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Intro to Sensors
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Overview
Sensors?
Commonly Detectable Phenomenon
Physical PrinciplesHow Sensors Work?
Need for Sensors
Choosing a Sensor Examples
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Sensors?
American National Standards Institute A device which provides a usable output in response to a specified measurand
A sensor acquires a physical quantity and converts it into a signal
suitable for processing (e.g. optical, electrical, mechanical)
Nowadays common sensors convert measurement of physical
phenomena into an electrical signal
Active element of a sensor is called a transducer
Sensor
Input Signal Output Signal
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Transducer?A device which converts one form of energy to another
When input is a physical quantity and output electrical Sensor
When input is electrical and output a physical quantity Actuator
ActuatorsSensors
Physicalparameter
Electrical
Output
ElectricalInput
Physical
Output
e.g. Piezoelectric:
Force -> voltage
Voltage-> Force
=> Ultrasound!
Microphone, Loud Speaker
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Commonly Detectable Phenomena
Biological
Chemical
Electric
Electromagnetic
Heat/Temperature
Magnetic
Mechanical motion (displacement, velocity, acceleration, etc.)
Optical
Radioactivity
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Common Conversion Methods
Physical
thermo-electric, thermo-elastic, thermo-magnetic, thermo-optic
photo-electric, photo-elastic, photo-magnetic,
electro-elastic, electro-magnetic
magneto-electric
Chemical
chemical transport, physical transformation, electro-chemical
Biological
biological transformation, physical transformation
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Commonly Measured Quantities
Stimulus Quantity
Acoustic Wave (amplitude, phase, polarization), Spectrum, Wave
Velocity
Biological & Chemical Fluid Concentrations (Gas or Liquid)
Electric Charge, Voltage, Current, Electric Field (amplitude, phase,
polarization), Conductivity, Permittivity
Magnetic Magnetic Field (amplitude, phase, polarization), Flux,
Permeability
Optical Refractive Index, Reflectivity, Absorption
Thermal Temperature, Flux, Specific Heat, Thermal Conductivity
Mechanical Position, Velocity, Acceleration, Force, Strain, Stress,
Pressure, Torque
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Physical Principles: Examples
Amperess Law
A current carrying conductor in a magnetic field experiences a force (e.g.
galvanometer)
Curie-Weiss Law
There is a transition temperature at which ferromagnetic materials exhibitparamagnetic behavior
Faradays Law of Induction
A coil resist a change in magnetic field by generating an opposing
voltage/current (e.g. transformer)
Photoconductive Effect
When light strikes certain semiconductor materials, the resistance of the material
decreases (e.g. photoresistor)
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Choosing a Sensor
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Need for Sensors
Sensors are pervasive. They are embedded inour bodies, automobiles, airplanes, cellulartelephones, radios, chemical plants, industrial
plants and countless other applications.
Without the use of sensors, there would be no
automation !!Imagine having to manually fill Poland Spring
bottles
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Motion Sensors
Monitor location of various parts in a system absolute/relative position
angular/relative displacement proximity
acceleration
Principle of operation Magnetic, resistive, capacitance, inductive, eddy current, etc.
Primary Secondary
LVDT Displacement Sensor
Optoisolator
Potentiometer
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Strain Gauge: Motion, Stress, Pressure
Strain gauge is used to measure deflection, stress, pressure, etc.
The resistance of the sensing element changes with applied strain
A Wheatstone bridge is used to measure small changes in the strain gauge resistance
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Temperature Sensor: Bimetallic Strip
Bimetallic Strip
Application
Thermostat (makes or
breaks electrical
connection withdeflection)
Metal A
Metal B
)]T-(T1[ 00 LL
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Temperature Sensor: RTD
Resistance temperature device
(RTD)
0
11
0
00 )]T-(T1[
TTeRR
RR
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Other Temperature Sensors
Thermistor
Thermocouple: Seeback effect to
transform a temperature difference to a
voltage difference
ResistorThermal
Therm istor
exp
2
gE
R
kT
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Capacitance TransducersI
Recall, capacitance of a parallel plate capacitor is:
A: overlapping area of plates (m2)
d: distance between the two plates of the capacitor (m)
: permittivity of air or free space 8.85pF/m
dielectric constant
0
0rA
Cd
:r
The following variations can be utilized to make capacitance-based sensors.Change distance between the parallel electrodes.
