Chapter 3 Sensors and Transducers[1]

Introduction to Instrumentation Engineering

Chapter 3: Sensors and Transducers

Department of Electrical and Computer Engineering

By Sintayehu Challa

Goals of the Chapter

• Define classification of sensors and some terminologies

• Introduce various types of sensors for measurement

purpose and their applications

• Example: Displacement, motion, level, pressure, temperature, …

2Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers

Overview

• Introduction

• Classification of sensors

• Passive sensors

• Active sensors

3Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers

Introduction

• Sensors

Sensing element

Signal conditioning

element

Signal conversion/processing

element

Output presentation

Non-electrical

quantity

Electrical

signal

• Sensors

• Elements which generate variation of electrical quantities (EQ) in response to variation of non-electrical quantities (NEQ)

• Examples of EQ

• Temperature, displacement, humidity, fluid flow, speed, pressure,…

• Sensor are sometimes called transducers


Introduction …

• Advantages of using sensors include

1. Mechanical effects such as friction is reduced to the minimum possibility

2. Very small power is required for controlling the electrical system

3. The electrical output can be amplified to any desired level

4. The electrical output can be detected and recorded remotely at a distance from the sensing medium and use modern digital computers computers

5. etc …


Introduction - Use of Sensors

1. Information gathering: Provide data for display purpose

• This gives an understanding of the current status of the system parameters

• Example: Car speed sensor and speedometer, which records the speed of a car against time

2. System control: Signal from the sensor is an input to a

controller controller

6

Controller

System

under

control Output signalDesires signal

Sensor

Introduction to Instrumntaion Eng‘g - Ch. 4 Sensors and Transducers

Introduction – Sensor Requirements

• The main function of a sensor is to respond only for the

measurement under specified limits for which it is designed

• Sensors should meet the following basic requirements

1. Ruggedness: Capable of withstanding overload

• Some safety arrangements should be provided for overload

protection

2. Linearity: Its input-output characteristics must be linear 2. Linearity: Its input-output characteristics must be linear

3. Repeatability: It should reproduce the same output signal when the same input is applied again and again

4. High output signal quality

5. High reliability and stability

6. Good dynamic response

7. No hysteresis, …


Overview

• Introduction


• Passive sensors

• Active sensors


Classification of Sensors

• Sensors can be divided on the basis of

• Method of applications

• Method of energy conversion used

• Nature of output signals

• Electrical principle

• In general, the classification of sensors is given by

•• Primary and secondary sensors

• Active and passive sensors

• Analog and Digital sensors


Primary and Secondary Sensors

• Classification is based on the method of application

• Primary sensor

• The input NEQ is directly sensed by the sensor

• The physical phenomenon is converted into another NEQ

• Secondary sensor

• The output of the primary sensor is fed to another (secondary) • The output of the primary sensor is fed to another (secondary) sensor that converts the NEQ to EQ

10

Load cell

Strain-gauge

NEQ

Secondary

sensor

Weight

(Force ∆∆∆∆F)

Primary

sensor

NEQ EQ

Displacement

∆∆∆∆dResistance

∆∆∆∆R


Active and Passive Sensor

• Classification based on the basis of energy conversion

• Active sensor

• Generates voltage/current in response to NEQ variation

• Are also called self-generating sensors

• Normally, the output of active sensors is in µV or mV

• Examples

•• Thermocouples: A change in temperature produces output voltage

• Photovoltaic cell: Change solar energy into voltage

• Hall-effect sensors, …

11

Active sensors

NEQ

Ex. Temperature

EQ

Voltage or current


Active and Passive ….

• Passive sensors

• Sensors that does not generate voltage or current, but produce element variation in R, L, or C

• Need an additional circuit to produce voltage or current variation

• Examples

• Thermistor: Change in temperature leads to change in resistance

• Photo resistor: Change in light leads to change in resistance• Photo resistor: Change in light leads to change in resistance

• Straingauge: Change in length or position into change in resistance)

• LVDT, Mic

12

Passive sensors

NEQ ∆∆∆∆R, ∆∆∆∆L, ∆∆∆∆C


Analog and Digital Sensors

• Classification based on the nature of the output signal

• Analog sensor

• Gives an output that varies continuously as the input changes

• Output can have infinite number of values within the sensor’s range

• Digital sensor

• Has an output that varies in discrete steps or pulses or sampled form and so can have a finite number of valuesand so can have a finite number of values

