.g. reading offset or bias. Such
errors can be reduced by proper calibration.
Random errors:
They are caused by several parameters and change in time in an
unpredictable fashion. They can be quantified by mean errors, standard
Deviation. Can be reduce by averaging several measurements
Sensor properties
output
input
Ideally, the sensor characteristic is a straight line should take
no time convert the input.
But that is never the case.
SENSOR CHARACTERISTIC
Accuracy : Error measurement
Sensitivity: change in output for unit change in
input
Resolution: the smallest change in the signal that
can be detected and accurately indicated by a
sensor.
Linearity: the closeness of the calibration curve to
a straight line.
Drift: the deviation from the null reading of the
sensor when the value is kept constant for a
long time.
SENSOR CHARACTERISTIC
Hysteresis: the indicated value depends on
direction of the test (increasing and decreasing)
Repeatability (precision): the maximum deviation
from the average of repeated measurements of
the same static variable.
Dynamic Characteristics: A sensor may have
some transient characteristic. The sensor can be
tested by a step response where the sensor
output is recorded for a sudden change of the
physical variable. The rise time, delay time, peak
time, settling time, percentage overshoot should
be as small as possible.
ACCURACY
Accuracy
• Accuracy is a degree of conformity of an indicated value
to recognized accepted standard value or ideal value
• Measured accuracy is the maximum positive and
negative deviation observed during a testing a device
under specified condition and procedure.
• Accuracy rating is a number or quantity that defines a
limit that error will not exceed when the device is used
under specific condition.
• When the operating conditions are not specified
reference operating condition should be assumed.
• In specification sheet term accuracy should be assumed
to mean accuracy rating.
• Accuracy rating includes the combines effect of
conformity, hysteresis, dead band, and repeatability.
Accuracy
output
downscale calibration
Specified characteristic
Low permissible error limit
Accuracy, rating
Upscale calibration
Max actual negative deviation
Max actual negative deviation
Low permissible error limit
0
Input
100%
Accuracy, rating
Accuracy rating can be expressed in number of form, e.g.:
1. In term of measured variable e.g.: ±2o C
2. In percent of span e.g.: ±0.5% of span
3. In percent of upper range e.g.: ±0.5% of upper range
4. In percent of scale length e.g.: ±0.5% of scale length
5. In percent of output reading e.g.: ±0.5% of output
-10
110
Range -10 to 110, upper range 110, lower range -10
Span = length = 120
Measuring Accuracy
Create calibration table by
1. Set 50% input (the input must be
secondary standard source)
2. Read the output
3. Compute the percentage deviation and
write it down in the table
4. Repeatedly increase the input until 100%
is reach then decrease until 0%, increase
and decrease again and again.
CALIBRATION TEST TABLE
Input
%
actual error %
up
down
up
down
up
0.05
down
up
0
-0.04
0.06
10
0.14
0.04
0.15
0.05
0.16
0.06
20
0.23
0.08
0.26
0.09
0.26
0.13
30
0.24
0.09
0.25
0.10
0.26
0.11
40
0.13
-0.07
0.15
-0.04
0.17
-0.04
-0.13
50
-0.18
-0.02
-0.16
0.01
-0.13
0.01
60
-0.27
-0.12
-0.25
0.10
-0.23
-0.08
70
-0.32
-0.17
-0.30
-0.16
-0.28
-0.12
80
-0.27
-0.17
-0.26
-0.15
-0.72
-0.13
90
-0.16
-0.06
-0.15
-0.05
-0.14
-0.04
100
0.09
0.11
0.1
Measured Accuracy
actual error % of span
Input %
up
down
up
down
up
0.05
down
up
0
-0.04
0.06
10
0.14
0.04
0.15
0.05
0.16
0.06
20
0.23
0.08
0.26
0.09
0.26
0.13
30
0.24
0.09
0.25
0.10
0.26
0.11
40
0.13
-0.07
0.15
-0.04
0.17
-0.04
-0.13
50
-0.18
-0.02
-0.16
0.01
-0.13
0.01
60
-0.27
-0.12
-0.25
0.10
-0.23
-0.08
70
-0.32
-0.17
-0.30
-0.16
-0.28
-0.12
80
-0.27
-0.17
-0.26
-0.15
-0.72
-0.13
90
-0.16
-0.06
-0.15
-0.05
-0.14
-0.04
100
0.09
0.11
0.1
Measured accuracy is the greatest positive and negative deviation of the
recorded values. Measured accuracy is -0.32% to +0.26%
DEAD BAND
Dead band.
