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)

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

    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

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

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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

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

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

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

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

    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