• E.g., Revolution counter: A cam, attached to a revolving body whose motion is being measured, opens and closes a switch

• The switching operations are counted by an electronic counter


Sensor Classification

• Sensors can also be classified according to the application

• Example

• Measurement of displacement, motion, temperature, intensity, sensors


Overview

• Introduction


• Passive sensors

• Resistive sensors

• Potentiometers, temperature dependent resistors, strain gauge,

photoconductors (photoresistors), Piezoresistive

• Capacitive sensors• Capacitive sensors

• Inductive sensors

• Active sensors


Resistive Sensors - Potentiometer

• Examples: Displacement, liquid level (in petrol-tank level

indicator) using potentiometer or rheostat

• Convert s linear (translatory) or angular (rotary) displacement into a change of resistance in the resistive element provided with a movable contact

• Petrol-tank level indicator

• Change in petrol level moves a

16

• Change in petrol level moves a potentiometer arm

• Output signal is proportion to the external voltage source applied across the potentiometer

• The energy in the output signal comes from the external power source


Resistive Sensors – Potentiometer …

• A linear or rotary movement of a moving contact on a slide

wire indicates the magnitude of the variable as a change in

resistance which can easily be converted by a proper

electrical circuit into measurements of volt or current


Resistive Sensors – Temperature Dependent Resistors

• Two classes of thermal resistors are

• Metallic element

• Semiconductor

• For most metals, the resistance increases with increase in

temperature

]1[...]1[)(R 0

2

210 TRTTRT ααα +≈+++=

• Where α is the temperature coefficient of resistance and given as

• Example: Platinum

• Has a linear temperature-resistance characteristics

• Reproducible over a wide range of temperature

• Platinum Thermometers are used for temperature measurement

18

]1[...]1[)(R 0210 TRTTRT ααα +≈+++=

0

1

R

R

T

∆

∆=α


Resistive Sensors – Temperature Dependent…

• Semiconductor based resistance thermometers elements

• The resistance of such elements decreases with increasing temperature

• Example: Thermistor

• The resistance-temperature relationship is non-linear and

governed by

)11

( −β

• Where R0 is the resistance at absolute temp (in Kelvin) and β is material constant expressed in degree Kelvin

• Most semiconductor materials used for thermometry

possess high resistivity and high negative temperature

coefficients

19

KTeRTTT 0

0

)(

0 300;)(R 0 ==−β


Resistive Sensors – Temperature Dependent…

• The temperature coefficient of resistance is

• β is typically 4000 k and for T = 300k,

2

0

1

TR

R

T

βα −=

∆

∆=

4000−=−=−=

βα

20

044.0300

400022

−=−=−=T

βα


Resistive Sensors – Strain Gauges

• Is a secondary transducer that senses tensile or

compressive strain in a particular direction at a point on

the surface of a body or structure

• Used to measure force, pressure, displacement

• ∆

)(R eR=

• Where e=∆l/l is the strain

• The resistance of an unstrained conductor is given as

• Under strained condition, resistance of conductor changes

by ∆R because of ∆l, ∆A, and/or ∆ρ

21

A

lρ=R


Resistive Sensors – Strain Gauges …

• To find the change in resistance ∆R,

• Dividing both sides by R, we get the fractional change as

ρρρ

ρρ

∆+∆−∆=

∆∂

∂+∆

∂

∂+∆

∂

∂=∆

A

lA

A

ll

A

RA

A

Rl

l

R

2

R

• Dividing both sides by R, we get the fractional change as

• Let us define eL = ∆l/l as the longitudinal stain and eT as

the transversal strain

• Also assume that eT = -νeL ,where ν is the Poisson’s Ratio

22

ρ

ρ∆+

∆−

∆=

∆

A

A

l

l

R

R



• Then, Gauge Factor, G is defined as

• G is also known as Strain-Sensitivity factor; rearranging

terms, we get

Gρρ

υ/

)21(∆

++=

LellG

R/R

/

R/R ∆=

∆

∆=

• Where is the Piezoresistive term

• For most metals, the Piezoresistive term is about 0.4 and 0.2 < ν < 0.5

• Thus, Gauge factor for metallic stain gauges is in the range 2.0–2.5 (not sensitive)

