TRANSDUCERS

Introduction

Basically transducer is defined as a device, whichconverts energy or information from one form toanother.These are widely used in measurement workbecause not all quantities that need to bemeasured can be displayed as easily as others. A better measurement of a quantity can usuallybe made if it may be converted to another form,which is more conveniently or accuratelydisplayed.

Introduction(cont’d)

For example, the common mercury thermometerconverts variations in temperature into variationsin the length of a column of mercury. Since thevariation in the length of the mercury column israther simple to measure, the mercurythermometer becomes a convenient device formeasuring temperature.

Manometer, which detects pressure and indicatesit directly on a scale calibrated in actual units ofpressure.

Introduction(cont’d)

Thus the transducer is a device, which provides ausable output in response to specific inputmeasured, which may be physical or mechanicalquantity, property or condition. The transducer may be mechanical, electrical,magnetic, optical, chemical, acoustic, thermalnuclear, or a combination of any two or more ofthese.

 

Mechanical transducers

Advantages:-simple and rugged in constructioncheaper in costaccurate andoperate without external power supplies

Disadvantage:-

poor frequency responserequirement of large forces to overcomemechanical frictionin compatibility when remote control or indicationis required

ELECTRICAL TRANSDUCERS

Mostly quantities to be measured are non-electrical such as temperature, pressure,displacement, humidity, fluid flow, speed etc., butthese quantities cannot be measured directly.Hence such quantities are required to be sensedand changed into some other form for easymeasurement.Electrical quantities such as current, voltage,resistance. inductance and capacitance etc. canbe conveniently measured, transferred and stored,and therefore, for measurement of non-electricalquantities these are to be converted intoelectrical quantities first and then measured.

ELECTRICAL TRANSDUCERS(cont’d)

The function of converting non-electrical quantityinto electrical one is accomplished by a devicecalled the electrical transducer.Basically an electrical transducer is a sensingdevice by which a physical, mechanical or opticalquantity to be measured is transformed directly,with a suitable mechanism, into an electricalsignal (current, voltage or frequency).The production of these signals is based uponelectrical effects which may be resistive,inductive, capacitive etc in nature.

BASIC REQUIREMENTS OF A TRANSDUCER

The main function of a transducer is to respond only forthe measurement under specified limits for which it isdesigned. It is, therefore, necessary to know therelationship between the input and output quantities andit should be fixed.

Transducers should meet the following basicrequirements:-

Ruggedness: It should be capable of withstandingoverload and some safety arrangement should beprovided for overload protection.

Linearity: Its input-output characteristics should belinear and it should produce these characteristics insymmetrical way.

Basic Requirements Of a Transducer(cont’d)

 Repeatability: It should reproduce same output signalwhen the same input signal is applied again and againunder fixed environmental conditions e.g.temperature, pressure, humidity etc.

High Output Signal Quality: The quality of outputsignal should be good i.e. the ratio of the signal to thenoise should be high and the amplitude of the outputsignal should be enough.

High Reliability and Stability: It should give minimumerror in measurement for temperature variations,vibrations and other various changes in surroundings.  

Basic Requirements Of a Transducer(cont’d)

No Hysteretic: It should not give any hystereticduring measurement while input signal is variedfrom its low value to high value and vice-versa.Residual Deformation: There should be nodeformation on removal of local after long periodof application.Range: The range of the transducer should belarge enough to encompass all the expectedmagnitudes of the measurand.

Basic Requirements Of a Transducer(cont’d)

Sensitivity: The transducer should give a sufficientoutput signal per unit of measured input in orderto yield meaningful data.Electrical Output Characteristics: The electricalcharacteristics-the output im pedance, thefrequency response, and the response time of thetransducer output signal should be compatiblewith the recording device and the rest of themeasuring system equipment.Physical Environment: The transducer selectedshould be able to withstand the environmentalconditions to which it is likely to be subjectedwhile carrying out measurements and tests. 

Basic Requirements Of a Transducer(cont’d)

Errors: The errors inherent in the operation of thetransducer itself, or those errors caused byenvironmental conditions of the measurement,should be small enough or controllable enoughthat they allow meaningful data to be taken.

