ei ch 1 (sensors for transducers)

8/6/2019 Ei Ch 1 (Sensors for Transducers)

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SENSORS & TRANSDUCERS

EXTC-III Electronic Instrumentation


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Wh at is sensor?` Sensor is a measurement device that produces a signal in response

to its detecting the physical variable of interest, for exampletemperature, force, pressure, position, speed, acceleration andvibration. Eg thermometer

` Still, in measurement system, one will also encounter the term

transducer .` Transducer converts the previously mentioned physical variable into

another form by quantifying the variable into a signal that can beinterpreted as the value of the measured variable.

` Some of signal conditioning devices are also classified as transducer.

` However, in some cases, sensor and transducer can be the samedevice.` For instance, in the case of a limit switch that converts mechanical

movement of lever to close an electrical contact.


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Element of Measurement System


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Classification of transducers

1 . Based on principle of transduction2. Primary & Secondary3. Active & passive4. Analog & digital5. Transducer & Inverse transducer


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B ased on principle used

` R esistive- Potentiometer, R esistance strain guage` Inductive- differential transformer` Capacitive- pressure Guage, Capacitor microphone

Depending upon how they input quantity intofollowing terms.


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P assive & Active transducer

Passive-- Device which derive power reqd. fortransduction from auxiliary power source.Externally poweredEg : resistive, inductive, capacitive Without power they

will not work

Active-- No extra power reqd. to produce I/p Self generating.Draw power from input appliedEg. Piezo electric xtal used for accelartionmeasurement


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Anolog & Digital Transducers

Anolog-- convert I/p quantity into an analog o/p i.e Analogo/p- a continuous fn. Of timeEg.Strain gauge, L VD T, thermocouple

Digital-- Converts I/p into an electrical o/p in the form of pulses


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Transducers & inverse transducers

TransducerWhich converts non-electrical quantity intoelectrical signal.

Inverse transducer which converts electrical quantuty innon-electrical quantuty.eg piezoelectric crystal.


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F actor influencing t h e c h oice of transducers` Operating principle- T he transducers are many a times

selected on the basis of operating principle used by themlike resistive, capacitive, inductive etc.

` Sensitivity- it is the ratio of output s/g or response of theinstrument to a change of input or measured variable.T hus transducer must be sensitive enough to producedetectable output.

` Operating range- the transducer should maintain therange requirements and have good resolution over itsentire range.

` Cont


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` Resolution-The smallest c hange in measured v alue to whic hthe transducer will respond. ` Accurac y - Degree of closeness to the true v alue of the quantity

under test.Thus hi g h degree of accurac y is required if frequent

cal ibration is not required. ` Error-The transducer should maintain the ex pec ted

input/ output relation as descr ibed by transfer func tion so as av oid the error.

` Load ing effec t-The transducer should hav e a hi g h inputimpedance and low output impedance to av oid load ing effec ton transducer.

C ont


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` Environmental stability-` R uggedness-` R eliability-

` Precision- a measure of the lack of random errors (scatter)


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potentiometer


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potentiometer` resistive potentiometers are one of the most widely used forms of

position sensor` can be angular or linear` consists of a length of resistive material with a sliding contact

onto the resistive track ` when used as a position transducer a potential is placed across

the two end terminals, the voltage on the sliding contact is thenproportional to its position

` an inexpensive and easy to use sensor` T he potentiometer can be used as a voltage divider to obtain a

manually adjustable output voltage at the slider (wiper) from afixed input voltage applied across the two ends of thepotentiometer. T his is the most common use of them.


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Different type of potentiometer- translatory.


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Different type of potentiometer- rotational


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Advantage & Disadvantage

Advantage--` T hey are inexpensive.` Simple to operate and very useful for application where

the requirment are not particularly severe.` T hey are useful for the measurement of large of

displacement.Disadvantage `

When using a linear potentiometer, a large force isrequired to move the sliding contacts.` T he sliding contacts can wear out, became misaligned and

generate noise.


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L inear variabledifferential transformer


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L inear variable differential transformer` Inductive transducer translate line motion into electrical

signal

construction


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Construction` A single primary winding` Two secondary windings wound on a former equal

turns placed on either side of primary winding andconnected in serier opposition.

`

Primary winding connected to AC` Movable soft iron core is placed inside the` Former` Core Made of high permeability, annealed nickel

hydrogen gives high sensitivity, low null voltage-slotted longitudinally reduces eddy I losses

Entire assembly in stainless steel housing


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W orking

O/p of secondaryindividual voltages in S1 = E1individual voltages in S2 = E2E= E1

- E

2(since connected in series opposition o/p vtg is

difference of two vtg)At normal (NULL) position, flux in both coils equal-

so E =0If core is moved left E 1 > E2 ( depends on flux

linkage). E in phase with E 1 ie in phase with primaryIf core is moved right E 1 < E2 , E in phase with E 2 ieout of phase with primary


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` Hence applitude is function of the distance moved andphase indicates the direction of movement

Displacement Vs O/p V


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Disadvantages

` Large displacement required for appreciable differentialoutput.

` Sensitive to stray magnet (but shielding is possible)

` Performance affected by temp` Dynamic response limited mechanically by the mass of

the core and electrically by applied voltage.


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Strain T h eory

Strain is the amount of deformation of a body due to anapplied force or in other words strain is the fractionalchange in length and lateral dimension shown in the figureabove. Strain can be positive (tensile) or negative(compressive).

Cont..

Measuring Strain with Strain Gauges


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` T he resistance of the conductor is proportional to thelength and inversely proportional to cross section area.

` T hus there is change in the resistance of conductor dueto strain and this property is called piezoelectric effect

and thus strain guage is sometimes also calledpiezoelectric guage.


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Expression for gauge factor` For a wire of cross-sectional area A, resistivity , and

length L the resistance is given by

L R

A

V!

When the wire is stretched, the cross-sectionalarea A is reduced, which causes the total wireresistance to increase. In addition, since the latticestructure is altered by the strain, the resistivity of the material may also change, and this, in general,causes the resistance to increase further.


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Divide t h e above equation by l/l


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` T he strain is measured in term of microstrain.` 1 microstrain = 1 um/m

` From gauge factor equtaion-` 1 st term = resistance change due to change in length.` 2nd term = resistance change due to change in area.` 3rd trem = resistance change due to change in

piezoelectric effect.` Now change in resistivity due to strain in almost

negligible and if neglected. T he gauge factor in given by:Gf = 1+ 2v


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Type of strain gauges


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B ounded metal wire strain gauges


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` T he bonded metallic type of strain gauge consists of astrain sensitive conductor (wire) mounted on a smallpiece of paper or plastic backing.

` this gauge is cemented firmly to the surface of the

structural member to be tested by epoxy .` T he wire grid may be flat type or wrap-around.` Flat for plane surface and wrap around for cylindrical

surface.


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G rid and Rosette & h elical type


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ei ch 1 (sensors for transducers)

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