EEE 432

Measurement and Instrumentation

Lecture 9

Sensor Technologies

Prof. Dr. Murat Aşkarİzmir University of EconomicsDept. of Electrical and Electronics Engineering

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Transducers

Transducer

A device which converts one form of energy to another

Sensor

When input is a physical quantity and output electrical

Actuator

When input is electrical and output a physical quantity

ActuatorsSensors Physical

parameter

Electrical

Output

Electrical

Input

Physical

Output

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Sensors over Human Beings

Human beings are equipped with 5 different types of sensors.

Eyes detect light energy, ears detect acoustic energy.

Tongue detects certain chemicals related with tasting

Nose detect certain chemicals related with smelling

Skin detects pressures and temperatures.

The eyes, ears, tongue, nose, and skin receive these signals then send

messages to the brain which outputs a response.

For example, when you touch a hot plate, it is your brain that tells you it is

hot, not your skin.

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Conversion Methods

• Physical Properties

– thermo-electric, thermo-elastic, thermo-magnetic, thermo-optic

– photo-electric, photo-elastic, photo-magnetic,

– electro-elastic, electro-magnetic

– magneto-electric

• Chemical Properties

– chemical transport, physical transformation, electro-chemical

• Biological Properties

– biological transformation, physical transformation

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Why do we use Sensors

Sensors are used widely in system control..

Without the use of sensors, there would be no automation!

They are embedded in our bodies, automobiles, airplanes,

cellular telephones, radios, chemical plants, industrial plants

and countless other applications.

Signals obtained from sensors will used to control the system

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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

Amperes’s 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

exhibit paramagnetic behavior

Faraday’s 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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How to Choose A Sensor

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Classification of Sensors

Where is the information coming from?

Inside: Proprioceptive sensors motor speed, wheel load, heading of the robot, battery status

Outside: Exteroceptive sensors distances to objects, intensity of the ambient light, unique

features

How does it work? Requires energy emission?

No: Passive sensors temperature probes, microphones, CCD

Yes: Active sensors Controlled interaction -> better performance Interference

Simple vs. composite (sonar vs. wheel sensor)

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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 with

deflection)

Metal A

Metal B

δ

)]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 differenceResistorThermal

Therm istor

exp2

gER

kT

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Capacitive Transducers

• Capacitance of a parallel plate capacitor :

– A: overlapping area of plates (m2)

– d: distance between the two plates of the

capacitor (m)

– : permittivity of air or free space 8.85 pF/m

– dielectric constant

0

0r AC

d

: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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Accelerometer–I

Accelerometers are used to measure acceleration along one or more axis and are relatively insensitive to orthogonal directions

Applications

– Motion, vibration, blast, impact, shock wave

Vibrating Base

m

k b

Position Sensor

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Accelerometer–II

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 objectundergoing acceleration through a spring and a damper;

– Piezoelectric accelerometers: a microscopic crystal structure is mountedon a mass undergoing acceleration; the piezo crystal is stressed byacceleration forces thus producing a voltage

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Accelerometer–III– Capacitive accelerometer: consists of two microstructures

(micromachined features) forming a capacitor; acceleration forcesmove one of the structure causing a capacitance changes.

– Piezoresistive accelerometer: consists of a beam or micromachinedfeature whose resistance changes with acceleration

– Thermal accelerometer: tracks location of a heated mass duringacceleration by temperature sensing

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Accelerometer Applications

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

Helmet: Impact

Detection

Segway

2 axis joystick

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Gyroscope

Heading sensors, that keep the orientation to a fixed frame

absolute measure for the heading of a mobile system.

Two categories,

Mechanical Gyroscopes

Standard gyro

Rated gyro

Optical Gyroscopes

Rated gyro

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Mechanical Gyroscopes Concept: inertial properties of a fast spinning rotor

– gyroscopic precession

Angular momentum associated with a spinning wheel keeps the axis of the gyroscope inertially stable.

Reactive torque t (tracking stability) is proportional to the spinning speed w, the precession speed W and the wheels inertia I.

No torque can be transmitted from the outer pivot to the wheel axis

– spinning axis will therefore be space-stable

Quality: 0.1° in 6 hours

If the spinning axis is aligned with the north-south meridian, the earth’s rotation has no effect on the gyro’s horizontal axis

If it points east-west, the horizontal axis reads the earth rotation

I

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Global Positioning System (GPS) -I

• Developed for military use

• Recently it became accessible for commercial applications

• 24 satellites (including three spares) orbiting the earth every 12 hours at a

height of 20.190 km.

• Four satellites are located in each of six planes inclined 55 degrees with

respect to the plane of the earth’s equators

• Location of any GPS receiver is determined through a time of flight

measurement

Technical challenges:

• Time synchronization between the individual satellites and the GPS

receiver

• Real time update of the exact location of the satellites

• Precise measurement of the time of flight

• Interferences with other signals

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Global Positioning System (GPS) -II