Dead band is the range through which an input can be
varied without initiating observable response.
Dead band is usually expressed in percent of span
Dead band
Dead band.
To measure dead band proceed as follows:
1. Slowly increase the input until a detectable output
change is observed
2. Observe the input value
3. Slowly decrease the input until a detectable output
change is observed
4. Observe the input value
The difference between step 2 and 4 is the dead band.
Those steps is repeated for input from 0% to 100%.
The highest number is reported
Example: the dead band is 0.10% of the input span
DRIFT, POINT
Drift, Point
Drift is change of input-output relation over a period of time
Point drift is the maximum change in recorded output
during the test period, expressed in percent of output span.
Example: The point drift is 0.1% of output span for 24 hour
test
To measure drift proceed as follows:
1. Adjust the input to the desired values without overshoot
and record the output value. The test device should be
permitted to warm up before recording the initial output
value.
2. Maintain a fixed input value and fixed operating
condition for the duration of the test.
3. Record the output value during the test.
HYSTERESIS
Hysteresis
A property of element evidenced by the dependence of the output value
for the given excursion of input, upon the history of prior excursions and
the current direction of the traverse.
output
Hysteresis
input
Hysteresis + dead band
output
output
Dead band
Hysteresis
input
input
output
Hysteresis + dead band
input
Hysteresis
• Hysteresis is usually determined by subtracting the value
of dead band from the maximum separation between
upscale going and down scale going indication of
calibration report.
• This measurement is sometimes called hysteresis error
or hysteretic error
Hysteresis
Input %
actual error %
up
down
up
down
0
-0.04
0.05
0.06
10
0.14
0.04
0.15
0.05
0.16
0.06
20
0.23
0.08
0.26
0.09
0.26
0.13
30
0.24
0.09
0.25
0.10
0.26
0.11
40
0.13
-0.07
0.15
-0.04
0.17
-0.04
-0.13
50
-0.18
-0.02
-0.16
0.01
-0.13
0.01
60
-0.27
-0.12
-0.25
0.10
-0.23
-0.08
70
-0.32
-0.17
-0.30
-0.16
-0.28
-0.12
80
-0.27
-0.17
-0.26
-0.15
-0.22
-0.13
90
-0.16
-0.06
-0.15
-0.05
-0.14
-0.04
100
0.09
0.11
Hysteresis + dead band = 0.22%
If the dead band is 0.1% the hysteresis is 0.12%
0.1
LINEARITY
Linearity
The closeness to which a curve is approximates a straight
line
• The linearity of curve a is better
then curve b.
output
• It is usually measured as a
a
b
nonlinearity and is expressed as
linearity e.g. a maximum
deviation between an average
curve and a straight line.
• There are 3 type of linearity i.e.
independent, terminal based, and
input
zero based linearity
Independent Linearity
output
• It is the maximum deviation of
calibration curve (averaged of
upscale and down scale reading)
from a straight line so positioned
as to minimized the maximum
deviation.
input
Max ± deviation are minimized
And equal
Terminal based Linearity
output
• It is the maximum deviation of
calibration curve (averaged of
upscale and down scale reading)
from a straight line coinciding with
the calibration curve at the upper
and lower range values
input
Max deviation
Zero based Linearity
output
input
Max ± deviation are minimized
• It is the maximum deviation of
calibration curve (averaged of
upscale and down scale reading)
from a straight line so positioned
as to minimized the maximum
deviation and coincide with the
lower range value.
Measuring Linearity
1.
Take the average deviation for
every input
2. Find the straight line for
independent, terminal based,
and zero based linearity.
