Engineering Q&A
State the working principle and specification of LVDT 
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Working principle: It works on the principle of mutual induction.to convert linear displacement into equivalent voltage. LVDT has a single primary winding P and two secondary windings S1 and S2 wound on a cylindrical former. S1 and S2 have equal number of turns and are connected in series opposition. A movable soft iron core is placed inside the cylindrical former. When a.c. supply is given to the primary winding, voltages are induced in both the secondary windings. When a displacement is applied to the movable core the flux linking with both the secondary winding changes and produces output voltage which is proportional to the displacement applied. The output voltage is Vo = (VS1 -VS2) where VS1 is voltage induced in S1 and VS2 is voltage induced in S2. 


Specifications of LVDT 

High sensitivity Very good linearity Ruggedness Less friction Low hysteresis Low power consumption

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Working principle:- 

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When primary winding is excited by suitable a.c. source.It tries to produces magnetic flux. This mutual flux link with two secondary winding and as flux linkages change emf is induced in both secondaries. When core is centrally located, equal voltage is induced in both secondaries. But when core is displaced flux linkages changes and hence more voltage is induced in on secondary than the other one. The variation of o/p voltage with core position is shown fig. (a) when core is centrally located, net o/p voltage Vo is zero (theoretically). This core position is called as null position.

As core moves on either side from null positon, output voltage increases primarily dependent upon length of secondary coils. Beyond proportionality limit o/p increases at a decreasing rate until it reaches a maximum from which it drops again to the balnced condition when the core is removed. There is a phase shift as core moves both to and from from central position, so that phase measurement can be related to the direction of core motion voltage variation with core is shown fig (b).

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Diagram:

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A Linear Variable Differential Transformer (RLVDT) is an electromechanical transducer that provides a variable alternating current (AC) output voltage that is linearly proportional to the linear displacement of its input shaft. LVDT is a passive inductive transducer to translate the linear motion into electrical signals. It consists of a single primary winding and two secondary windings. Secondary windings have equal number of turns and are identically placed on either side the primary winding. The primary winding is connected to an A.C. source. A movable iron core is placed inside the winding. The displacement to be measured is applied to the arm attached to the soft iron core. Since the primary coil is excited by an A.C. source, it produces an alternating magnetic field, which in turn induces alternating current or voltage in the two secondary windings. Two secondary windings are connected in series opposition to get differential output voltage. When displacement is applied to the core, inductance of one secondary winding increase and other decreases, giving differential output. The Differential output voltage= V1-V


Applications of LVDT: Measurement of displacement Measurement of weight or pressure exerted by liquid in a tank. Measurement and control of thickness of a meat sheet being rolled. Acting as a secondary transducer it can be used as device to measure force, weight and pressure

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