Good Dynamic Response: Its output should befaithful to input when taken as a function of time.The effect is analyzed as the frequency response. 

Classification Of Transducers

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The transducers may be classified in various wayssuch as on the basis of electrical principles involved,methods of application, methods of energyconversion used, nature of output signal etc.

1. On the basis of transduction principleinvolved:ResistiveInductiveCapacitive etc.

Classification Of Transducers(cont’d)Primary and Secondary Transducers: Transducers, on the basis of methods of

applications, may be classified into primary andsecondary transducers. When the input signal isdirectly sensed by the transducer and physicalphenomenon is converted into the electrical formdirectly then such a transducer is called theprimary transducer.

 

2-Primary and SecondaryTransducers(cont’d) For example a thermistor used for the

measurement of temperature fall in this category.The thermistor senses the temperature directlyand causes the change in resistance with thechange in temperature. When the input signal issensed first by some detector or sensor and thenits output being of some form other than inputsignals is given as input to a transducer forconversion into electrical form, then such atransducer falls in the category of secondarytransducers.

Primary and Secondary Transducers(cont’d)

For example, in case of pressure measurement,bourdon tube is a primary sensor which convertspressure first into displacement, and then thedisplacement is converted into an output voltage by anLVDT. In this case LVDT is secondary transducer. 

3-Active and PassiveTransducers.

Transducers, on the basis of methods of energyconversion used, may be classified into active andpassive transducers.

Active transducers:- Self-generating type transducers i.e. thetransducers, which develop their output the formof electrical voltage or current without anyauxiliary source, are called the active transducers.Such transducers draw energy from the systemunder measurement. Normal such transducersgive very small output and, therefore, use ofamplifier becomes essential. E.g. Thermocouple.

 

Active and PassiveTransducers(cont’d)

Passive transducers:-Transducers, in which electrical parameters i.e.resistance, inductance or capacitance changeswith the change in input signal, are called thepassive transducers. These transducers requireexternal power source for energy conversion. Insuch transducer electrical parameters i.e.resistance, inductance or capacitance causes achange in voltages current or frequency of theexternal power source. These transducers maydraw sour energy from the system undermeasurement. Resistive, inductive and capacitivetransducer falls in this category.

4-Analog and Digital Transducers

Transducers, on the basis of nature of outputsignal, may be classified into analog and digitaltransducers.

Analog transducer:- converts input signal into output signal, which is acontinuous function of time such as thermistor,strain gauge, LVDT, thermo-couple etc.

Digital transducer:-

converts input signal into the output signal of theform of pulse e.g. it gives discrete output.

Analog and Digital transducers(cont’d)

These transducers are becoming more and morepopular now-a-days because of advantagesassociated with digital measuring instrumentsand also due to the effect that digital signals canbe transmitted over a long distance withoutcausing much distortion due to amplitudevariation and phase shift. Sometimes an analogtransducer combined with an ADC (analog-digitalconvector) is called a digital transducer.

  

5. Transducers and Inverse Transducers.

Transducer:-Transducer, as already defined, is a device thatconverts a non-electrical quantity into anelectrical quantity. For example a thermo-couple,photoconductive cell, pressure gauge, straingauge etc.

Inverse transducer:-

An inverse transducer is a device that converts anelectrical quantity into a non-electrical quantity. Itis a precision actuator having an electrical inputand a low-power non-electrical output.

Transducers and InverseTransducers(cont’d)

For examples a piezoelectric crystal andtransnational and angular moving-coil elementscan be employed as inverse transducers. Manydata-indicating and recording devices arebasically inverse transducers. An ammeter orvoltmeter converts electric current intomechanical movement and the characteristics ofsuch an instrument placed at the output of ameasuring system are important. A most usefulapplication of inverse transducers is in feedbackmeasuring systems.

 

STRAIN GAUGE

DEFINITION•

•

A strain gauge is an example of passivetransducer that converts a mechanicaldisplacement into a change of resistance. A strain gauge is a thin wafer like device thatcan be attached to a variety of materials tomeasure applied strain.