3. Compute the linearity
Input % dev %
0
10
-0.050
0.100
20
0.175
30
40
50
60
70
80
90
100
0.175
0.050
-0.075
-0.175
-0.225
-0.200
-0.100
0.100
1. Independent linearity =.18%
2. Terminal based linearity =.28%
3. Zero based linearity =0.21%
0.4
0.3
0.2
terminal based
straight line
0.1
0.0
-0.1
-0.2
-0.3
-0.4
2
zero based
straight line
1
3
independent
straight line
REPEATABILITY
Repeatability
The closeness of agreement among number of consecutive measurement
for the output of the same value of the input under the same operating
condition approaching from the same direction. It is usually measured in
non-repeatability and measured as repeatability in percent of span
output
Down scale calibration curve
Upscale calibration curve
Repeatability
input
Repeatability
actual error % of span
Input %
up
down
up
down
up
0.05
down
up
0
-0.04
0.06
10
0.14
0.04
0.15
0.05
0.16
0.06
20
0.23
0.08
0.26
0.09
0.26
0.13
30
0.24
0.09
0.25
0.10
0.26
0.11
40
0.13
-0.07
0.15
-0.04
0.17
-0.04
-0.13
50
-0.18
-0.02
-0.16
0.01
-0.13
0.01
60
-0.27
-0.12
-0.25
0.10
-0.23
-0.08
70
-0.32
-0.17
-0.30
-0.16
-0.28
-0.12
80
-0.27
-0.17
-0.26
-0.15
-0.72
-0.13
90
-0.16
-0.06
-0.15
-0.05
-0.14
-0.04
100
0.09
0.11
Repeatability =0.05%
0.1
Typical specification
SENSORS
Motion sensors
• These transducers measure the following
variables: displacement, velocity, acceleration,
force, and stress.
• Such measurements are used in mechanical
equipment such as servo-systems, robots, and
electrical drive systems.
• Motion sensors include the following types of
devices: potentiometers, resolvers, optical
encoders, variable inductance sensors
(displacement), tachometers (velocity), piezoresistive sensors (strain).
POTENTIOMETER
CAPACITIVE SENSORS
C
 0WL
d
C
 0W ( L  X )
C
d
 0WL
d 
Resolver
• Resolvers are used in
accurate servo and robot
systems to measure angular
displacement. Their signal
can be differentiated to
obtain the velocity.
• The rotor is connected with
the rotating object and
contains a primary coil
supplied by an alternating
current from a source voltage
vref. The stator consists of
two windings separated by
90o, with induced voltages
V01= K vref sin θ
V02= K vref sin θ
Tachometer
• The permanent magnet
generates a steady and
uniform magnetic field.
Relative
• motion between the field and
the rotor induces voltages,
which is proportional
• to the speed of the rotor.
• The inductance gives the
tachometer a certain time
constant so that the
• tachometer cannot measure
fast transient accurately.
Optical encoders
• These are optical devices to
measure angular displacement
and angular velocity.
• A disk of an optical encoder is
connected to the rotating shaft.
• The disk has patterns (holes).
• On one side of the disk there is
a light source and on the other
photo-detectors. When the disk
rotates the light is going
through the holes and the
photo-detectors generate
series of pulses.
• There are two types of optical
encoders: incremental and
absolute.
Optical encoders
• The incremental encoder provides a pulse each
time the shaft has rotated a defined distance.
• The disc of an absolute encoder has several
concentric tracks, with each track having an
independent light source and photo detector.
• With this arrangement a unique binary or Gray
coded number can be produced for every shaft
position.
LVDT
• The two secondary
coils are connected in
the opposite phase.
When the core is in the
middle there is no
output voltage.
• Moving the core from
the central position
unbalances the
secondary coils,
developing an output.
Vout
displacement
LVDT
Strain gauge
Strain gauge
• When external forces
are applied to a
stationary object,
stress and strain are
the result.
• Stress is defined as
Strain gauge
• Strain is defined as the amount of
deformation per unit length of an object
when a load is applied.
Strain (ε) = ΔL/L
• Typical values for strain are less than
0.005 inch/inch and are often expressed
in micro-strain units:
1 μstrain = 106 strain
Strain gauge
• Strain may be compressive or tensile and
is typically measured by strain gages.
• It was Lord Kelvin who first reported in
1856 that metallic conductors subjected
to mechanical strain exhibit a change in
their electrical resistance.
• This phenomenon was first put to
practical use in the 1930s.
Strain gauge
• Fundamentally, all strain gages are
designed to convert mechanical motion
into an electronic signal.
• A change in capacitance, inductance, or
resistance is proportional to the strain
experienced by the sensor.
Strain gauge
• If a wire is held under tension, it gets
slightly longer and its cross-sectional
area is reduced. This changes its
resistance (R) in proportion to the strain
sensitivity (S) of the wire's resistance.
