transducers lecture

7/26/2019 Transducers Lecture

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TRANSDUCERTRANSDUCER

SS


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A transducer is a device that convert one form of energyto other form. It converts the measurand to a usableelectrical signal.

In other word it is a device that is capable of converting

the physical quantity into a proportional electricalquantity such as voltage or current.

Pressure Voltage


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BLOCK IA!"A# O$ %"A&'(C)"'


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*. "uggedness+. Linearity

,. "epeatability

-. Accuracy

. /igh stability and reliability0. 'peed of response

1. 'ensitivity

2. 'mall si3e


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The transducers can be classified as:I. Active and passive transducers.II. Analog and digital transducers.III.On the basis of transduction principle used.I4.5rimary and secondary transducer4. %ransducers and inverse transducers.


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Active transducers :

%hese transducers do not need any e6ternal source of power fortheir operation. %herefore they are also called as selfgenerating type transducers.

I. %he active transducer are self generating devices whichoperate under the energy conversion principle.

II. As the output of active transducers we get an equivalentelectrical output signal e.g. temperature or strain to electric

potential7 without any e6ternal source of energy being used.

AC%I4) A& 5A''I4) %"A&'(C)"'


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Passive Transducers :

I. %hese transducers need e6ternal source of powerfor their operation. 'o they are not selfgenerating type transducers.

II. A C power supply or an audio frequency

generator is used as an e6ternal power source.

III.%hese transducers produce the output signal inthe form of variation in resistance7 capacitance7inductance or some other electrical parameter in

response to the quantity to be measured.

AC%I4) A& 5A''I4) %"A&'(C)"'


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'ome transducers contain the mechanical as well aselectrical device. %he mechanical device converts the physical

quantity to be measured into a mechanical signal. 'uch

mechanical device are called as the primary transducers7

because they deal with the physical quantity to be measured.

8%he electrical device then convert this mechanical signal into

a corresponding electrical signal. 'uch electrical device are

9nown as secondary transducers.


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"ef fig in which the diaphragm act as primarytransducer. It convert pressure :the quantity to be

measured; into displacement:the mechanical signal;.

%he displacement is then converted into change in

resistance using strain gauge. /ence strain gauge acts as

the secondary transducer.


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CLA''I$ICA%IO& O$ %"A&'(C)"'According to %ransduction 5rinciple


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CAPACITIVE TRANSDUCER:

In capacitive transduction transducers the measurand is converted to

a change in the capacitance.

8 %he relationship between the capacitance and the si3e of capacitor

plate7 amount of plate separation7 and the dielectric is given by

C < =>=rA ? d

d is the separation distance of plates :m;C is the capacitance (F, Farad)

=>@ absolute permittivity of vacuum

=r@ relative permittivity

A is the effective (overlapping) area of capacitor plates (m2)


d

Area=A

Either A, d or can bevaried.


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ELECTROA!NETIC TRANSDUCTION:

In electromagnetic transduction7 the measurand is

converted to voltage induced in conductor by change in

the magnetic flu67 in absence of e6citation.

%he electromagnetic transducer are self generating activetransducers

%he motion between a piece of magnet and an

electromagnet is responsible for the change in flu6



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INDUCTIVE TRANSDUCER:

In inductive transduction7 the measurand is converted

into a change in the self inductance of a single coil. It isachieved by displacing the core of the coil that is

attached to a mechanical sensing element



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PIE"O ELECTRIC INDUCTION :

In pie3oelectric induction the measurand is converted

into a change in electrostatic charge q or voltage 4

generated by crystals when mechanically it is stressedas shown in fig.


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P#OTOVOLTAIC TRANSDUCTION :

In photovoltaic transduction the measurand is

converted to voltage generated when the unction

between dissimilar material is illuminated as shown infig.


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n-typesemiconductor

p-type

semiconductor

+ + + + + + + + + + + + + ++- - - - - - - - - - - - - - - - - -

Physics of Photovoltaic Generation

Depletion Zone


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P#OTO CONDUCTIVE TRANSDUCTION :

In photoconductive transduction the measurand is

converted to change in resistance of semiconductor

material by the change in light incident on the material.


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CLA''I$ICA%IO& O$ %"A&'(C)"'%ransducer and Inverse %ransducer

TRANSDUCER:

%ransducers convert non electrical quantity to

electrical quantity.

