sensor and instrumentation by Yatika

LVDT (LINEAR VARIABLE DIFFERENTIAL TRANSFORMER)

This is the most widely used inductive transducer for translating linear motion into an electric signal. The transformer consist of single frequency windings and two secondary widings wound on a cylindrical former. The secondary windings have equal number of turns and are identically placed on either sides of primary windings.The primary windins are connected to an ac 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. The core is made up of high permeablity nickel iron which is hydrogen annealed.
Since the primary windings is connected by an ac source.It produces an alternating magnetic field which in turn induces an alternating current voltage in the two secondary winding.The output voltage of secondary is ES1 and ES2. In order to converts the outputs into single frequency signal,the two windings are connected in series opposition.The output voltage are

E0=ES1-ES2

When the core is at null position, the flux linking with both the secondary windings are equal.Hence equal emf induced in them.

ES1=ES2

Now if the core moves towards the left of null position,the more flux link with S1 and less with S2

ES2>ES1,   E0=ES2-ES1

Ideally, output voltage should be 0 at null position but practically there is a small voltage at output when ES1=ES2  i.e. due to the presence of harmonics in the input supply voltage and also due to the harmonics present in output voltage supply on account of iron core. There may be electric or magnetic unbalance.This voltage is known as Residual voltage