23

LeG υ )21( ++=

Le

ρρ/∆



• Sensitive measurements require very high Gauge factors in

the range of 100-300

• Such factor can be obtained from semiconductor strain

gauges

• Due to the significant contribution from the Piezoresistive term


Piezoresistive Pressure Sensor

• Piezoresistivity is a strain dependent resistivity in a single

crystal semiconductor

• When pressure is applied to the diaphragm, it causes a

strain in the resistor

• Resistance change is proportional to this strain, and hence change in pressure


Resistive Sensors – Photoconductor

• Are light sensitive resistors with non-linear negative

temperature coefficient

• Are resistive optical radiation transducers

• Photoconductors have resistance variation that depends

on illumination

• The resistance illumination characteristics is given by

• Where RD is Dark Resistance and E is illumination level in Lux

• Photoconductors are used in

• Cameras, light sensors in spectrophotometer

• Counting systems where an object interrupts a light beam hitting the photoconductor, etc.

26

E

DeRα−=R


Photoconductive Transducers

• A voltage is impressed on the semiconductor material

• When light strikes the semiconductor material, there is a decrease in the resistance resulting in an increase in the current indicated by the meter

• They enjoy a wide range of applications and are useful for measurement of radiation at all levels

• The schematic diagram of this device is shown below • The schematic diagram of this device is shown below


Photovoltaic Cells

• When light strikes the barrier between the transparent metal layer

and the semiconductor material, a voltage is generated

• The output of the device is strongly dependent on the load

resistance R

• The most widely used applications is the light exposure meter in

photographic work

Schematic of a photovoltaic cell.


Overview

• Introduction


• Passive sensors


• Capacitive sensors


• Active sensors • Active sensors


Capacitive Transducers

• The parallel plate capacitance is given by

• d= distance between plates

• A=overlapping area

• ε0 = 8.85x10-12 F/m is the absolute permittivity, εr =dielectric constant (ε =1 for air and ε =3 for plastics)

d

AC rεε 0=

constant (εr =1 for air and εr =3 for plastics)

• Displacement

measurement can be

achieved by varying d,

overlapping area A

and the dielectric

constantSchematic of a capacitive transducer.


Capacitive Transducers – Liquid Level Measurement

• A simple application of

such a transducer is for

liquid measurement

• The dielectric constant

changes between the

electrodes as long as there

is a change in the level of is a change in the level of

the liquid

Capacitive transducer for liquid level

measurement.


Capacitive Transducers – Pressure Sensor

• Use electrical property of a capacitor to measure the

displacement

• Diaphragm: elastic pressure senor displaced in proportion

to change in pressure

• Acts as a plate of a capacitor


Capacitive Transducers – Pressure Sensor

• Components: Fixed plate, diaphragm, displaying device,

dielectric material (air)

• When the diaphragm deflects, there is change in the gap

between the two plates which in turn deflects the meter

• Capacitance C of the

capacitor is inversely

proportional to distance d

dC

1α

33

proportional to distance d

between the plates, i.e.,


Capacitive Transducers - Linear Displacement

• Variable area capacitance displacement transducer


Overview

• Introduction


• Passive sensors


• Capacitive sensors


• Active sensors • Active sensors


Inductive Sensors

• For a coil of n turns, the inductance L is given by

• Where

• n: Number of turns of the coil

• l: Mean length of the magnetic path

• A: Area of the magnetic path

R

n

l

nAL

22

== µ

• A: Area of the magnetic path

• µ: Permeability of the magnetic material

• R: Magnetic reluctance of the circuit

• Application of inductive sensors

• Force, displacement, pressure, …

• Inductance variation can be in the form of

• Self inductance or

• Mutual inductance: e.g., differential transformer


• Input voltage (alternating current): One primary coil

• There will be a magnetic coupling between the core and the coils

• Output voltage: Two secondary coils connected in series

• Operates using the principle of variation of mutual

inductance

Linear Variable Differential Transformer (LVDT)

• The output voltage is

Schematic diagram of a differential transformer

• The output voltage is

a function of the

core’s displacement

• Widely used for

translating linear

motion into an

electrical signal


LVDT - Output Characteristics

Output characteristics of an LVDT


LVDT – Applications

• Measure linear mechanical displacement

• Provides resolution about 0.05mm, operating range from ± 0.1mm to ± 300 mm, accuracy of ± 0.5% of full-scale reading

• The input ac excitation of LVDT can range in frequency from 50 Hz to 20kHz

• Used to measure position in control systems and precision

manufacturing manufacturing

• Can also be used to measure force, pressure, acceleration,

etc..


