Transcript Document 7196276

Sensors and Transducers
• Characteristic
• Transducers converts the form of energy
• A microphone coverts sound energy into electrical energy
• A speaker converts electrical energy into sound
• Sensors are transducers that used to detect
and/or measure something
• Used to convert mechanical, thermal, magnetic,
chemical, or etc variations into electrical voltages and
currents
Sensors and Transducers
• This course only focuses on a limited number
of sensor types. Specifically:
• Some Temperature sensors
• Some light sensors
• Some strain gages
• Many other types exist
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Smart Sensors
Infrared
Switches
RFID
etc
Sensors and Transducers
• Temperature Sensors
• Types of Temperature Sensors
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Thermocouple
Resistance temperature device(RTD)
Thermistor
Monolithic IC Sensors
• Thermocouple
• Characteristics
• Most common sensor
• A pair of dissimilar wires welded together at the sensing location
• A temperature difference from the welded end and the other end
causes a DC voltage at the non welded end
• Can be used under extreme conditions
» Ovens, Furnaces, Nuclear tests
Sensors and Transducers
• Temperature Sensors
Sensors and Transducers
• Temperature Sensors
• Thermocouple
• Operation
• When wires made of dissimilar metals are
» Welded together at both ends
» With different temperatures at both ends
Current flows
Sensors and Transducers
• Temperature Sensors
• Thermocouple
• Operation
• Open the pair of wires in
between the two ends a
voltage develops
» Called Seebeck Voltage
» Proportional to
temperature difference
V  s  T
Where
V  OpenCircui tVolts
s  SeebeckTempCoefficient
T  TempDifferenceBetweenEnds
Sensors and Transducers
• Temperature Sensors
• Thermocouple
• Operation
• Equation is linear over only a small range of temperatures
• Tables of corrected voltages in 10 increments is available from
the NBS for each type
• Reference Junction
• Voltage developed is dependent upon the temperature
difference between ends – NOT Absolute Temperature of the
welded end
» Where’s the
cold junction
• It’s at room temp
• Voltage will be wrong
• Need a 00C ref
Sensors and Transducers
• Temperature Sensors
• Thermocouple
• Reference Junction
• Lab Set up
» Not practical for most situations
• Practical Reference Junction
solutions
• Electronic Ice points
» Available for All types
of thermocouples
» Encased electronic device
that balances an internal bridge
circuit which generates a voltage to cancel out effect that
the measurement end isn’t at OOC
Sensors and Transducers
• Temperature Sensors
• Thermocouple
• Practical Reference Junction
solutions
• Isothermal block
» Usually used with
computerized (also microcontrollers) data collection systems
» The isothermal block is a good conductor of heat not
electrical current
» However it’s resistance is effected measurably by changes in
temperature
» Block is always near the point were the voltages are
measured
» Computerized measuring system calculates cool end
temperature based on the block resistance and corrects the
voltage reading
Sensors and Transducers
• Temperature Sensors
• Thermocouple
• Typical Problems
• A short of the two wires
» Junction then will be at the point of the short
» Temperatures readings will be incorrect
• No Reference Junction Compensation
» Temperatures readings will be incorrect
» Test – Short the inputs to the compensator and room
temperature should be the new reading
• If extensions of the thermal couple wires are used they should
be of a larger size and material
» Different materials create Incorrect readings since the
connection of dissimilar materials creates a new junction
» Larger size is needed for IR drops
Sensors and Transducers
• Temperature Sensors
• Thermocouple
• Typical Problems
• Noise pick-up
» Long leads form an antenna – uses shielding. e.g., grounded
over braiding of copper
• Extreme Temperature Gradient
» Can damage the thermocouple should have protection
• Environment can change the metal and it’s thermal
characteristics
» Chenicals
» molten metals
– new alloys
Sample Commercial
thermocouple assemblies
Sensors and Transducers
• Temperature Sensors
• Resistance Temperature Device
• Key principle
• As the temperature of a resistor increases so does its resistance
• Measure the change in the resistance of a known resistor –
calculate the temperature change
» Linear relation ship for smaller changes – more linear than
thermocouples – NBS has correction tables for the typical
types of measurement resistors
