Linear Variable Differential Transformer (LVDT)

The Linear Variable Differential Transformer (LVDT) is a type of sensor which converts linear displacement into a variable electrical signal. An LVDT consists of a sliding ferromagnetic core which sits inside 3 electrical coils, a primary coil in the middle and two identical secondary coils at either side.

Since the design of an LVDT is based on the transformer principle of Mutual Inductance, an LVDT device will only function with a AC voltage supplied to the primary coil.

The secondary coils are interconnected so that any current induced by the primary coil will be 180 degrees out of phase between the two secondary coils, and the resulting output will be zero when there is no displacement. If the core is moved to the left or the right, the mutual inductance between the primary and secondary coils will increase in one and decrease in the other, creating a differential voltage across the two secondary coils. The differential voltage output produced is directly proportional to the amount of displacement of the core.

There is very little friction generated by the movement of the core which makes it ideal for incorporating into sensor technologies where high precision and repeatability are important requirements. The lack of friction also reduces mechanical fatigue, extending the working life of the sensor.

The resolution of an LVDT is considered to be infinite and the null point is extremely repeatable, these qualities make it possible to incorporate the technology into other devices such as pressure sensors without compromising on measurement precision or stability.

LVDT technology is used by some manufacturers of pressure measurement instrumentation. An LVDT is incorporated into a pressure measurement device by connecting the core of the LVDT to the centre of a pressure sensing diaphragm, so that any flexing of the diaphragm due to changes in pressure will be detected as a position change by the LVDT.