INSTRUMENTATIONECE Fourth Semester

Presented By:-Sumit Grover

Lect., Deptt. of ECE

Detailed Contents

• Objectives• Sensors and transducer• Classification of transducers• Temperature transducers• Resistance transducers• Capacitive transducers• Inductive transducers

• Objectives• Sensors and transducer• Classification of transducers• Temperature transducers• Resistance transducers• Capacitive transducers• Inductive transducers

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Objectives

• Ability to understanding the definition,functions & categories of transducers.

• List the classes and types and examplesof transducers.

• Operations and applications for eachtransducers

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• Ability to understanding the definition,functions & categories of transducers.

• List the classes and types and examplesof transducers.

• Operations and applications for eachtransducers

Sensor and transducer• ‘Sensor' is `a device that detects a change in a physical

stimulus and turns it into a signal which can be measured orrecorded.E.g. : Thermistor

• ‘Transducer' is 'a device that transfers power from one systemto another in the same or in the different form'.E.g. Thermistor with it associate circuit convert heat toelectricity.

As a comparison……

‘Sensor' for the sensing element itself and 'transducer' for thesensing element plus any associated circuitry. All transducerswould thus contain a sensor and most (not all) sensors wouldalso be transducers.

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• ‘Sensor' is `a device that detects a change in a physicalstimulus and turns it into a signal which can be measured orrecorded.E.g. : Thermistor

• ‘Transducer' is 'a device that transfers power from one systemto another in the same or in the different form'.E.g. Thermistor with it associate circuit convert heat toelectricity.

As a comparison……

‘Sensor' for the sensing element itself and 'transducer' for thesensing element plus any associated circuitry. All transducerswould thus contain a sensor and most (not all) sensors wouldalso be transducers.

Sensing process

Definition of atransducer

Transducer is any device that converts energy in one formto another energy. The majority either convert electricalenergy to mechanical displacement or convert somenon-electrical physical quantity, such as temperature,sound or light to an electrical signal.

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Transducer is any device that converts energy in one formto another energy. The majority either convert electricalenergy to mechanical displacement or convert somenon-electrical physical quantity, such as temperature,sound or light to an electrical signal.

Functions of transducer1. To sense the presence, magnitude, change in, and frequency

of some measurand.

2. To provide an electrical output that, when appropriatelyprocessed and applied to readout device, gives accuratequantitative data about the measurand

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1. To sense the presence, magnitude, change in, and frequencyof some measurand.

2. To provide an electrical output that, when appropriatelyprocessed and applied to readout device, gives accuratequantitative data about the measurand

Transducer Electricaloutput

Measurand

Excitation

Measurand – refers to the quantity, property or condition which thetransducer translates to an electrical signal.

Classification of transducersTransducer can be classified according to their application,based primarily on the physical quantity, property, orcondition that is measured.

The transducer can be categories into:

A) Passive transducer:- requires an external power- output is a measure of some variation, such resistance andcapacitance. E.g. : condenser microphone

B) Self generating transducer:- not require an external power, and they produce analogvoltage or current when stimulated by some physical form ofenergy. E.g. : Thermocouple

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Transducer can be classified according to their application,based primarily on the physical quantity, property, orcondition that is measured.

The transducer can be categories into:

A) Passive transducer:- requires an external power- output is a measure of some variation, such resistance andcapacitance. E.g. : condenser microphone

B) Self generating transducer:- not require an external power, and they produce analogvoltage or current when stimulated by some physical form ofenergy. E.g. : Thermocouple

Selecting a transducers1. Operating range2. Sensitivity3. Frequency response and resonant frequency4. Environmental compatibility -5. Minimum sensitivity measurand.6. Accuracy7. Usage and ruggedness8. Electrical parameter

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1. Operating range2. Sensitivity3. Frequency response and resonant frequency4. Environmental compatibility -5. Minimum sensitivity measurand.6. Accuracy7. Usage and ruggedness8. Electrical parameter

Transducers to be covered

• Temperature transducers• Resistive Position Transducer• Capacitive Transducer• Inductive Transducer• Strain Gauge• LVDT• Photoelectric

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• Temperature transducers• Resistive Position Transducer• Capacitive Transducer• Inductive Transducer• Strain Gauge• LVDT• Photoelectric

Temperature Transducers

Temperature transducers can be divided intofour main categories:

1. Resistance Temperature Detectors (RTD)2. Thermocouples3. Thermistor4. Ultrasonic transducers

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Temperature transducers can be divided intofour main categories:

1. Resistance Temperature Detectors (RTD)2. Thermocouples3. Thermistor4. Ultrasonic transducers

Detectors of wire resistance temperature common employ platinum,nickel or resistance wire elements, whose resistance variation withtemperature has high intrinsic accuracy. They are available in manyconfigurations and size and as shielded or open units for bothimmersion and surface applications.The relationship between temperature and resistance of conductors canbe calculated from the equation:

whereR = the resistance of the conductor at temperature t (0C)R0 = the resistance at the reference temperature, usually