WORKING

• ‐ The strain gauge isconnected into aWheatstone Bridgecircuit. The changein resistance isproportional toapplied strain and ismeasured withWheatstone bridge.

WORKING

TYPES OF STRAIN GAUGES••••

•

•

Unbonded metal strain gaugesBonded metal wire strain gaugesBonded metal foil strain gaugesVacuum deposited thin metal film straingaugesSputter deposited thin metal film straingaugesSemiconductor strain gauges

ADVANTAGES &DISADVANTAGES

•

•

Advantages: There is no moving part. It issmall and inexpensive. Disadvantages: It is non-linear. It needs to becalibrated.

APPLICATIONS

• Residual stress and Vibration measurement,Torque measurement, Bending and deflectionmeasurement, Compression and tensionmeasurement, Strain measurement

LINEAR VARIABLEDIFFERENTIAL TRANSFORMER

(LVDT)

CONTENTS:

LVDTCONSTRUCTIONPRINCIPLEOPERATIONUSES

LINEAR VARIABLE DIFFERENTIALTRANSFORMER (LVDT)Linear variable-differential transformer

is the most widely used Inductive transducer

to translate linear motion into electricalsignal.

CONSTRUCTION :

A differential transformer consistsof a primary winding and twosecondary windings. The windingsare arranged concentrically andnext to each other.A ferromagnetic core(armature) inthe shape of a rod or cylinder isattached to the transducer sensingshaft.The core slides freely within thehollow portion of

the bobbin.

PRINCIPLE :

Any physical displacement of the corecauses the voltage

of one secondary winding to increase whilesimultaneously

reducing the voltage in the other secondarywinding. The difference of the two voltages appearsacross the output

terminals of the transducers and gives ameasure of the physical

position of the core and hence displacement.

working:

When the core is in the neutral or zero position and two secondary windings are

equal and opposite and the net output isnegligible. By comparing the magnitude and phase of

output with input source, the amount and direction of movement of core and hence displacement may be determined.

USES

The LVDT can be used in all applications where displacements ranging from fraction of a mm to few cm have to be measured.

Acting as a secondary transducer it can be

used as a device to measure force, weight,pressure etc.

It is defined as the force acting on a body which tends toproduce rotation.Mathematically ,torque is given as :

T = F x D

Where, T = torqueF = forceD = perpendicular distance from the axis of rotation of the line ofaction of force

A number of devices which can be used for themeasurement of torque is :

1. Strain gauge torque meter2. Inductive torque transducer

3. Magnetostrictive transducer4. Digital methods

1. STRAIN GAUGE TORQUEMETER :-

The torque is given by the relation ;

Where, G = modulus of rigiditymeasured in N/m².R = outer radius of the shaftmeasured in m.r = inner radius of the shaftmeasured in m.L = length of the shaft measured inm.

ADVANTAGE :-

1. They are fully temperature compensated. 2. they give a maximum sensitivity for a given torque.

A strain gauge is generally measured by electrical means,In this arrangement , two strain gauges are subjected totensile stresses while the other two experienceCompressive stresses to indicated the torque.

THEORY:-

.

The gauges must be at 45 with the shaft axis. Gauge 1 and2 must be diametrically opposite , as must gauge 3 and 4 .

2. Inductive torque transducer

In inductive torque transducer ,flange A carries a coil andflange B, an iron core. This core is move IN an OUT of thecoil according to the relative displacement of the twoflanges.

The coil used an arm of A.C bridge .

The output of the A.C bridge is depend upon theinductance of the coil which in turn depends upon theposition of core and hence on the displacement.

The displacement is depend upon torque.

The bridge output can be directly calibrated to read the torque.

THEORY:

3. Magnetostrictive transducers

These are based on the principle that the permeability ofmagnetic material changes when they are subjected tostrain . The permeability decreases with positive strain andincreases with Negative strain .

THEORY :-Magnetostrictive transducer is used for the measurementof torque.The inductance of one of the coil increases due to theincrease in permeability.

