transducers lecture01 (2)

7/26/2019 Transducers LECTURE01 (2)

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TRANSDUCERS


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Instrumentation System

Three Major Elements

1. An Input Device

2. A signal Conditioning or rocessingDevice

!. An "utput Device


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Instrumentation Systemcont..The input device receives the #uantity under

measurement and delivers a proportionalelectrical signal to the signal conditioning device.

$ere the signal is modi%ed& ampli%ed or %ltered toa 'ormat that is accepta(le to the output device.

The output device may(e a)

1. Simple indicating Meter

2. An oscilloscope!. A Magnetic Tape *ecorder

+. A Computer


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WHAT IS A TRANSDUCER ?

A transducer is a device ,hich trans'orms a non-electrical physical #uantity i.e. temperature& sound orlight/ into an electrical signal i.e. voltage& current&

capacity0/In other ,ord it is a device that is capa(le o'converting the physical #uantity into a proportionalelectrical #uantity such as voltage or current.

Pressure Voltage


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BLOCK DIAGRAM O TRANSDUCERS

Transducer contains t,o parts that areclosely related to each other i.e. thesensing element and transduction

element. The sensing element is called as the

sensor. It is device producing measura(leresponse to change in physical conditions.

The transduction element convert thesensor output to suita(le electrical 'orm.


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6

Alternating De%nitions

De%nition) Technically0 A device that converts one energy 'orm to

another eg& mechanical to electrical/. Any device or component that converts an input

signal o' one 'orm to an output signal o' another'orm An element or device ,hich receives in'ormation

in the 'orm o' one #uantity and converts it toin'ormation in the same or an other #uantity or

'orm. A device 'or translating the magnitude o' one

#uantity into another #uantity.


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Selecting a Transducer

Selecting an appropriate transducer is the %rstand most 'undamental step to o(tain accurateresults.

A num(er o' #uestions should (e ased (e'orean appropriate transducer is selected& 'oreample&

a/ 3hat is the physical #uantity that needs to (emeasured4

(/ 3hich transducer principle can (e used tomeasure this physical #uantity4

c/ 3hat accuracy is re#uired 'or the measurement4


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Selecting a transducercont.. 5irst #uestions can (e ans,ered (y

determining the type and range o' thetransducer.

Second #uestion re#uires that the inputand output characteristic o' thetransducer must (e compati(le ,ith therecording or measurement system.

roper transducer is selected (y theaddition o' accuracy tolerance.


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TRANSDUCERS SELECTION ACTORS

1. Sensitivity: The transducer must (e sensitive enough to producedetecta(le output.

2. Operating Range:The transducer should maintain the range re#uirement

and have a good resolution over the entire range.4. Accuracy:$igh accuracy is assured.

5. Crss sensitivity: It has to (e taen into account ,hen measuringmechanical #uantities. There are situation ,here the actual #uantity is(eing measured is in one plane and the transducer is su(jected to variationin another plan.

6. Errrs: The transducer should maintain the epected input-outputrelationship as descri(ed (y the trans'er 'unction so as to avoid errors.

!. Operating "rincip#e: The transducer are many times selected on the(asis o' operating principle used (y them. The operating principle used


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!. Transient an$ %re&uency respnse : The transducershould meet the desired time domain speci%cation lie peaovershoot& rise time& setting time and small dynamic error.

'. (a$ing E)ects: The transducer should have a high inputimpedance and lo, output impedance to avoid loadinge7ects.

*. Envirn+enta# C+pati,i#ity: It should (e assured thatthe transducer selected to ,or under speci%edenvironmental conditions maintains its input- output

relationship and does not (rea do,n.

1-. nsensitivity t un/ante$ signa#s: The transducershould (e minimally sensitive to un,anted signals andhighly sensitive to desired signals.


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*eduction in Measurement Error

8se in-place system cali(ration ,ith correctionper'ormed in the data reduction

Simultaneously monitoring the environment andcorrecting the data accordingly

Arti%cially controlling the environment tominimi6e possi(le errors

Arti%cially controlling the environment means

physically moving the transducer to more'avora(le position or providing the re#uiredisolation 'rom the environment (y a heaterenclosure& vi(ration isolation& or similar means.