200Cα = the temperature coefficient of resistanceΔT = the difference between the operating and the

reference temperature

1) Resistance Temperature Detector (RTD)

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Detectors of wire resistance temperature common employ platinum,nickel or resistance wire elements, whose resistance variation withtemperature has high intrinsic accuracy. They are available in manyconfigurations and size and as shielded or open units for bothimmersion and surface applications.The relationship between temperature and resistance of conductors canbe calculated from the equation:

whereR = the resistance of the conductor at temperature t (0C)R0 = the resistance at the reference temperature, usually

200Cα = the temperature coefficient of resistanceΔT = the difference between the operating and the

reference temperature

)1(0 TRR

2) Thermocouple

It consists of two wires of different metals are joined togetherat one end, a temperature difference between this end and theother end of wires produces a voltage between the wires. Themagnitude of this voltage depends on the materials used for thewires and the amount of temperature difference between thejoined ends and the other ends.

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It consists of two wires of different metals are joined togetherat one end, a temperature difference between this end and theother end of wires produces a voltage between the wires. Themagnitude of this voltage depends on the materials used for thewires and the amount of temperature difference between thejoined ends and the other ends.

Cont’dThe emf of the thermocouple :

E = c(T1 – T2) + k(T12 – T2

2)

Wherec and k = constant of the thermocouple

materialsT1 = The temperature of the “hot”

junctionT2 = The temperature of the “cold” or

“reference” junction

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Wherec and k = constant of the thermocouple

materialsT1 = The temperature of the “hot”

junctionT2 = The temperature of the “cold” or

“reference” junction

3) Thermistor

A thermistor is a semiconductor made by sinteringmixtures of metallic oxide, such as oxides of manganese,nickel, cobalt, copper and uranium.Termistors have negative temperature coefficient (NTC).That is, their resistance decreases as their temperaturerises.

Types of thermistor ResistanceDisc 1 to 1MΩWasher 1 to 50kΩRod high resistance

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A thermistor is a semiconductor made by sinteringmixtures of metallic oxide, such as oxides of manganese,nickel, cobalt, copper and uranium.Termistors have negative temperature coefficient (NTC).That is, their resistance decreases as their temperaturerises.

Types of thermistor ResistanceDisc 1 to 1MΩWasher 1 to 50kΩRod high resistance

This figure showsresistance versustemperature for afamily thermistor.The resistance valuemarked at the bottomend of each curve is avalue at 250C

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This figure showsresistance versustemperature for afamily thermistor.The resistance valuemarked at the bottomend of each curve is avalue at 250C

Note!The resistancedecreases as theirtemperature rises-NTC

Advantages of thermistor• Small size and low cost

• Fast response over narrow temperature range

• Good sensitivity in Negative Temperature Coefficient(NTC) region

• Cold junction compensation not required due to dependenceof resistance on absolute temperature.

• Contact and lead resistance problems not encountered dueto large resistance

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• Small size and low cost

• Fast response over narrow temperature range

• Good sensitivity in Negative Temperature Coefficient(NTC) region

• Cold junction compensation not required due to dependenceof resistance on absolute temperature.

• Contact and lead resistance problems not encountered dueto large resistance

Limitations of thermistor

• Non linearity in resistance vs temperaturecharacteristics

• Unsuitable for wide temperature range• Very low excitation current to avoids self heating• Need of shielded power lines, filters, etc due to high

resistance

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• Non linearity in resistance vs temperaturecharacteristics

• Unsuitable for wide temperature range• Very low excitation current to avoids self heating• Need of shielded power lines, filters, etc due to high

resistance

The principle of the resistance transducer is that the physicalvariable under measurement causes a resistance change in thesensing element.

A common requirement in industrial measurement and controlwork is to be able to sense the position of an object or distance ithas moved.

.

Resistive Position Transducer

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The principle of the resistance transducer is that the physicalvariable under measurement causes a resistance change in thesensing element.

A common requirement in industrial measurement and controlwork is to be able to sense the position of an object or distance ithas moved.

. A

LR

Potentiometer

R: resistance change

: density

L: Length

A: area

Cont’d

FIG 1 (a) FIG 1 (b)

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Figure shows the construction of a displacement transducer uses aresistance element with a sliding contact or wiper linked to the objectbeing monitored.