• Typical construction
• Wire wound resistor in on a ceramic core using platinum wire
» Stable (linear) over a wide range of temperatures
» Temperature coefficient = 0.00385/0C
• Typical Values: 10 – several kilo-ohms
• Most common value 100Ω
Sensors and Transducers
• Temperature Sensors
• Resistance Temperature Device
• Measuring Circuit Types
• RTD Bridge circuit
• Constant Current Source
• RTD Bridge circuit
• Platinum resistor is remote from
the bridge circuit which is isolated
from the sensing point
• Bridge is balanced at a known
temperature
» Eliminates consideration of
the connecting leads
• Voltage developed is proportional
to the temperature change
Sensors and Transducers
• Temperature Sensors
• Resistance Temperature Device
• Constant Current through RTD
• Voltage across the RTD rises and
the resistance increase with the
rise in temperature
• The constant current also increases
the temperature of the resistance
and effects the temperature
reading
» The correction factor for
common platinum RTDs has
been determined
TC  0.50C / mW
Sensors and Transducers
• Temperature Sensors
• Thermistor
• Resistors with high negative temperature coefficients
• Resistance decreases with an increase in temperature
• High temperature coefficients means that there is a significant
change in resistance for a small temperature change
• Construction
• Semiconductor material
» In either tube or bead shapes
• Can be used as a plain resistor in circuits such as a bridge or voltage
divider
» Come in a Wide range of values
» Also come with manufacturer provided resistance vs
temperature curves
Sensors and Transducers
• Temperature Sensors
• Thermistor
• Construction
• Also come manufacturer provided resistance vs temperature curves
• Sample for thermistor with nominal value of 5kΩ at 00C
Sensors and Transducers
• Temperature Sensors
• Monolithic IC Sensors
• Current or voltage types are available
• They have linear output voltages or currents with temperature
changes
• Typical values: 1µA/0K; 10mV/0K
» 1 0K = 1 0C
Sensors and Transducers
• Light Sensors
• Typical uses of the sensors
• Measure intensity of the light
• Detect the presence or absence of light
• Types of Light Sensors
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Photovoltaic Cells
Photoconductive Cells
Photo Diodes
Phototransistors
• Photovoltaic Cells
• aka, Solar Cells
• Semiconductor material that generates a voltage when light
shines on it
• 2.5 by 5 cm cell can produce 0.4 V with 180mA of current
Sensors and Transducers
• Light Sensors
• Photovoltaic Cells
• Sometimes used to detect the presence of light
• Photoconductive Cells
• aka, photoresistors
• Characteristics of Photoresisters
• Uses bulk resistivity which decreases with increasing illumination,
allowing more photocurrent to flow.
• Signal current from the detector can be varied over a wide range by
adjusting the applied voltage.
• Thin film devices made by depositing a
layer of a photoconductive material
on a ceramic substrate.
Sensors and Transducers
• Light Sensors
• Photoconductive Cells
• Characteristics of
Photoresisters
• Metal contacts with
external connection.
These thin films have a
high sheet resistance.
Therefore, the space
between the two
contacts is made narrow
and long for low cell
resistance at moderate
light levels.
Sensors and Transducers
• Light Sensors
• Photoconductive Cells
• Light Intensity Application
• With little or no light the
voltage at point X is low
• As the intensity of the
light on the sensor increases
the voltage at X will increase
• By adjusting Rf,a usable output range of voltages that the is
proportional to the light intensity can be obtained
• Presence or Absence of
Light application
• Activates a electromechanical
counter when the light is blocked
Sensors and Transducers
• Light Sensors
• Photoconductive Cells With a Microcontroller
• Critical aspect of this application a BASIC command for
measuring the RC decay time on a connected circuit
• RCTIME command is designed to measure RC decay time on a
circuit like the one below. The lower the count recorded the
brighter the light measured
• RCTIME Pin, State, Duration
» Pin argument is the number of
the I/O pin that you want to measure
» State argument - 1 if the voltage across the capacitor starts
above 1.4 V and decays downward. 0 if the voltage across
the capacitor starts below 1.4 V and grows upward
» Duration argument has to be a variable that stores the time
measurement, which is in 2 μs units
• Very simple circuit – range of measured light is limited only
by the size of the variable used to store the count.
