Principle
LVDT works on the principle of conversion of linear motion of an object to which it is mechanically coupled into a corresponding electrical signal.
Construction
The basic construction of LVDT is shown in Fig.1. The transformer consists of a single primary winding P and two secondary windings S1 and S2 wound on a cylindrical former. The secondary winding has equal number of turns and is identically placed on either side of the primary winding. The primary winding is connected to an alternating current source. A movable soft iron core is placed inside the former. The displacement to be measured is applied to the arm attached to the soft iron core. In practice the core is made of high permeability, nickel iron which is hydrogen annealed. This gives low harmonics, low null voltage and a high sensitivity. This is slotted longitudinally to reduce eddy current losses. The assembly is placed in stainless steel housing and the end lids provide electrostatic and electromagnetic shielding. The frequency of AC applied to primary winding may be between 50Hz to 20Hz. Since the primary winding is excited by an alternating current source, it produces an alternating magnetic field which in turn induces alternating current voltages in the two secondary windings.
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