INVERSE TRANSDUCER:

Inverse transducers convert electrical quantity to anon electrical quantity


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Resistive transducers @ "esistive transducers are those transducers in which the

resistance change due to the change in some physical

phenomenon.

%he resistance of a metal conductor is e6pressed by a simple

equation.

" < L?A

here " < resistance of conductor in D

L < length of conductor in m

A < cross sectional area of conductor in m2

< resistivity of conductor material in DEm.


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There are $ t%&e 'f resistive transducers(

*. 5otentiometers :5O%;+. 'train gauge

,. %hermistors-. "esistance thermometer


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%he potentiometer are used for voltage division. %hey consist of aresistive element provided with a sliding contact. %he slidingcontact is called as wiper.

%he contact motion may be linear or rotational or combination ofthe two. %he combinational potentiometer have their resistive

element in heli6 form and are called helipots. $ig shows a linear pot and a rotary pot.


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%he strain gauge is a passive7 resistive transducer whichconverts the mechanical elongation and compression into a

resistance change.

%his change in resistance ta9es place due to variation in length

and cross sectional area of the gauge wire7 when an e6ternalforce acts on it.


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%he type of strain gauge are as

*. ire gauge

a; (nbonded

b; Bonded

c; $oil type

+. 'emiconductor gauge


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An unbonded meter strain gauge is shown in fig %his gauge consist of a wire stretched between twopoint in an insulating medium such as air. %he wiresmay be made of various copper7 nic9el7 crome nic9le ornic9le iron alloys.

In fig the element is connected via a rod to diaphragm

which is used for sensing the pressure. %he wire aretensioned to avoid buc9ling when they e6perience thecompressive force.


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%he unbounded meter wire gauges used almost e6clusively intransducer application employ preloaded resistance wireconnected in heatstone bridge as shown in fig.

At initial preload the strain and resistance of the four arms arenominally equal with the result the output voltage of the

bridge is equal to 3ero. Application of pressure produces a small displacement 7 thedisplacement increases a tension in two wire and decreases itin the other two thereby increase the resistance of two wirewhich are in tension and decreasing the resistance of theremaining two wire .

%his causes an unbalance of the bridge producing an outputvoltage which is proportional to the input displacement andhence to the applied pressure .


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%he bonded metal wire strain gauge are used for both stressanalysis and for construction of transducer.

A resistance wire strain gauge consist of a grid of fineresistance wire. %he grid is cemented to carrier which may

be a thin sheet of paper ba9elite or teflon.

%he wire is covered on top with a thin sheet of material soas to prevent it from any mechanical demage.

%he carrier is bonded with an adhesive material to thespecimen which permit a good transfer of strain fromcarrier to grid of wires.


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%his class of strain gauge is only an e6tension of thebonded metal wire strain gauges. %he bonded metal wire starin gauge have been completely

superseded by bonded metal foil strain gauges. #etal foil strain gauge use identical material to wire

strain gauge and are used for most general purpose stressanalysis application and for many transducers.


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'emiconductor gauge are used in application where ahigh gauge factor is desired. A high gauge factor meansrelatively higher change in resistance that can bemeasured with good accuracy.

%he resistance of the semiconductor gauge change as

strain is applied to it. %he semiconductor gauge dependsfor their action upon the pie3oEresistive effect i.e. changein value of resistance due to change in resistivity.

'ilicon and germanium are used as resistive material forsemiconductor gauges.


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"esistance of metal increase with increases in

temperature. %herefore metals are said to have apositive temperature coefficient of resistivity.

$ig shows the simplest type of open wire constructionof platinum rFsistance thermometer. %he platinum

wire is wound in the form of spirals on an insulatingmaterial such as mica or ceramic. %his assembly is then placed at the tip of probe %his wire is in direct contact with the gas or liquid

whose temperature is to be measured.


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%he resistance of the platinum wire changes with thechange in temperature of the gas or liquid

%his type of sensor have a positive temperaturecoefficient of resistivity as they are made from metalsthey are also 9nown as resistance temperaturedetector

"esistance thermometer are generally of probe typefor immersion in medium whose temperature is to bemeasured or controlled.


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%hermistor is a contraction of a term Gthermal resistorH.%hermistor are temperature dependent resistors. %hey are

made of semiconductor material which have negative

temperature coefficient of resistivity i.e. their resistance

decreases with increase of temperature.%hermistor are widely used in application which involve

measurement in the range of >E0> %hermistor are composed

of sintered mi6ture of metallic o6ides such as magnese7

nic9le7 cobalt7 copper7 iron and uranium


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Contd.

