SENSORS MEETING THE NEEDS OF THE DAY

 A device which converts energy or information in one form to another  In other words, a device which provides a usable output in response a to specific input measurand which may be physical or mechanical quantity,property or condition.

Basic requirements of a transducers The main function of a transducer is to respond only for the measurement under specified limits for which it is designed.  RUGGEDNESS (Capability of withstanding overload)  LINEARITY (input – output characteristics should be linear)  REPEATABILITY (should reproduce same output signal when the same input signal is applied again and again)  HIGH OUTPUT SIGNAL QUALITY (quality of output signal should be good)  HIGH RELIABILITY & STABILITY  GOOD DYNAMIC RESPONSE (output should be faithful to input when taken as a function of time)  NO HYSTERESIS (should not give any hysteresis during measurement)  RESIDUAL DEFORMATION (should be no deformation on removal of local after long period of application)

1: PRIMARY AND SECONDARY TRANSDUCERS 2: ACTIVE AND PASSIVE TRANSDUCERS 3: ANALOG AND DIGITAL TRANSDUCERS 4: TRANSDUCERS AND INVERSE TRANSDUCERS (An inverse transducer is a device which that converts an electrical quantity into a non-electrical quantity)

 RESISTIVE PRESSURE DISPLACEMENT TRANSDUCER  It is used to sense position of an object or the distance it has moved.  The physical variable being measured is made to cause a resistance change in sensing element.  A resistance element AB with a sliding contact or shaft(w) linked with the object being monitored. Resistance between W and B,R depends on the position of shaft.L1/L XR = R2.  Output voltage V is fn of shaft position  V=V X R2/R1+R2.

ADVANTAGES  Electrical efficiency high  Provides sufficient output for control operations.  Device may be excited by ac or dc supply. DISADVANTAGES  Mechanical friction between W and resistive elements AB reduces its life.

 RESISTIVE PRESSURE TRANSDUCER  INDUCTIVE PRESSURE TRANSDUCER  CAPACITIVE PRESSURE TRANSDUCER

 It is based on an arrange- ment in which a change in pressure results in the change of the resistance of a sensing element.  When applied pressure changes, the spring bellow extends or contracts thereby moving the resistance slider up and down.Consequently changes in resistance of sensing resistive element become proportional to the changes in pressure.

INDUCTIVE PRESSURE TRANSDUCER INDUCTIVE PRESSURE TRANSDUCER  In this case, change in pressure produces a change in the self inductance of a sensing element.  When pressure being measured increases, the bellow expands and, hence, the movable magnetic core is pushed more inside the coil thereby increasing in self inductance. Similarly, when applied pressure decreases, the bellows contract and thus the magnetic core is pulled out of the coil and, as a result, coil inductance is decrease.

 The changes in self inductance can be made a part of an inductance bridge and converted into an electrical signal for pressure measurement. As compared to the previous resistive arrangement the advantages of this inductive type are as under : No moving parts are used thereby avoiding wear & tear, No extra friction load is imposed on the measuring system.

CAPACITIVE PRESSURE TRANSDUCER In this arrangement, changes in pressure are detected by the variation in the capacitance of a parallel plate capacitor whose one plate is free to move with change in pressure. The capacitance of the capacitor is given by C= ε A/d, where A=plate area, d= plate separation When pressure increases the moving plate comes inwards, thereby decreasing d and consequently increasing C. However when the pressure decreases exactly opposite happens. The changes in capacitance can be used for pressure determination

ADVANTAGES:-  Inexpensive and easy to construct.  Particularly effective for high frequency variations.  Capable of measuring both static and dynamic phenomena. DISADVANTAGES:-  Sensitivity to temperature variation.  Possibility of erratic or distorted signals due to long leads.

 It is an inductive transducer (active) that utilizes the basic generator principle that when there is any relative motion between a conductor and a magnetic field or flux, an emf is induced in the conductor.  The relative motion between flux and conductor is provided in tachometer by the physical quantity under measurement(velocity).  The object whose angular velocity is to be measured is directly coupled to the rotor made of permanent magnet having N and S poles.

 As the rotor rotates, its alternating flux cuts the stationary coils thereby inducing an alternating voltage ‘E’ in them. The magnitude of ‘E’ depends on the angular velocity of the rotor directly. Polarity of output voltage indicates direction of rotation.  This emf can be measured by using a moving coil voltmeter having a uniform scale calibrated directly in terms of rpm(speed).

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