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TRANSDUCERS

Classification of transducers

Selection of transducers

Resistive transducers

Capacitive transducers

Inductive transducers Piezoelectric transducers

Optical transducers

Digital transducers

Elements of data acquisition system - A/D, D/A

converters.


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TRANSDUCERS

Its a device which convert one form of

energy to another form

Non electrical quantity is converted into an

electrical form by a transducer.

Another name is pick up


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Advantage of Electrical

Transducers

Electrical amplification and attenuation can

be done easily.

Mass inertia effects are minimized.

The effect of friction is minimized.

The electrical or electronic systems can be

controlled with a very small power level.


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Conti..

The electrical output can be easily used ,

transmitted and processed for the purpose

of measurement.

Telemetry (aerospace remote indication /

recording)

Miniaturization on account of use of ICs.


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Two parts/element of transducer

Sensing or detector element.A detector or a sensing element is that

part of a transducer which responds to a

physical phenomenon or a change in a

physical phenomenon.

Transduction element.

A transduction element transforms the

output of a sensing element to an electricaloutput. The transduction element in a way

acts as a secondary transducer.


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Classification ofTransducers

On the basis of transduction form used.

As primary and secondary transducers

As passive and active transducer.

As analog and digital transducer.

As transducers and inverse transducers.


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Classification based upon principle of

transduction Resistive

Inductive

Capacitive etcDepending upon how they convert the

input quantity into resistance, inductance

or capacitance respectively.

Eg piezoelectric, thermoelectric, magneto

restrictive, electro kinetic and optical


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Primary and SecondaryTransducers

LVDT (Linear

Variable

Differential

Transformer)

Primary- Pressure to displacement(bourdon tube)

Secondary-Displacement into analogous

voltage (LV

DT).


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Passive and Active Transducer

Active Transducer:

Also known as self generating type,

develop their own voltage or current from

the physical phenomenon being

measured.

Velocity , temperature , light intensity andforce can be transduced with the help of

active transducer.


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Conti..

Passive Transducer:

Also known as externally powered

transducers, i.e., derive the power

required for energy conversion from an

external power source.

e.g. POT (Potentiometer)-used for the

measurement of displacement .


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Analog and Digital Transducer.

Analog Transducers : It converts the input

quantity into an analog output which is a

continuous function of time.

E.g. LVDT, Thermocouple or a thermistor

(gives output which is continuous function of

time)


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Conti..

Digital Transducer: Converts input quantity

into an electrical output which is in the

form of pulse.


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Characteristics and Choice of

Transducer Input Characteristics

Transfer Characteristics

Output Characteristics.


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Input Characteristics

Type of Input and Operating Range

Loading effect.

Type of Input :The type of input, whichcan be any physical quantity, is generallydetermined in advance .

Operating Range : Choice of transducer

depends upon the useful range of inputquantity.


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Conti..

Loading Effect : The transducer, that is

selected for a particular application should

ideally exact NO force, power or energyfrom the quantity under measurement in

order that is measured accurately.


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Transfer Characteristics

1. Transfer function.

2. Error.

3.R

esponse of transducer to environmentalinfluences.


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Transfer function.

The transfer function of a transducer defines a

relationship between the input quantity and the

output. The transfer function is

Where

are respectively output and input of the

transducer.

)(0 iqfq !

iandqq0


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Conti..

Sensitivity,

Scale Factor, Inverse of sensitivity.i

o

dq

dqS!

i

o

dq

dq

S

!1


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Error

The error in transducer occur because they do

not follow, the input output relationship.

Example.. Instead of qo, we might get a output as

qo, then the error of the instrument is

0

,qqo !I


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Three components of error

1. Scale error.

2. Dynamic error

3.E

rror on account of noise and drift.


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Scale error.

Zero error.

Sensitivity error

Non conformity. Hysteresis.


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Zero error

Output deviates from the correct value by

a constant factor over the entire range ofthe transducer.

I

Theorectical Curve.

Practical Curve.

Input

Output


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SensitivityError

Observed output deviates from the correctvalue by a constant value.

I

Theorectical Curve.