When a strain is introduced, the strain
sensitivity, which is also called the gage
factor (GF), is given by:
GF = (ΔR/R)/(ΔL/L)
Strain gauge
• The ideal strain gage would change
resistance only due to the deformations
of the surface to which the sensor is
attached.
• However, in real applications,
temperature, material properties, the
adhesive that bonds the gage to the
surface, and the stability of the metal all
affect the detected resistance.
Strain gauge
• Because most materials do not have the
same properties in all directions, a
knowledge of the axial strain alone is
insufficient for a complete analysis.
Poisson, bending, and torsion strains
also need to be measured. Each requires
a different strain gage arrangement.
Strain gauge
• The deformation of an object can be
measured by mechanical, optical,
acoustical, pneumatic, and electrical
means.
• The earliest strain gages were
mechanical devices that measured strain
by measuring the change in length and
comparing it to the original length of the
object.
Strain gauge
• The most widely used characteristic that varies in
proportion to strain is electrical resistance. Although
capacitance and inductance-based strain gages have
been constructed, these devices' sensitivity to vibration,
their mounting requirements, and circuit complexity have
limited their application.
• The photoelectric gage uses a light beam, two fine
gratings, and a photocell detector to generate an
electrical current that is proportional to strain. The gage
length of these devices can be as short as 1/16 inch, but
they are costly and delicate.
Strain gauge
• The first bonded, metallic wire-type strain
gage was developed in 1938. The metallic
foil-type strain gage consists of a grid of
wire filament (a resistor) of approximately
0.001 in. (0.025 mm) thickness, bonded
directly to the strained surface by a thin
layer of epoxy resin
Strain gauge
Strain gauge
Application of Strain gauge
• Strain gages are used to measure displacement, force,
load, pressure, torque or weight. Modern strain-gage
transducers usually employ a grid of four strain elements
electrically connected to form a Wheatstone bridge
measuring circuit.
• The strain-gage sensor is one of the most widely used
means of load, weight, and force detection.
• As the force is applied, the support column experiences
elastic deformation and changes the electrical resistance
of each strain gage. By the use of a Wheatstone bridge,
the value of the load can be measured. Load cells are
popular weighing elements for tanks and silos and have
proven accurate in many other weighing applications.
Application of Strain gauge
• Strain gages may be bonded to cantilever
springs to measure the force of bending.
• The strain gages mounted on the top of the
beam experience tension, while the strain gages
on the bottom experience compression. The
transducers are wired in a Wheatstone circuit
and are used to determine the amount of force
applied to the beam.
Application of Strain gauge
• Strain-gage elements also are used widely in the
design of industrial pressure transmitters. Using
a bellows type pressure sensor in which the
reference pressure is sealed inside the bellows
on the right, while the other bellows is exposed
to the process pressure.
• When there is a difference between the two
pressures, the strain detector elements bonded
to the cantilever beam measure the resulting
compressive or tensile forces.
Application of Strain gauge
• A diaphragm-type pressure transducer is
created when four strain gages are attached to a
diaphragm.
• When the process pressure is applied to the
diaphragm, the two central gage elements are
subjected to tension, while the two gages at the
edges are subjected to compression.
• The corresponding changes in resistance are a
measure of the process pressure. When all of
the strain gages are subjected to the same
temperature, such as in this design, errors due
to operating temperature variations are reduced.
Piezoelectric Materials
• Many polymers, ceramics,
and molecules such as water
are permanently polarized:
some parts of the molecule
are positively charged, while
other parts of the molecule
are negatively charged.
Piezoelectric Materials
• When an electric field is applied to these
materials, these polarized molecules will align
themselves with the electric field, resulting in
induced dipoles within the molecular or crystal
structure of the material.
Piezoelectric Materials
Furthermore, a permanentlypolarized material such as
quartz (SiO2) or barium titanate
(BaTiO3) will produce an
electric field when the material
changes dimensions as a result
of an imposed mechanical
force.
These materials are
piezoelectric, and this
phenomenon is known as the
piezoelectric effect.
Piezoelectric Materials
• Conversely, an applied electric
field can cause a piezoelectric
material to change
dimensions.
• This phenomenon is known as
electrostriction, or the reverse
piezoelectric effect.
• Piezoelectric Effect Reverse
Piezoelectric Effect