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    GENERALIZED PERFORMANCE

    CHARACTERISTICS OFINSTRUMENTS

    RESISTIVE TRANSDUCERS

    Lecture 10Instructor : Dr Alivelu M Parimi

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    Problem

    2

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    Solution

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    Problem contd,,

    What is the actual temperature at 3000 m

    4

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    Problems

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    Solution

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    y(t) = AK (1e- t/)

    Error = em= inputoutput = // )1(/)()( tt AeeAAKtytx

    em = M- Me-t/= M( 1- e

    -t/)

    Steady state error = em,ss( emas t ) = M, Transient error = Me-t/

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    Solution

    7

    )(1

    1

    22

    Kx

    y

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    Example

    A first order measurement system with a time constant of =

    0.1 seconds is used to measure a harmonically varying voltage.

    The voltage has a peak-to-peak amplitude of 6 Volts and a

    frequency of 5 Hz. Calculate the following:

    Frequency of the observed signal Mean-to-peak amplitude of the observed signal

    Phase (in degrees) of the observed signal

    Time delay (lag) of the observed signal.

    8

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    Solution

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    Solution

    (i) The input and observed signals both have the same frequency. Therefore, observed

    frequency = 5 Hz.

    (ii)Mean-to-peak amplitude of input = (peak-to-peak)/2 = 6/2 = 3 V

    = 2 f =31.42 rad/s

    Observed value of mean to peak = V910.0)(1

    1

    22

    (appreciable attenuation!)

    (iii) Phase angle = tan-1

    (- ) = - 72.30

    (iii) One cycle takes 1/5 = 0.2 seconds which corresponds to 3600.

    Therefore 72.30correspond to

    0.2*72.30/360

    0=0.0402sec

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    Outline RESISTIVE TRANSDUCERS

    Introduction

    10

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    Introduction

    Instrument Transducers are devices which for the purpose of

    measurement convert physical input quantities into an

    electrical output signal.

    Measurand is converted into electrical signal becauseelectrical signals are easy to manipulate, process, transmit,

    store and display.

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    Radiant to Electrical Photodiode/phototransistor/LDR

    Mechanical to Mechanical Gear Box

    Mechanical to Electrical Piezoresistance

    Thermal to Thermal Heat exchanger

    Thermal to Electrical Seebeck Efefct

    Electrical to Chemical Electroplating

    Electrical to Radiant LED/LCD

    Electrical to Mechanical Piezoelectricity

    Electrical to Thermal Peltier effect

    Electrical to Electrical Transistor

    Electrical to Magnetic Coil

    Magnetic to Magnetic Magnetic circuit

    Chemical to Electrical pH

    Chemical to Chemical Chemical reaction

    Some examples of energy transformation

    devices/ effects are as follows:

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    Transducers examples

    Any energy conversion device is not a transducer. Only those

    which convert for the purpose of measurement are called

    Transducers.

    LED/LCD are not transducers since the energy conversionprocess in them is utilized for display purpose, while

    photodiode which is made to measure light intensity is a

    transducer.

    Thermocouple utilizing Seebeck effect is a transducer for

    temperature measurement.

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    Transducers are also classified

    on the basis of: Transduction principle

    This classification is based on the transduction principle used for converting non-electrical signal to electrical signal. For example inductive transducer will change itsinductance upon change in measurand.

    Examples : Resistive, Capacitive, Inductive transducers

    Primary Input quantity

    This classification depends on the measurand for which they are used, like motiontransducer are the ones used to measure motion.

    Example : Flow, Temperature, Motion, Pressure transducers

    Material and technology

    This classification depends on technology used in making transducers, for examplefibre optic transducers use principle and components of fiber optics to sense, SMTsuse IC technology to make sensors.

    Example : SMT (Silicon Micro Technology), MEMS ( Microelectomechanicsystems), Fiber Optic transducers

    Application

    This classification is based on the broad area of application in which transducers areused. For example biomedical transducers cover all types of transducer used infield of biomedical.

    Example : Environmental, Biomedical , Industrial etc.

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    RESISTIVE transducers

    The major types of resistive transducers are Potentiometers,

    Strain Gages Resistance Temperature Detector (RTD),

    Thermistors, Light Dependent Resistor (LDR) etc.

    Resistive transducers have been used to measuredisplacement, load, pressure, temperature, flow, light

    intensity, humidity etc.

    This chapter mainly deals with Strain Gages and

    Potentiometers.

    RTDs and Thermistors would be covered under temperature

    measurement and LDR would be covered under Photoelectric

    Transducers. 15

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    RESISTIVE transducers

    Three main parameters of an electrical system are

    Conductance (Resistance), Inductance and Capacitance.

    All these parameters depend on (i) Geometrical

    configuration and (ii) Material properties.

    Resistance R is given by where L(length) & A (area)

    are geometrical properties while (resistivity) is a

    material property. 16

    ,A

    L

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    RESISTIVE

    POTENTIOMETERS Resistive potentiometer consists of a resistance element

    provided with a movable contact.

    The contact motion can be translational, rotational, or a

    combination of the two such as helical thus allowingmeasurement of rotatory, translatory and helical

    displacements.

    This displacement could be caused by load, force, vibration,sound pressure etc. The resistance element is excited with

    either dc or ac voltage, and the output voltage is a linear

    function of the input displacement. 17

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    RESISTIVE

    POTENTIOMETERS Resistance elements in common use may be classified as wire-

    wound, slide wire, conductive plastic, carbon film, thin film

    metal resistor, conducting plastic.

    Figure shows different types of potentiometer configuration

    like slide type, wire wound, linear, rotary, helical; and graphicalrelation of open circuit output voltage vs displacement (linear

    or angular).

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