Linear Variable Differential Transformer (LVDT) the most widely used inductive transducer to converts the linear motion or displacement into electrical signals.

Construction

The transformer consists of a single primary winding and two secondary winding S1 and S2 wound on a cylindrical former. Both the secondary winding have an equal number of turns and are identical placed on either side of the primary winding. The secondary windings are connected in series opposition so that the emfs induced in the coils oppose each other. 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. The linear voltage differential transformer measures force in terms of output voltage. The frequency of a.c. applied to the primary winding may be between 50Hz to 200 kHz.

Working

When primary winding is connected to an a.c. supply and a soft iron core is placed inside the former; e.m.fs. ES1, and ES2 are induced in two secondary winding S1 and S2 respectively. Their magnitudes depend upon the flux linking with them. The working of LVDT is explained in three cases

Case-I

When the core is in the centre position, the induced e.m.fs. in the two secondaries are equal. Since the windings are cross connected, their e.m.fs. oppose each other, the output voltage will be zero volt.

Case-II

When an externally applied force moves the core to the left hand side position, more magnetic flux links with the coil S1 than coil S2. The induced e.m.f. of coil S1, (i.e. ES1) is, therefore, larger than the induced emf. of Coil S2 (i.e. ES2). The magnitude of the output voltage is then equal to the difference between the two secondary voltages (i.e.ES1 – ES2) and it is in phase with the voltage of the coil S1.Case-III

When the core is forced to move to the right hand side position, more flux links with the coil S2 than coil S1. The induced emf. in coil S2 (i.e. ES2) is, therefore, larger than the induced e.m.f. in coil S1, (i.e. ES1). The magnitude of the output voltage is then equal to the difference between the two secondary voltage (i.e. ES1 – ES2) which is in phase with the voltage of the coil S2.Thus. the output voltage of Linear Variable Differential Transformer (LVDT) is the function of the core position. This output voltage is measured to determine the displacement or applied force. The output signal may also be applied to a recorder or to a controller that can restore the moving system to its normal position. The output voltage of an LVDT is a linear function of core displacement within a limited range of motion (say about 5mm from its reference position) and hence the name linear variation differential transformer. Beyond this range. the curve starts to deviate from a straight line.