The resistance between the slider and one end of the resistance elementdepends on the position of the object. The output voltage depends onthe wiper position and therefore is a function of the shaft position

FIG 1 (a) FIG 1 (b)

Consider Fig 1 (b), if the circuit is unloaded, the output voltage V0 is a certainfraction of VT, depending on the position of the wiper:

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RR

R

V

V

T

This equation shows that the output voltage is directly proportional tothe position of the wiper, if the resistance of the transducer is distributeduniformly along the length of travel of the wiper

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This equation shows that the output voltage is directly proportional tothe position of the wiper, if the resistance of the transducer is distributeduniformly along the length of travel of the wiper

EXAMPLE 1

A displacement transducer with a shaft stroke of 4 in. is used in thecircuit of figure 1 (b). R1 +R2 is 1000 Ω and VT = 4 V.The wiper is 1.5 in from B. Find V0?

Capacitive Transducer

The capacitance of a parallel plate capacitor is given by

wherek = dielectric constantA = the area of the plate, in m2

εo = 8.854 x 10-12 F/md = the plate placing in m

)(0 Faradsd

kAC

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The capacitance of a parallel plate capacitor is given by

wherek = dielectric constantA = the area of the plate, in m2

εo = 8.854 x 10-12 F/md = the plate placing in m

Cont’dForms of Capacitance Transducers

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Rotary plate capacitorRectilinear CapacitanceTransducer

Thin diaphragm

Cont’d

Rotary plate capacitor:

The capacitance of this unit proportional to theamount of the fixed plate that is covered, thatshaded by moving plate. This type of transducerwill give sign proportional to curvilineardisplacement or angular velocity.

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Rotary plate capacitor:

The capacitance of this unit proportional to theamount of the fixed plate that is covered, thatshaded by moving plate. This type of transducerwill give sign proportional to curvilineardisplacement or angular velocity.

Cont’d

Rectilinear capacitancetransducer:

It consists of a fixed cylinder anda moving cylinder. Thesepieces are configured so themoving piece fits inside thefixed piece but insulated fromit.

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Rectilinear capacitancetransducer:

It consists of a fixed cylinder anda moving cylinder. Thesepieces are configured so themoving piece fits inside thefixed piece but insulated fromit.

Cont’d

Thin diaphragm:

A transducer that varies thespacing between surfaces. Thedielectric is either air or vacuum.

Often used as Capacitancemicrophones.

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Thin diaphragm:

A transducer that varies thespacing between surfaces. Thedielectric is either air or vacuum.

Often used as Capacitancemicrophones.

Advantages:1. Has excellent frequency response2. Can measure both static and dynamic phenomena.

Disadvantages:1. Sensitivity to temperature variations2. the possibility of erratic or distortion signals owing to

long lead length

Applications:1. As frequency modulator in RF oscillator2. In capacitance microphone3. Use the capacitance transducer in an ac bridge circuit

Cont’d

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Advantages:1. Has excellent frequency response2. Can measure both static and dynamic phenomena.

Disadvantages:1. Sensitivity to temperature variations2. the possibility of erratic or distortion signals owing to

long lead length

Applications:1. As frequency modulator in RF oscillator2. In capacitance microphone3. Use the capacitance transducer in an ac bridge circuit

Inductive TransducerInductive transducers may be either of the self generating orpassive type. The self generating type utilises the basicelectrical generator principle, i.e, a motion between aconductor and magnetic field induces a voltage in theconductor (generator action). This relative motion betweenthe field and the conductor is supplied by changes in themeasurand.

An inductive electromechanical transducer is a device thatconverts physical motion (position change) into a change ininductance. Transducers of variable inductance type workupon one of the following principles:1. Variation of self inductance2. Variation of mutual inductance

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Inductive transducers may be either of the self generating orpassive type. The self generating type utilises the basicelectrical generator principle, i.e, a motion between aconductor and magnetic field induces a voltage in theconductor (generator action). This relative motion betweenthe field and the conductor is supplied by changes in themeasurand.

An inductive electromechanical transducer is a device thatconverts physical motion (position change) into a change ininductance. Transducers of variable inductance type workupon one of the following principles:1. Variation of self inductance2. Variation of mutual inductance

Cont..

Inductive transducers are mainly used for the measurement ofdisplacement. The displacement to be measured isarranged to cause variation in any of three variables:

1. Number of turns2. Geometric configuration3. Permeability of the magnetic material or magnetic circuits

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Inductive transducers are mainly used for the measurement ofdisplacement. The displacement to be measured isarranged to cause variation in any of three variables:

1. Number of turns2. Geometric configuration3. Permeability of the magnetic material or magnetic circuits

Strain GaugeThe strain gauge is an example of a passive transducer thatuses electric resistance variation in wires to sense the strainproduced by a force on wires. It is a very versatile detectorand transducer for measuring weight, pressure, mechanicalforce, or displacement.

The construction of a bonded straingauge (see figure) shows a fine wireelement looped back and forth on amounting plate, which is usuallycemented to the member undergoingstress. A tensile stress tends toelongate the wire and therebyincrease its length and decrease itscross-sectional area.

