department of electrical and computer engineering introduction to instrumentation engineering...

Department of Electrical and Computer Engineering

Introduction to Instrumentation Engineering

Chapter 3: Sensors and Transducers

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. 3 Sensors and Transducers

Overview

Introduction Classification of sensors Passive sensors Active sensors


Introduction

Sensors Elements which generate variation of electrical quantities (EQ) in

response to variation of non-electrical quantities (NEQ)

Examples of NEQ Temperature, displacement, humidity, fluid flow, speed, pressure,…

Sensor are sometimes called transducers

4

Sensing element

Signal conditioning

element

Signal conversion/processing

element

Output presentation

Non-electrical quantity

Electricalsignal

Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and 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

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

6

Controller

System under control Output signalDesires signal

Sensor


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

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



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)

sensor that converts the NEQ to EQ

10

Load cell

Strain- gauge

NEQ

Secondary sensor

Weight(Force F)

Primary sensor

NEQ EQ

Displacementd

Resistance 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 V 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 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 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 Classification of sensors Passive sensors

Resistive sensors Potentiometers, temperature dependent resistors, strain gauge,

photoconductors (photoresistors), Piezoresistive

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

16

Petrol-tank level indicator 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


Instrum. & Control Eng. for Energy Systems - Ch. 4 Sensors and Transducers 17

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

Where 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

19

]1[...]1[)(R 02

210 TRTTRT

0

1

R

R

T


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

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

Most semiconductor materials used for thermometry possess high resistivity and high negative temperature coefficients

21

KTeRT TT 00

)11

(

0 300;)(R 0


Resistive Sensors – Temperature Dependent…

The temperature coefficient of resistance is

is typically 4000 k and for T = 300k,

22

20

1

TR

R

T

044.0300

400022

T


ECEg535:-Instrumentation Eng'g By Sintayehu Challa

strain gauge

If a strip of conductive metal is stretched, it will become skinnier and longer, both changes resulting in an increase of electrical resistance end-to-end.

Conversely, if a strip of conductive metal is placed under compressive force (without buckling), it will broaden and shorten. Such a device is called a strain gauge.

ECEg535:-Instrumentation Eng'g By Sintayehu Challa

Contd.

Most strain gauges are smaller than a postage stamp, and they look something like this

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

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

26

)(R eR

A

lR


Resistive Sensors – Strain Gauges …

To find the change in resistance 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 = -eL ,where is the Poisson’s Ratio

27

A

lA

A

ll

A

RA

A

Rl

l

R

2

R

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

Where is the Piezoresistive term

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

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

28

LeG

/)21(

Le

/

LellG

R/R

/

R/R



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.

31

EDeR

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


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


Resistive sensors Capacitive sensors Inductive sensors

Active sensors


Capacitive Transducers

The parallel plate capacitance is given by

d= distance between plates A=overlapping area 0 = 8.85x10-12 F/m is the absolute permittivity, r =dielectric

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

d

AC r 0

Displacement measurement can be achieved by varying d, overlapping area A and the dielectric constant

Schematic 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 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 sensor 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

dC

1

38

Capacitance C of the capacitor is inversely proportional to distance d between the plates, i.e.,


Capacitive Transducers - Linear Displacement

Variable area capacitance displacement transducer


Overview


Resistive sensors Capacitive sensors Inductive 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 : 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

41

R

N

l

NAL

22


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)

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

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 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

Where: d: distance moved by the bellows in m, A is cross sectional area of the bellow in m2

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

LVDT – Bellows

The bellow is held inside a protective casing Differential pressure sensor

Used to measure low pressure

46

A

dPunknown


LVDT and Bellow Combination

Bellows produce small displacement Amplified by LVDT and potentiometer


Overview


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 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

49

)(....)()( 212

22

121 TTTTTTEemfInduced


Introduction to Instrumntaion Eng‘g - Ch. 3 Sensors and Transducers 50

Active Sensors – Thermocouple …

Typical material combinations used as thermocouples

To get higher output emf Connect two or more Thermocouples in series

For measurement of average temperature Connect Thermocouples in parallel

51

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

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 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 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.

2rmsI


Overview




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 transistor Also operate in reverse-biased Responds to light intensity on its lens instead of base current


Overview




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 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

The piezoelectric effect

With certain 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)

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

62

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 the piezoelectric crystal

The crystal generates voltage at the output

This voltage is proportional to the applied pressure


Overview




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

B: 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

65

t

BIKV CHH


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, ) Mobility of carriers Very small amount of power

66Introduction to Instrumntaion Eng‘g – Ch.3 Sensors and Transducers

Hall-effect Transducers …

Magnetic field forces electrons to concentrate on one side of the conductor (mainly uses semiconductor)

This accumulation creates emf, which is proportional to the magnetic field strength

Used in proximity sensors


Overview




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, 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

70

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


NoticeMid Semester Exam On December

1,2012@8:30 A.M(Saturday) In Room 109 and 113


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