LVDT – Bourdon Tube Pressure Gauge

• LVDT can be combined

with a Bourdon tube

• LVDT converts

displacement into an

electrical signal

• The signal can be

displayed on an electrical displayed on an electrical

device calibrated in

terms of pressure


• When pressure is applied via the hole,

the bellow expands a distance d

• This displacement can be calibrated in

terms of pressure

LVDT – Bellows

• The bellow is held inside a protective casing

• Differential pressure sensor

• Used to measure low pressure

terms of pressure

• Where:

• d: distance moved by the bellows in m,

• A is cross sectional area of the bellow in m2

• λ is the stiffness of the below in N.m-1

41

λA

dPunknown =


LVDT and Bellow Combination

• Bellows produce small displacement

• Amplified by LVDT and potentiometer


Overview

• Introduction


• Passive sensors

• Active sensors

• Thermoelectric transducers

• Photoelectric transducers

•• Piezoelectric transducers

• Hall-effect transudes

• Tachometric generators


Active Sensors - Thermocouple

• Thermoelectric transducers provide electrical signal in

response to change in temperature

• Example: Thermocouple

• Thermocouple: Converts thermal energy into electrical

energy

• Application: To measure temperature

• Contains a pair of dissimilar metal wires joined together at • Contains a pair of dissimilar metal wires joined together at

one end (sensing or hot junction) and terminated at the

other end (reference or cold junction)

• When a temperature difference exists b/n the sensing

junction and the reference, an emf is produced

44

)(....)()( 21

2

2

2

121 TTTTTTEemfInduced −≈+−+−== αβα


Active Sensors – Thermocouple …

• Typical material combinations used as thermocouples

Type Materials Temp. Range Output voltage (mV)

T Copper-Constantan -2000C to 3500C -5.6 to 17.82

J Iron-Constantan 0 to 7500C 0 to 42.28

E Chromel-Constantan -200 to 9000C -8.82 to 68.78

K Chromel-Alumel -200 to 12500C -5.97 to 50.63

• To get higher output emf

• Connect two or more Thermocouples in series

• For measurement of average temperature

• Connect Thermocouples in parallel

45

K Chromel-Alumel -200 to 12500C -5.97 to 50.63

R Platinum = 13%

Rhodium = 87%

0 to 14500C 0 to 16.74


Active Sensors – Thermocouple …

• Applications

• Temperature measurement

• Voltage measurement

• Rectifier based rms indications are waveform dependent

• They are normally designed for sinusoidal signals

• Hence, error for non-sinusoidal signals

• Use thermocouple based voltmeters• Use thermocouple based voltmeters

• Here, temperature of a hot junction is proportional to the true rms

value of the current


Active Sensors – Thermocouple Meter

• The measured a.c. voltage signal is

applied to a heater element

• A thermocouple senses the

temperature of the heater due to

heat generated ( )

• The d.c. voltage generated in the

thermocouple is applied to a

2

rmsI

thermocouple is applied to a

moving-coil meter• The thermocouple will be calibrated to read

current (Irms)

• AC with frequencies up to 100 MHz may

be measured with thermocouple meters

• One may also measure high frequency

current by first rectifying the signal to DC

and then measuring the DC

Schematic of a

thermocouple meter.