PracticalCurve.

Input

Output


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Non conformity

Transfer function deviates from the

theoretical transfer function for almost everyinput.

I

Theorectical Curve.

PracticalCurve.

Input

Output


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Hysteresis

Increasing input

Decreasing input

Input

Output


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Response of transducer to

environmental influences.

It should not be subjected to any

disturbances like stray electromagnetic and

electrostatic fields, mechanical shocks andvibrations temperature changes, pressure

and humidity changes, changes in supply

voltage and improper mechanical

mountings.


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Output Characteristics

Type ofElectrical Output.

Output Impedance

UsefulR

ange.


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Type ofElectrical Output.

The type of output which may be available

from the transducers may be available

from the transducers may be a voltage,current , impedance or a time function of

these amplitudes.


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Output Impedance

Ideally the value of output impedanceshould be zero if no loading effects are

there on the subsequent stage.

Since zero output impedance is notpossible , it should be kept as low as

possible, since it determines the amount of

power that can be transferred to the

succeeding stages of the instrumentationsystem.


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Useful Output Range

The output range of a transducer is limited

at the lower end by noise signal.

The upper limit is set by the maximumuseful input level.


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Factors Influencing the choice of

Transducer.

1. Operating Principle

2. Sensitivity

3. Operating Range4. Accuracy

5. Cross sensitivity

6. Errors7. Transient and frequency response


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Conti..

8. Loading effects.

9. Environmental compatibility

10. Insensitivity to unwanted signals

11. Usage and Ruggedness

12. Electrical aspects

13. S

tability andR

eliability14. Static characteristics.


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1. Operating Principle: The transducer are many

times selected on the basis of operatingprinciple used by them. The operating principle

used may be resistive, inductive, capacitive ,

optoelectronic, piezo electric etc.

2. Sensitivity: The transducer must be sensitiveenough to produce detectable output.

3. Operating Range: The transducer should

maintain the range requirement and have a

good resolution over the entire range.


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4. Accuracy: High accuracy is assured.

5. Cross sensitivity: It has to be taken into accountwhen measuring mechanical quantities. There

are situation where the actual quantity is being

measured is in one plane and the transducer is

subjected to variation in another plan.6. Errors: The transducer should maintain the

expected input-output relationship as described

by the transfer function so as to avoid errors.


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7. Transient and frequency response : Thetransducer should meet the desired time domain

specification like peak overshoot, rise time,

setting time and small dynamic error.

8. Loading Effects: The transducer should have ahigh input impedance and low output impedance

to avoid loading effects.


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9. Environmental Compatibility: It should beassured that the transducer selected to work

under specified environmental conditions

maintains its input- output relationship and does

not break down.

10. Insensitivity to unwanted signals: The

transducer should be minimally sensitive to

unwanted signals and highly sensitive to desired

signals.


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11. Usage and Ruggedness: The ruggedness both

of mechanical and electrical intensities of transducer versus its size and weight must be

considered while selecting a suitable

transducer.

12. Electrical aspects: The electrical aspects thatneed consideration while selecting a transducer

include the length and type of cable required.

13. Stability and Reliability : The transducer should

exhibit a high degree of stability to be operativeduring its operation and storage life.


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14. Static Characteristics :Apart from low static

error, the transducer should have a low non-

linearity, low hysteresis, high resolution and a

high degree of repeatability.


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Resistive Transducers

m-material;conductoroyresistivit

mconductor;oareasectional-cross

m;conductorolengthL;resistance

,

2

;!

!

!

;!

!

V

V

R

A

LR


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Any method of varying one of the quantities

involved in the above relationship can be thedesign basis of an electrical resistive transducer.

The translational and rotational

potentiometers which work on the basis of change in the value of resistance with change in

length of the conductor can be used for

measurement of translational or rotary

displacement.


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Strain gauge work on the principle that the

resistance of the conductor or a semiconductor

changes when strained. This property can be

used for measurement of displacement, force

and pressure. The resistivity of the material changes with

change oftemperature thus causing a change

of resistance. This property may be used for

measurement of temperature.

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