Sensors and Transducers
• Light Sensors
• Photodiodes
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A diode that is forward biased by light
Very fast reactions to changing light levels
Same physical size as LEDs
Have small windows through which light is
sensed
• Testing is simple
• When the window is blocked
» High resistance is read
• Shine a bright light (several footcandles) on it while still
connected to an ohmmeter
» The resistance will drop significantly
• Phototransistors
• Usually used instead of photodiodes when low light levels are
measured
Sensors and Transducers
• Light Sensors
• Phototransistors
• Usually used instead of photo resistors when low
light levels are broken at high rates
• Typical ratings
• Like low power transistors
» 30-50V maximum collector to emitter voltages
» Max collector currents of 25mA
Sensors and Transducers
• Light Sensors
• Replacement Considerations
• Best option is an exact replacement
• If not possible match the following characteristics :
• Voltage, current, & power ratings; physical size
• Light sensitivity
» Can be specified nm (human sight 400 -700) nm (700nm –
red light)
» Called spectral response
» Can also be specified in angstroms Å. 10 Å = 1nm
• Light Insensitivity
» For photoresistors – X-kΩ at Y-footcandles
» 1 Foot candle = light falling on 1 square foot – one foot
from a standardcandle
» For phototransistors: Collector current at a specified light
level
Sensors and Transducers
• Light Sensors
• Other Problems with light sensing systems
• Burned out, weak, or obstructed light sources
• Can be a simple problem of dirty light filters or lens
• Light shields may have been misaligned by a bump
• Mechanical Sensors
• Characteristics
• Used to measure:
• Force
• motion
• position
• The chapter covers Strain gages
• They measure Forces
• Weight is a common force
Sensors and Transducers
• Strain gages
• Characteristics
• Sensors used to measure change in the dimensions of solid
objects caused by forces
• Information is critical to designs of mechanical systems
• Used in load cells which are used to measure weights of
objects
• Measurements can range from a few pounds to the weight of a
fully loaded tractor trailer rig
• Strain and Stress
• Strain = ΔL/L0 , where ΔL = change in length due to a force
and L0 = the original length before the force
was applied
• Can be caused by tension or compression forces
Sensors and Transducers
• Strain gages
• Strain and Stress
• Strain = ΔL/L0 , where ΔL = change in length due to a force
and = the original length before the force was
applied
• Can be caused by tension or compression forces
• Stress is a measure of the force applied that has been
normalized to a unit area
• Stress = F/A , where F= the total force applied and A= crosssectional area
• The ratio of Stress/Strain is a constant value for each
material
• Called Young’s Modulus and has been tabulated for many
material
• Most metals won’t stretch beyond 0.5% without deforming
Sensors and Transducers
• Strain gages
• Strain and Stress
• Resistor conductance can be determined from: R=ρL/A
• Where R= resistance in ohms, ρ (rho) is the resistivity of the
material, L= length of the material, & A is the cross-sectional
area of the material
• If the gage material under stress increases it length by0.4% - it’s
resistance will increase by 0.4%
» Some commercial gages have been designed to yield
multiples of the change in length in change of resistance – A
Gage Factor
• Construction
• Metal or semiconductor foil woven back and forth to increase
the length
• Range of common values 30 -3000 Ω
• Most common sizes 120 Ω and 350 Ω
Sensors and Transducers
• Strain gages
• Strain and Stress
• Calculations
L
R  R0 (1 
 GF )
L
• Where R =resistance of the gage under stress, R0 = Original
resistance of the gage, ΔL = change in length of the gage, L =
original length of the gage, GF = gage factor
• Typical Bridge configurations
Sensors and Transducers
• Strain gages
• Typical Bridge configurations
• The 1/4 bridge has a gain factor of 1
• Change of resistance causes the bridge to unbalance
• The ½ bridge has two strain gages
• One in tension mode and one in compression mode, like in the
metal beam drawing –bottom right of previous slide
» rg1 is stretched and rg2 is compressed
» Changes double the resistance change
• GF = 2
• The full bridge has four gages and a GF of four
• Problems with Strain Gages
• Temperature changes
• If outside the circuitry must have temperature compensation
» e.g., Las Vegas temperatures range from the 20’s – 115+
Sensors and Transducers
• Strain gages
• Typical
Some Smart Sensors
• Smart Sensor Characteristics
• Inside every smart sensor
• One or more primitive sensors
• Primitive sensors are devices or materials that have some electrical
property that changes with some physical phenomenon
• Additional, built-in electronics makes a smart sensor "smart
• Smart sensors able to do one or more of the following:
• Pre-process their measured values into meaningful quantities
• Communicate their measurements Orchestrate the actions of
primitive circuits and sensors to "take" measurements
• Make decisions and initiate action based on sensed conditions,
independent of a microcontroller
• Remember calibration or configuration settings
• Three samples of smart sensors follow
Ultrasonic Distance Detector
• Example – Parallax Ping)))
• The Ping))) sensor interfaced
with a Controller measures
distances to objects
• Range of 3 centimeters to 3.3 meters
• Accurate to the centimeter
• How it works
• The controller starts by sending the Ping))) sensor a pulse to
start the measurement.
• Then, the Ping))) sensor waits a short period of time, enough for
the controller to start a elapsed time counter.
• Then, at the same time the Ping))) sensor chirps its 40 kHz tone,
it sends a high signal to the controller.
Ultrasonic Distance Detector
• Characteristics of the Parallax part
• How it works
• When the Ping))) sensor detects the echo with its ultrasonic
microphone, it changes that high signal back to low.
• The controller uses a variable to store how long the high signal
from the Ping))) sensor lasted.