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The construction of a bonded straingauge (see figure) shows a fine wireelement looped back and forth on amounting plate, which is usuallycemented to the member undergoingstress. A tensile stress tends toelongate the wire and therebyincrease its length and decrease itscross-sectional area.

The combined effect is an increase in resistance:

A

LR

Where,

ρ: the specific resistance of the conductor material in ohm metersL : length of conductor (meters)A : area of conductor (m2)

As consequence of strain, 2 physical qualities are particular interest:

1) The change in gauge resistance2) The change in length

The relationship between these two variables called gauge factor,K, is expressed mathematically as

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Where,

ρ: the specific resistance of the conductor material in ohm metersL : length of conductor (meters)A : area of conductor (m2)

As consequence of strain, 2 physical qualities are particular interest:

1) The change in gauge resistance2) The change in length

The relationship between these two variables called gauge factor,K, is expressed mathematically as

LL

RRK

/

/

Where

K= the gauge factorR=the initial resistance in ohms (without strain)∆R= the change in initial resistance in ohmsL= the initial length in meters (without strain)∆L=the change in initial length in meters

∆L/L same unit with G, therefore

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Where

K= the gauge factorR=the initial resistance in ohms (without strain)∆R= the change in initial resistance in ohmsL= the initial length in meters (without strain)∆L=the change in initial length in meters

∆L/L same unit with G, therefore

G

RRK

/

From Hooke theory, stress, S, is defined as internal force/area.

WhereS= the stress in kilograms per square meterF= the force in kilogramsA= area in square meters

Then the modulus of elasticity of material E or called Young’s modulus(Hooke’s Law) is written as:

A

FS

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From Hooke theory, stress, S, is defined as internal force/area.

WhereS= the stress in kilograms per square meterF= the force in kilogramsA= area in square meters

Then the modulus of elasticity of material E or called Young’s modulus(Hooke’s Law) is written as:

G

SE

Where,E= Young modules in kg per square meterS= the stress in kilograms per square meterG= the strain (no units)

Metallic strain gauge – formed from thinresistance wire or etched from thin sheets of metalfoil.

Wire gauge (small) – to minimum leakage – forhigh T applications

Semiconductor strain gauge – high outputtransducers as load cells

Strain gauge is generally used as one arm ofbridge

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Metallic strain gauge – formed from thinresistance wire or etched from thin sheets of metalfoil.

Wire gauge (small) – to minimum leakage – forhigh T applications

Semiconductor strain gauge – high outputtransducers as load cells

Strain gauge is generally used as one arm ofbridge

It consists basically of a primary winding andtwo secondary windings, wound over a hollowtube and positioned so the primary winding isbetween two secondaries. In figure shows theconstruction of the LVDT.

LINEAR VARIABLE DIFFERENTIALTRANSFORMER (LVDT)

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It consists basically of a primary winding andtwo secondary windings, wound over a hollowtube and positioned so the primary winding isbetween two secondaries. In figure shows theconstruction of the LVDT.

An iron core slides within the tube and therefore affects the magnetcoupling between the primary and the two secondaries. When the core is inthe centre, voltage induced in the two secondaries is equal. When the coreis moved in one direction from centre, the voltage induced in one windingis increased and that in the other is decreased. Movement in the oppositedirection reverses this effect

Cont..

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Cont..

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• What’s Photoelectric Effect?-is the emission of electrons from matter uponthe absorption of electromagnetic radiation, suchas ultraviolet radiation or x-rays.-refers to theemission, or ejection, of electrons from thesurface of, generally, a metal in response toincident light.

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• What’s Photoelectric Effect?-is the emission of electrons from matter uponthe absorption of electromagnetic radiation, suchas ultraviolet radiation or x-rays.-refers to theemission, or ejection, of electrons from thesurface of, generally, a metal in response toincident light.

Photoelectric TransducerCan be categorized as: photoemissive, photoconductive, or

photovoltaic.

No. Types Characteristics

1. Photoemmisive radiation falling into a cathode causeselectrons to be emitted from cathodesurface.

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radiation falling into a cathode causeselectrons to be emitted from cathodesurface.

2. Photoconductive the resistance of a material is changewhen it’s illuminated.

3. Photovoltaic Generate an output voltageproportional to radiation intensity

• Examples of Photoelectric Transducer

• (i) The Photomultiplier Tube• (ii) Photoconductive Cells OR Photocells

the electrical resistance of the materialsvaries with the amount of light striking.

• (iii) The Photovoltaic Cell or solar cell- produce an electrical current whenconnected to the load.

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• Examples of Photoelectric Transducer

• (i) The Photomultiplier Tube• (ii) Photoconductive Cells OR Photocells

the electrical resistance of the materialsvaries with the amount of light striking.

• (iii) The Photovoltaic Cell or solar cell- produce an electrical current whenconnected to the load.

THANKS

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