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CLASSIICATION O

TRANSDUCERS

Analog and digital transducers. rimary and secondary transducer. Transducers and inverse

transducers. "n the (asis o' transduction

principle used. Active and assive Transducers


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Ana#g trans$ucers converts input signal intooutput signal& ,hich is a continuous 'unction o'time such as T$E*MIST"* &strain gauge& 9:DT &

thermocouple etc. Digita# trans$ucers converts input signal into

the output signal in the 'orm o' pulses e.g. itgives discrete output. These transducers are

(ecoming more popular no,adays (ecause o'advantages associated ,ith digital measuringinstruments and also due to the 'act that digitalsignals can (e transmitted over a long distance,ithout causing much distortion due to

ANALOG AND DIGITAL TRANSDUCERS

Transducers &on the (asis o' nature o' output signal& may (e classi%ed into analog and digital transducers.


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PRIMAR! TRANSDUCERStrans$ucers n t0e ,asis % +et0$s % app#icatins +ay ,e c#assie$ intpri+ary an$ secn$ary trans$ucers.

3hen the input signal is directly sensed (ythe transducers and physical phenomenonis converted into the electrical 'orm directly

then such a transducer is called the primarytransducer.

5or eample) A T$E*MIST"* used 'or themeasurement o' temperature 'all I this

category. The T$E*MIST"* senses thetemperature directly and causes the changein resistor ,ith the change in temperature.


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SECONDAR! TRANDUCERS

3hen the input signal is sensed %rst (y somedetector or sensor and then its output (eing o'some 'orm other than input signal is given as inputto a transducer 'or conversion into electrical 'rom&

them such a transducer 'alls in the category o'secondary transducers.

5or eample & in case o' pressure measurement &(ourdon tu(e is a primary sensor ,hich converts

pressure %rst into displacement& then thedisplacement is converted into an output voltage(y an (3DT. In this case 9:DT is a secondarytransducer.


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INVERSE TRANSDUCERS

It is a device that converts an electrical#uantity into a non electrical #uantity .itis a precision actuator having an

electrical input and a lo, po,er nonelectrical output.

5or eample a pie6oelectric crystal and

translational and angular moving coilelements can (e employed as inversetransducers. A most use'ul application o'inverse transducers is in 'eed (ac

measuring systems.


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1!

Transducer operating characteristics areusually de%ned (y a num(er o'parameters.

Some o' the main parameters to (econsidered are) Sensitivity ; *ange ; Span

9inearity ; $ysteresis ; Accuracy

recision *eproduci(ility& *epeata(ility/

And others.

Transducer parameters


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1'

$ysteresis

A transducer shouldproduce the sameoutput ,hether thevalue has (eenreached due to acontinuallyincreasing input or acontinually

decreasing input.


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1*

Accuracy


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2-

recision *eproduci(ility& *epeata(ility/


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Sensitivity


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*ange

The highest and lo,est values that thetransducer is designed to measure.

E.g. A Temperature transducer may have arange o' ;


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2

The di7erence (et,een the upper andlo,er values the transducer is designed tomeasure.

E.g. A Temperature transducer that has arange o' ;


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9inearity re'ers to the change in outputcompared to the change in input. I' thechange in output is proportional to the

change in input& the transducer is said to(e linear.

9inearity


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25

8nits ,e need to no,.


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26


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2!

Measuring temperature


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ON THE BASIS O TRANSDUCTION

PRICIPLE USED

*esistive Transducers.

Capacitive Transducers.

Inductive Transducers. :oltage and current ?eneratingTransducers.


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ACTIVE TRANSDUCERSTrans$ucers n t0e ,asis % +et0$s % energy cnversin use$ +ay ,e c#assie$ int active an$ passive trans$ucers.

Se#% generating type trans$ucers i.et0e trans$ucers /0ic0 $eve#p t0eir

utput in t0e %r+ % e#ectrica#v#tage r current /it0ut any

aui#iary surce. Nr+a##y suc0 trans$ucers give very s+a## utput t0ere%re use %

a+p#ier ,ec+es essentia#.

r ea+p#e TAC7O generatrs use$ %r +easure+ents % angu#arve#city t0er+cup#es use$ %r +easure+ent % te+perature

pie8e#ectric crysta# use$ %r +easure+ent % %rce.