Ultrasonic Distance Detector
• Characteristics of the Parallax part
• How it works
• This time measurement is how long it took sound to travel to the
object and back.
• Using this measurement and the speed of sound in air, you can
make your program calculate the object's distance in centimeters,
inches, feet, etc.
The Ping))) sensor's chirps are not audible because 40 kHz is ultrasonic.
What we consider sound is our inner ear's ability to detect the variations in air pressure
caused by vibration. The rate of these variations determines the pitch of the tone. Higher
frequency tones result in higher pitch sounds and lower frequency tones result in lower pitch
tones.
Most very young people can hear tones that range from 20 Hz, which is very low pitch, to 20 kHz,
which is very high pitch. Subsonic is sound with frequencies below 20 Hz, and ultrasonic is sound
with frequencies above 20 kHz. Since the Ping))) sensor's chirps are at 40 kHz, they are
definitely ultrasonic, and not audible to people.
Accelerometer
• Acceleration
• A measure of how quickly speed changes.
• Just as a speedometer is a meter that measures speed
• An accelerometer is a meter that measures acceleration.
• Some of the measurements that an
accelerometer can take:
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Acceleration
Tilt and tilt angle
Incline
Rotation
Vibration
Collision
Gravity
Accelerometer
• Accelerometers are already used in many
different devices, including personal
electronics, specialized equipment and
machines. Here are just a few examples:
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Self-balancing robots
Tilt-mode game controllers
Model airplane autopilots
Car alarm systems
Crash detection/airbag deployment systems
Human motion monitoring systems
Leveling tools
RFID
• Definition
• Radio-frequency identification (RFID) is an automatic
identification method, relying on storing and remotely
retrieving data using devices called RFID tags or
transponders.
• An RFID tag is an object that can be applied to or incorporated
into a product, animal, or person for the purpose of
identification using radiowaves. Some tags can be read from
several meters away and beyond the line of sight of the
reader.
• Most RFID tags contain at least two parts. One is an integrated
circuit for storing and processing information, modulating and
demodulating a (RF) signal, and other specialized functions.
The second is an antenna for receiving and transmitting the
signal.
RFID
• A significant thrust in RFID use is:
• In enterprise supply chain management
improving the efficiency of inventory
tracking and management.
• Types of Tags
Walmart RFID Tag
• RFID tags come in three general
varieties:- passive, active, or semi-passive (also known as
battery-assisted). Passive tags require no internal power
source, thus being pure passive devices (they are only
active when a reader is nearby to power them), whereas
semi-passive and active tags require a power source,
usually a small battery.
RFID
• Simple Security System based on RFID
• The system enables the reader
• Waits for a Tag to be read
• Compares the read identification to a list of known IDs
• It there is a match the an electronic door lock is opened
» We will use a Red LED
» In addition the name associated with that Tag will be printed
in the debug window
» And the Piezo electric speaker beeps
• If there isn’t a match – the speaker groans
Contact and Touch Sensors
• Characteristics
• As complex as a touch screen on a CRT or LCD monitor
• Use in such varied applications as Customer service Kiosks in
stores to video poker EGMs in casinos
• To a tactile sensors
• Used as input devices to video games, to the joy stick in the
cockpit of a F-16 fighter …….
• Contact sensors can be as simple as a switch
• Used in numerous systems to detect a changed condition
Touchscreens
• Overview
• Touchscreens systems come in three technologies each
having three components: The technologies are:
• Surface Acoustic Wave (SAW)
• Resistive and,
• Capacitive
• Information provided to the system
• X, Y coordinates of the point touched
• Used as substitute for button activation
• Surface Acoustic Wave (SAW)
• It is based on sending acoustic waves across a clear glass
panel with a series of transducers and reflectors.
• When a finger touches the screen, the waves are absorbed,
causing a touch event to be detected at that point (X, Y
coordinates)
Tactile Sensors
• Can be used to detect a wide range of stimuli:
• Presence or absence of a grasped object
• Successfully pick up object, or failed to grasp object
• To discovering a complete tactile image.
• Texture, impact, slip and other contact conditions generate
specific force and position patterns
• This information again can be used to identify the state of
manipulation.
• Objects size, shape and mass can be used to test whether the
object has been rotated ninety degrees and flipped , etc
• Measure contact forces
• Critical for Joy Stick implementations
Tactile Sensors
• Can be used to detect a wide range of stimuli:
• Measure contact forces
• Many of these sensors are on input devices that don’t move
• i.e., a joy stick
• The greater the force in a direction the greater of the output
• The output verses input levels can be either linear or
exponential
• Control stick on an F-16
• For easy and accurate
control of the aircraft during
high G-force combat
maneuvers, a side stick
controller is used