sensors-introduction

8/2/2019 Sensors-Introduction

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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
http://us.st11.yimg.com/us.st.yimg.com/I/yhst-64180292248885_1984_322755


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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

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