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CLASSIICATION O ACTIVE TRANSDUCERS


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PASSIVE TRANSDUCERS

Transducers in ,hich electrical parameters i.eresistance& inductance or capacitance changes,ith the change in input signal& are called the

passive transducers. These transducers re#uireeternal po,er source 'or energy conversion.In such transducers electrical parameters i.eresistance inductance or capacitance causes a

change in voltage &current or 're#uency o' theeternal po,er source.

5or eample resistive capacitive inductivetransducers.


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CLASSIICATION O PASSIVE

TRANSDUCERS


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STRAIN GUAGE

The Strain ?auge is aneample o' a passivetransducer that useselectrical resistancevariation in ,ires tosense the strainproduced (y a 'orce onthe ,ire. It is a veryversatile detector andtransducer 'or

measuring ,eight&pressure& mechanical'orce or displacement.

Strain gauge) ho, they loolieThe construction o' a (onded straingauge sho,s a %ne ,ire looped (ac

and 'orth on a mounting plate& ,hichis usually cemented to the element


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Sensitivity o' Strain ?age

Sensitivity o' a strain gage is descri(edin terms o' a characteristic called ?age5actor& @& de%ned as the unit change o'

resistance per unit change o' length.

?age 5actor @

ROM THE E"UATION O


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R= resistance

= specifc resistance o the conductor material L = the length o the conductor in meters

A= the area o the conductor in square meters

3hen a strain produced (y a 'orce is applied on the ,ires& 9increase and A decrease.

ROM THE E"UATION O

RESISTANCE#


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Eample

A resistance strain gage ,ith a gage'actor o' 2 is 'astened to a steel mem(ersu(jected to a stress o' 1&=


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Solution

Sol)G


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T ERMISTOR

A T7ER9STORis a type o' resistor ,hose resistance variessigni%cantly ,ith temperature. Semi-conductor thermistors have aegative Temperature CoeJcient TC/. i.e. as temperatureincreases& the resistance decreases.

The ,ord is a contraction o' t0er+a# an$ resistr.Thermistorsare ,idely used as inrush current limiters& temperature sensors& sel'-resetting over current protectors& and sel'-regulating heating


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Thermistors

Thermistors are made'rom semi-conductormaterials.

Semi-conductor

thermistors have aegative TemperatureCoeJcient TC/. i.e.as temperature

increases& theresistance decreases.


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Thermistor construction

Thermistors comein a variety o' si6esand shapes.

Keads& diss& rodsand pro(es aresome o' the morecommon styles.


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Thermistors Cont/

9ie *TDs&thermistors areo'ten enclosed in a

housing suita(le 'oreither contact ornon-contactapplications inindustry.


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THERMOCOUPLE

.

As the junction temperature increases a small voltage is created inthe loop. The voltage produced at the junction o' the dissimilar

metals is due to a phenomenon called the LSee(ec E7ect.

N The higher the temperature at the junction& the greater the voltageproduced (y that junction.

N The relationship (et,een voltage and temperature is constant and

there'ore ,ill graph as a linear line.


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PROS AND CONS

Pros O They are inepensive.

O They are rugged and relia(le.

O They can (e used over a ,idetemperature range.

Cons

O lo, output voltage O lo, sensitivity

O non-linearity

O electrical connections


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D$''ere%t$al Tra%s'or(er

)LVDT*

N An iron core slides ,ithin the tu(e andthere'orea7ects the magnetic coupling (et,een theprimary and t,o secondaries.N 3hen the core is in the centre & thevoltageinduced in the t,o secondaries is e#ual.

N 3hen the core is moved in one directiono' centre& the voltage induced in one

N assive inductive transducers re#uirean eternal source o' po,er.N The Di7erential trans'ormer is apassive inductive trans'ormer& ,ellno,n as 9inear :aria(le Di7erentialTrans'ormer 9:DT/.It consists (asically o' a primary ,indingand t,o secondary ,indings& ,ound overa hollo, tu(e and positioned so that theprimary is (et,een t,o o' itssecondaries.


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ADVANTAGES

It has high accuracy and good sta(ility.

