Working of the LVDT
The input voltage is supplied to the primary coil from the external source of power and the output is obtained from the secondary coils. The amplitude and the phase of the output depend on the relative coupling between the two output coils and primary coil. The relative coupling is in turn dependent on the position of the solid core inside the hollow core.
The figure below shows the output characteristics of the typical differential transformer. As shown in the figure there is certain position of the core, called as the null position, for which the output voltage is zero, this is an ideal position and is very difficult to attain. Beyond the null position the core moves either to the left or to the right and there is certain output voltage obtained from the differential transformer.
Within certain limits on either side of the null position the output obtained from the differential transformer is proportional to the movement of the core. This means the output from the differential transformer is linear with respect to the motion of the core. The linear variable differential transformer or LVDT works within this range of motion of the core. Thus in LVDT the voltage output obtained is linear with respect to the motion of the core moving inside it.
In LVDT the linear range obtained through the device is dependent on the length of the secondary coils. The magnitude of the output voltage obtained across the sides of the null position is same but they are opposite in phase. Thus it is possible to distinguish the two outputs from LVDT by determining the phase difference between the output voltages.
The output obtained from the LVDT is calibrated against the input motion of the core. The body whose displacement is to be measured is connected to this core, thus any motion of the body gives direct output from the LVDT in the form of the displacement.