LVDT (Linear Variable Differential Transformer) is a passive transducer, that works on the principle of mutual induction and can be used to measure displacement, pressure, and force.
Construction of LVDT
LVDT consists of one primary coil and two secondary coils wounded on a cylindrical core. The core is made up of a ferromagnetic material such as iron core and is freely movable inside the coil and this movement measures the physical quantity.
The primary winding is connected to an AC source. The two secondary winding S1 and S2 have an equal number of turns and are set up in series opposition. So the e.m.f induced in these winding are 180° out of phase with each other and thus the net effect is cancelled.
Operation of LVDT
When an alternating voltage input is given in the primary winding, an alternating emf is induced in the secondary winding(S1 and S2).
Suppose V1 is the voltage induced across S1 and V2 is the voltage induced across S2. The overall output voltage across the secondary winding(V0) is the difference between V1 and V2.
So the differential output is
The value of V0 depends on the position of the core. Three possible cases are illustrated in the following figure.
When the core is positioned at the centre (Case 1), voltages induced across winding S1 and S2 are equal(but in reverse-phase). Then, the resultant voltage V0=0. In this case, we say there is no displacement.
If the core is displaced from the central position to more in S1(Case 2), then more emf generated in coil S1. That is V1>V2.
Similarly, if the core is displaced from the central position to more in S2(Case 3), then more emf generated in coil S2. That is V2>V1.
In case 2, we have upward displacement and in case 3, we have downward displacement. In both cases, the magnitude of V0 will be proportional to the position of the core relative to the centre.
The input output curve of a LVDT is shown in the figure.
Advantages of LVDT
- No physical contact between the core and the coils.
- Easy to modify.
- High accuracy.
- Fast response.
- Long operating life.