Easy to 'a(ricate and install.

The trans'er characteristics is linear.

Input is directly proportional to output.

They can ,ithstand high temperature.

It consume very less po,er.

It has lo, hysteresis loss and good repeata(ility. *ugged construction.

Its output is very high.


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DISADVANTAGES

9:DT is proo' to errors due totemperature.

5or getting the apprecia(le di7erentialoutput relatively large displacement isre#uired .

9:DT is sensitive to straight magnetic%elds.

The dynamic response is limited 'or 9:DTdue to mass o' core.


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4!

"ptical devices

Many measurement and control systemsutilise light and light-intensity as a ,ayo' detecting other physical properties.

8sing direct or rePected light canprovide an ideal non-contact sensingmechanism.


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"0te#ectric

Trans$ucers

hotoelectric transducers are devices thatproduce an electrical variation in response to a

change in light intensity& or produce a lightintensity variation due to a change in appliedelectrical energy. hotoelectric transducersoperate in three classi%cations& they are)

; "0tcn$uctive; "0tv#taic

; "0te+issive.


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"0tcn$uctive

The photoconductive device is a semiconductor cell,hich produces a change in itQs resistance inresponse to a change in light intensity.

The three most common photoconductivetransducers are the

; (ig0t Depen$ant Resistr


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*ESIST"*

9ight dependant resistors


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The 9D* is asemiconductordevice.

Its resistance isdependant onthe light

intensity that'alls on thedevice.

9ight dependant resistors9D*S


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9ight dependant resistors

As the lightintensity increases&the resistance o'the 9D* decreases.

The 9D* is a non-linear device ,ithresistance ranging'rom a(out 1= MRin completedarness to 1==Rin 'ull sunlight.


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5

hototransistor

The phototransistor isa three-layersemiconductor device,ith a light-sensitive

collector-(ase p-njunction.

The current Po,ingthrough the collector

emitter circuit ,ill (econtrolled (y theamount o' light 'allingon the collector-(ase

junction.

?unctin resistance % t0e p0ttransistr


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? p$ecreases. T0is $ecrease in resistanceincreases t0e ,ase current t0at in turnincreases t0e @/ % c##ectr current.

T0e re#atins0ip ,et/een #ig0t intensity an$current @/ is genera##y cnstant an$t0ere%re /i## grap0 as a #inear #ine. T0ese#inear trans%er c0aracteristics are s0/n

,e#/.


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MaterialElectromagnetic spectrum

,avelengthrange nm/Silicon 1=;11==

?ermanium +==;1==

Indium gallium arsenide U==;2==

9eadII/ sul%de V1===;!


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S#ar ce##

As the light protons/ intensity increases& an im(alance o' electrons and holes arecreated& ,hich gives an increase to the open circuit potential voltage di7erence andthere'ore a current Po, ,ithin a circuit. The relationship (et,een light intensity andopen circuit voltage is not constant and there'ore ,ill not graph as a linear line


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9ight Emitting Diode

This 9ED is a semi conductive - junction

enclosed in a coloured case to enhance thecolour o' the light output. Silicon is not used

as it produces mainly heat rather than light.

The semi conductive materials used in themanu'acture o' 9EDQs determines the colour

o' the emitted light. Ky using di7erentmaterials& such colours as red& yello,& green&

and even invisi(le light spectrums such asin'ra-red can (e produced.


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Optcup#ers ,e#ng t a %a+i#y % $evicesuse$ t e#ectrica##y is#ate circuits.

T0is is#atin +ay ,e re&uire$ t prtect

circuits %r+ surge v#tages an$ t #tercertain nise.

"0te#ectric trans$ucers are e)ective in

pr$ucing 0ig0 &ua#ity %ast respn$ingOptcup#ers /0ic0 can ,e use$ in +anyvarying app#icatins.

T0e ,asic Optcup#er cnsists % a p0te+issive $evice (ED an$ a 0tcn$uctive

Optcup#ers


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5*

"pto-coupler devices


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"pto-coupler devices

Kecause 9EDs can sense light in addition toemitting it& construction o' symmetrical&(idirectional opto-isolators is possi(le. Anoptocoupledsolid state relay contains a

photodiode opto-isolator ,hich drives a po,ers,itch& usually a complementary pairo' M"S5ETs. A slotted optical s,itch containsa source o' light and a sensor& (ut its optical

channel is open& allo,ing modulation o' light(y eternal o(jects o(structing the path o'light or rePecting light into the sensor.

