LVDT &Displacment sensors

Cutaway view of an LVDT. Current is driven through the primary coil at A, causing an induction current to be generated through the secondary coils at B.

 

The linear variable differential transformer (LVDT) is a type of electrical transformer used for measuring linear displacement. The transformer has three solenoidal coils placed end-to-end around a tube. The center coil is the primary, and the two outer coils are the secondaries. A cylindrical ferromagnetic core, attached to the object whose position is to be measured, slides along the axis of the tube.

An alternating current is driven through the primary, causing a to be induced in each secondary proportional to its mutual inductance with the primary. The frequency is usually in the range 1 to 10 kHz.

As the core moves, these mutual inductances change, causing the induced in the secondaries to change. The coils are connected in reverse series, so that the output is the difference (hence “differential”) between the two secondary . When the core is in its central position, equidistant between the two secondaries, equal but opposite are induced in these two coils, so the output LVDT is theoretically zero. In practice minor variations in the way in which the primary is coupled to each secondary means that a small LVDT is output when the core is central

When the core is displaced in one direction, the LVDT in one coil increases as the other decreases, causing the output LVDT to increase from zero to a maximum. This LVDT is in phase with the primary LVDT. When the core moves in the other direction, the output LVDT also increases from zero to a maximum, but its phase is opposite to that of the primary. The magnitude of the output LVDT is proportional to the distance moved by the core (up to its limit of travel), which is why the device is described as “linear”. The phase of the LVDT indicates the direction of the displacement.

Because the sliding core does not touch the inside of the tube, it can move without friction, making the LVDT a highly reliable device. The absence of any sliding or rotating contacts allows the LVDT to be completely sealed against the environment.

LVDTs are commonly used for position feedback in servomechanisms, and for automated measurement in machine tools and many other industrial and scientific applications.

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