When an torque is applied, then bridge is balanced andtwo coil have equal inductance.

When torque is applied , inductance of one coil increaseswhereas inductance of another coil is decreases and hence ,bridge is unbalanced..

4. Digital methods Types:- 1. single toothed wheel system

2. Multi- toothed wheel system

Single toothed wheel system :-Digital timing techniques are generally used for thedetermination of relative displacement between two flangesA and B .

THEORY :- When a torque is applied to the shaft, there is arelative there is a relative displacement betweenthe two flanges , and a phase shift is producedbetween the pulses in theinductive transducer C and D .when these pulses are compared with the help of anelectronic timer, a time interval will be displayedbetween the two pulses. these time interval isproportional to the relative displacement of the twoflanges which in turn proportional to the torque.

ADVANTAGE :-1. Errors are eliminated.2. There is no noise problem.

2.Multi toothed wheel system

Multi –toothed wheels will replace the single –toothedwheels .

THEORY :-

The transducer are generally magnetic or photoelectric. Inthis case, the output is perfectly sinusoidal .

.The two outputs are exactly in phase of the two wheelsare correctly.

The output voltage progressively becomes out of phase asthe torque increases because an increases in torqueresults in relative displacement of the two flanges.

Electromagnetic Flow Meter

Introduction A Electromagnetic flow meter is a volumetric

flow meter which does not have any movingparts and is ideal for wastewater applicationsor any dirty liquid which is conductive or waterbased.

Working Principle

The operation of a Electromagnetic flowmeter is based upon Faraday’s Law, whichstates that:

“The voltage induced across a conductor as itmoves at right angles through the magneticfield is proportional to the velocity of thatconductor.”

Faraday’s Law

According to this law the induced voltage E is proportional to VxBxD where: E= The voltage generated in a conductor V= The velocity of the conductor B= The magnetic field strength D= The length of the conductor (in this instance distance between theelectrodes)

• Electromagnetic flowmeter use Faraday’sLaw of electromagnetic induction todetermine the flow of liquid in a pipe. In amagnetic flowmeter, a magnetic field isgenerated and channeled into the liquidflowing through the pipe. FollowingFaraday’s Law, flow of a conductive liquidthrough the magnetic field cause a voltagesignal to be sensed by electrodes locatedon the flow tube walls.

Uses…

Pipelines… Refineries

Advantages & Disadvantages

Advantages:Minimum obstruction in the flow path yields minimum

pressure drop.It can measure forward as well as reverse flow with equal

accuracy.Low maintenance cost because of no moving parts.Corrosive or slurry fluid flow.

Disadvantage:

Requires electrical conductivity of fluid.Zero check can only be done when there is no flow.

PH MEASUREMENT

PH:-

The solution of alkalinity or acidity ofaqueous solution is determine by therelative concentration of hydrogen andhydroxyl ions in solution.Solution is acidic when hydrogen ions arein majority and alkaline when hydroxyl ionsare in majority .Hydrogen ion concentration is measuredon a scale known as pH scale pH= -log10(H+)

ELECTRICAL METHOD OF PHMEASUREMENTThis method is based upon the measurement

of electrical potential.

Theory of ELECTRICAL METHOD

In this method Electrode is immersed in thesolution .An electric potential is producedat the Electrode which forms an electrolytichalf cell.This is a measuring cell. A secondelectrode is required to provide the astandard potential and

to complete the cell. This is the referencecell.The algebraic sum of the potential of thetwo half

cells is proportional to the concentration ofthe hydrogen ion activity in water based

ELECTRONIC METHOD OF PHMEASUREMENT

In this type, pH value is read on a digital meteror indicating type meter.

Theory of ELECTRONIC METHOD

In this method there are two electrodes, one asa Reference electrode and other is measuringelectrode.The voltage difference between two is dependon the pH value of solution . the output voltageobtained here is not linear and is of very lowvalue.A pH amplifier is used to amplify and line arisethis voltage.The output of the amplifier can be sent to anyindicating instrument or can be sent to recorderor to computer for storage

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