"ie8 $evices


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"ie8 $evices

T0e princip#e % pie8e#ectric actin 0as ,eenn/n %r &uite s+e ti+e. 9ateria#s suc0 as&uart8 an$ +an +a$e pr$ucts suc0 as Bariu+Titanate an$ (ea$ ircnate $e+nstrate ac0aracteristic in t0at /0en pressure is app#ie$

ver ne ais t0ere ten$s t ,e a p#ari8atin %e#ectric c0arge ver t0e a$?acent ais.


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ie6o Devices

3hether they are ie6oelectric or ie6oceramicdevices& the application is very ,ide& almost ,herever,e ,ish to measure pressure you ,ill %nd these

devices (eing used. Although not ehaustive& someeamples includeW

; "ressure s/itc0es

; "ie8e#ectric pressure gauges; D?)erentia# pressure +easuring trans$ucers

an$; Snar trans$ucers

; 3i,ratin $etectrs etc; gnitin $evices

Acustic Trans$ucers


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Acustic Trans$ucers

Acoustic transducers are devices that convert avariation in electrical energy into a change inmechanical energy& physical vi(rations oroscillations& ie. sound ,aves/. "r conversely&convert a variation in sound ,ave energy intoelectrical energy.

Common eamples o' acoustic transducers arethe)

Acoustic speakers,

Acoustic microphone,

Piezoceramic transducers, and

Ma netostrictive transducers

c / en curren sapp#ie$ is at rig0t


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gang#es t t0e +agnetice#$ pr$uce$ ,y t0eper+anent +agnet.

T0ere%re t0e t/ e#$sattract r repe# eac0t0er $epen$ing n t0ep#arity % t0e signa#current. T0is attractin

an$ repu#sin causes anin/ar$ r ut/ar$+ve+ent % t0e viceci# an$ cne /0ic0resu#ts in sun$ /aves,eing pr$uce$.

T0e v#u+e an$%re&uency % t0e sun$pr$uce$ is $epen$antupn t0e a+p#itu$e an$%re&uency % t0e inputsigna# current.


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"ie8e#ectric Bu88er


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"ie8e#ectric Bu88er

ie6o electric (u66ers andspeaers are used in a ,idevariety o' applications 'romsimple lo, %delity

applications such as a,arning (u66er to high%delity& high 're#uency audiospeaer applications.

*egardless o' the application&the principle o' pie6oelectricoperation remains a constant.

Disp#ace+ent "sitin An$ "ri+ity


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Disp#ace+ent "sitin An$ "ri+ityTrans$ucers

5loat transducers are used intan level monitoringapplications. These devicesuse a sender that is either as,itch or some 'orm o'

resistive device. Acom(ination o' these devicescan (e seen in an automotiveapplication ,here the s,itchis used to indicate tan lo,

level and the potentiometersender provides aproportional indication o'actual tan level.

7 ## E) t T $


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The $all E7ect descri(es a condition i' current Po, in aconductor (eing a7ected (y the presence o' a magnetic %eld I'an electric current Po,s through a conductor in a magnetic %eld&the magnetic %eld eerts a transverse 'orce on the movingcharge carriers ,hich tends to push them to one side o' the

conductor. This is most evident in a thin Pat conductor. A (uildup o' charge at the sides o' the conductors ,ill (alance thismagnetic inPuence& producing a measura(le voltage (et,eenthe t,o sides o' the conductor. The presence o' this measura(letransverse voltage is called the $all e7ect a'ter E. $. $all ,hodiscovered it in 1U.

7a## E)ect Trans$ucers

$all e7ect


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$all e7ectMagnetic reed


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$all e7ect devices

$all e7ect devices can(e used to)

Measure the velocity o'charged particles in a

magnetic %eld Po,meter/Measure the proimityo' magnetic materials9inear displacement/

Detect pulses o'magnetism e.g. as in atachometer

transducers lecture01 (2)

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