Change the overlapping area of the parallel electrodes.
Change the dielectric constant.
Air escape hole
air
Fuel tankParallel plate
capacitor
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AccelerometerI
Accelerometers are used to measureacceleration along one or more axis andare relatively insensitive to orthogonaldirections
Applications
Motion, vibration, blast, impact, shockwave
Mathematical description is beyond thescope of this presentation.
Vibrating Base
m
k b
Position Sensor
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AccelerometerII Electromechanical device to measure acceleration forces
Static forces like gravity pulling at an object lying at a table
Dynamic forces caused by motion or vibration
How they work Seismic mass accelerometer: a seismic mass is connected to the object undergoing
acceleration through a spring and a damper;
Piezoelectric accelerometers: a microscopic crystal structure is mounted on a massundergoing acceleration; the piezo crystal is stressed by acceleration forces thusproducing a voltage
Capacitive accelerometer: consists of two microstructures (micromachined features)forming a capacitor; acceleration forces move one of the structure causing a capacitancechanges.
Piezoresistive accelerometer: consists of a beam or micromachined feature whoseresistance changes with acceleration
Thermal accelerometer: tracks location of a heated mass during acceleration bytemperature sensing
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Automotive: monitor vehicle tilt, roll, skid, impact, vibration, etc., to
deploy safety devices (stability control, anti-lock breaking system,
airbags, etc.) and to ensure comfortable ride (active suspension)
Aerospace: inertial navigation, smart munitions, unmanned vehicles
Sports/Gaming: monitor athlete performance and injury, joystick, tilt
Personal electronics: cell phones, digital devices
Security: motion and vibration detection
Industrial: machinery health monitoring Robotics: self-balancing
Accelerometer Applications
WII Nunchuk: 3 axis accelerometer2 axis joystick
Segway
Helmet: Impact Detection
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MX2125 Accelerometer: How it Works A MEMS device consisting of
a chamber of gas with a heating element in the center
four temperature sensors around its edge
Hold accelerometer levelhot gas pocket rises to the top-center of theaccelerometers chamberall sensors measure same temperature
Tilt the accelerometerhot gas pocket collects closer to one or twotemperature sensorssensors closer to gas pocket measure higher temperature
MX2125 electronics compares temperature measurements and outputs pulses(pulse duration encodes sensor o/p)
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Light Sensor
Light sensors are used in
cameras, infrared detectors, and
ambient lighting applications
Sensor is composed of
photoconductor such as a
photoresistor, photodiode, or
phototransistor
p n
I
+ V -
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Photoresistors Light sensitive variable resistors.
Its resistance depends on the intensity of light incident upon it.
Under dark condition, resistance is quite high (M: called dark resistance).
Under bright condition, resistance is lowered (few hundred). Response time:
When a photoresistor is exposed to light, it takes a few milliseconds, before itlowers its resistance.
When a photoresistor experiences removal of light, it may take a few seconds
to return to its dark resistance.
Photoresisotrs exhibit a nonlinear characteristics for incident optical illumination
versus the resulting resistance.
Symbol
10 10log logR P
R
101
103
102
101
104
102
103
104
Relative illumination (P)
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Magnetic Field Sensor
Magnetic Field sensors are
used for power steering,
security, and current
measurements ontransmission lines
Hall voltage is proportionalto magnetic fieldx x x x x x
x x x x x x
x x x x x x
+ + + + + + + + + + + + + + +
- - - - - - - - - - - - - - -
I (protons) +
VH-
B
tqn
BIVH
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Ultrasonic Sensor
Ultrasonic sensors are used
for position measurements
Sound waves emitted are in
the range of 2-13 MHz Sound Navigation And
Ranging (SONAR)
Radio Dection And
Ranging (RADAR)
ELECTROMAGNETIC
WAVES !!
15 - 20
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Photogate
Photogates are used in
counting applications (e.g.
finding period of period
motion)
Infrared transmitter and
receiver at opposite ends of
the sensor
Time at which light is broken
is recorded