Overview

• Introduction


• Passive sensors

• Active sensors







Photoelectric Transducers

• Versatile tools for detecting radiant energy or light

• Are extensively used in instrumentation

• Most known photosensitive devices include

1. Photovoltaic cells

• Semiconductor junction devices used to convert radiation energy into electrical energy


Photoelectric Transducers …

2. Photo diode

• A diode that is normally reverse-biased=> Current is very low

• When a photon is absorbed, electrons are freed so current starts to flow, i.e., the diode is forward biased

• Has an opening in its case containing a lens which focuses incident light on the PN junction

3. Photo transistor3. Photo transistor

• Also operate in reverse-biased

• Responds to light intensity on its lens instead of base current


Overview

• Introduction


• Passive sensors

• Active sensors







Piezoelelectric Transducers

• Convert mechanical energy into electrical energy

• If any crystal is subject to an external force F, there will be

an atomic displacement, x

• The displacement is related to the applied force in exactly the same way as elastic sensor such as spring

• Asymmetric crystalline material such as Quartz, Rochelle

Salt and Barium Tantalite produce an emf when they are Salt and Barium Tantalite produce an emf when they are

placed under stress

• An externally force, entering the sensor through its

pressure port, applies pressure to the top of a crystal

• This produces an emf across the crystal proportional to the magnitude of the applied pressure


Piezoelelectric Transducers

• A piezoelectric crystal is placed between two plate

electrodes

• Application of force on such a plate will develop a stress

and a corresponding deformation

• With certain

crystals, this

The piezoelectric effect

crystals, this

deformation will

produce a potential

difference at the

surface of the

crystal

• This effect is called

piezoelectric effect


Piezoelelectric Transducers …

• Induced charge is proportional to the impressed force

Q = d F

• d= charge sensitivity (C/m2)/(N/m2) = proportionality constant

• Output voltage E= g t P

• t= crystal thickness

• P = impressed pressure

• g=voltage sensitivity (V/m)/(N/m2) • g=voltage sensitivity (V/m)/(N/m2)

• Shear stress can also produce piezoelectric effect

• Widely used as inexpensive pressure transducers for

dynamic measurements


Piezoelelectric Transducers ….

• Piezoelelectric sensors have good frequency response

• Example: Accelerometer

55

Piezoelectric accelerometer


Piezoelelectric Transducers …

• Example: Pressure Sensors

• Detect pressure changes by

the displacement of a thin

metal or semiconductor

diaphragm

• A pressure applied on the

diaphragm causes a strain on diaphragm causes a strain on

the piezoelectric crystal

• The crystal generates voltage

at the output

• This voltage is proportional to

the applied pressure


Overview

• Introduction


• Passive sensors

• Active sensors







Hall-effect Transducers

• Hall voltage is produced when a material is

• Kept perpendicular to the magnetic field and

• A direct current is passed through it

• The Hall-voltage is expressed as

• Where t

IKV C

HH

β=

• Where

• Ic: Control current flowing through the Hall-effect sensor, in Amps

• β: Flux density of the magnetic field applied, in Wb/m2

• t: Thickness of the Hall-effect sensor, in meters

• KH : Hall-effect coefficient

• Hall-effect sensors are used to measure flux density

• Can detect very week magnetic fields or small change in magnetic flux density


Hall-effect Transducers …

• Like active sensors, it generates

voltage VH

• It also need an external control

current IC like passive sensors

• The sensor can be used for

measurement of

• Magnetic quantities (B, φ)• Magnetic quantities (B, φ)

• Mobility of carriers

• Very small amount of power


Hall-effect Transducers …

• Magnetic field forces

electrons to concentrate

on one side of the

conductor (mainly uses

semiconductor)

• This accumulation

creates emf, which is creates emf, which is

proportional to the

magnetic field strength

• Used in proximity

sensors


Overview

• Introduction


• Passive sensors

• Active sensors







Tachometric Generators

• Tachometer – any device used to measure shaft’s rotation

• Tachometric generator

• A machine, when driven by a rotating mechanical force, produces an electric output proportional to the speed of rotation

• Essentially a small generators

• Tachometric generators connect to the rotating shaft, • Tachometric generators connect to the rotating shaft,

whose displacement is to be measured, by, e.g.,

• Direct coupling or

• Means of belts or gears

• They produce an output which primarily relates to speed

• Displacement can be obtained by integrating speed

• Types of Tachometric generators: Generally a.c. or d.c.


Tachometric Generators

• Voltage generated is proportional to rotation of the shaft

63

D.C. tachometric generator A.C. tachometric generator


Documents

Chapter 3 Sensors and Transducers[1]