%he thermistor may be in the form of beads7 rods anddiscs.

%he thermistor provide a large change in resistance for

small change in temperature. In some cases the

resistance of themistor at room temperature may

decreases as much as 0J for each *C rise in

temperature.


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A thermocouple converts thermal energyinto electrical energy. The thermocouple

works by creating a voltage diferencebetween two wires that is proportional tothe temperature at the unction.

This is called the !eebeck efect in honor o"

Thomas !eebeck, who #rst noticed thephenomena in $%2$.

&t was also noticed that diferent metalcombinations have a diferent voltage

diference.


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An inductive electromechanicaltransducer is a transducer which convertsthe physical motion into the change ininductance.

8Inductive transducers are mainly used

for displacement measurement.


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The inductive transducers are o" the sel"generating or the passive type. The sel"generating inductive transducers use thebasic generator principle i.e. the motion

between a conductor and magnetic #eldinduces a voltage in the conductor.

The variable inductance transducers workon the "ollowing principles.

'ariation in sel" inductance 'ariation in mutual inductance


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Let us consider an inductive transducer having & turnsand reluctance ". when current I is passed through thetransducer7 the flu6 produced is

< &i ? "

ifferentiating w.r.t. to t7d?dt < &?" di?dt %he e.m.f. induced in a coil is given bye < & d?dt


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e < & &?" di?dt e < &+ ? " di?dt 'elf inductance is given by

L < e?di?dt < &+ ? "

%he reluctance of the magnetic circuit is " < ? MA%herefore L < &+ ? ? MA < &+ MA ? $rom eqn we can see that the self inductance may

vary due to

i. Change in number of turns &ii. Change in geometric configurationiii. Change in permeability of magnetic circuit


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$rom eqn we can see the output may vary with thevariation in the number of turns. As inductive

transducers are mainly used for displacement

measurement7 with change in number of turns the

self inductance of the coil changes inEturn changing

the displacement

$ig shows transducers used for linear and angular

displacement fig a shows an air cored transducer forthe measurement of linear displacement and fig b

shows an iron cored transducer used for angular

displacement measurement.


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An inductive transducer that wor9s on the principle of changein self inductance of coil due to change in the permeability is

shown in fig

As shown in fig the iron core is surrounded by a winding. If

the iron core is inside the winding then the permeabilityincreases otherwise permeability decreases. %his cause the self

inductance of the coil to increase or decrease depending on the

permeability.

%he displacement can be measured using this transducer

displacement

(erromagnetic "ormer

coil


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$ig shows a variable reluctance inductive transducer. As shown in fig the coil is wound on the ferromagnetic iron. %he

target and core are not in direct contact with each other. %hey areseparated by an air gap.

%he displacement has to be measured is applied to the ferromagnetic

core %he reluctance of the magnetic path is found by the si3e of the air

gap. %he self inductance of coil is given by L < &+ ? " < &+ ? )i * )a & @ number of turns " @ reluctance of coil "i @ reluctance of iron path "a @ reluctance of air gap


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)( O&eratin* Princi&le: %he transducer are many timesselected on the basis of operating principle used by them.%he operating principle used may be resistive7 inductive7capacitive 7 optoelectronic7 pie3o electric etc.

+( Sensitivit%: %he transducer must be sensitive enough to

produce detectable output.,( O&eratin* Ran*e: %he transducer should maintain the

range requirement and have a good resolution over theentire range.

$( Accurac%:/igh accuracy is assured.-( Err'rs: %he transducer should maintain the e6pected

inputEoutput relationship as described by the transferfunction so as to avoid errors.


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.( Transient and fre/uenc% res&'nse : %he transducer should

meet the desired time domain specification li9e pea9overshoot7 rise time7 setting time and small dynamic error.

0( L'adin* Effects: %he transducer should have a high inputimpedance and low output impedance to avoid loadingeffects.

1( Envir'n2ental C'2&atibilit%: It should be assured that thetransducer selected to wor9 under specified environmentalconditions maintains its inputE output relationship and doesnot brea9 down.

)3( Insensitivit% t' un4anted si*nals: %he transducer should beminimally sensitive to unwanted signals and highly sensitiveto desired signals.


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transducers lecture

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