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 centre 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. Image File history File links Download high resolution version (800x800, 59 KB)Illustration of a Linear variable differential transformer. ... Image File history File links Download high resolution version (800x800, 59 KB)Illustration of a Linear variable differential transformer. ... Transformers - Typical electrical configurations. ... In engineering, a solenoid is a mechanical device that converts energy into linear motion. ...

An alternating current is driven through the primary, causing a voltage 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. It has been suggested that this article or section be merged with AC power. ... This article may be too technical for most readers to understand. ... Inductance is a physical characteristic of any system of conductors (including an inductor), which creates a voltage proportional to the instantaneous rate of change in current flowing through it. ... Sine waves of various frequencies; the lower waves have higher frequencies than those above. ... A kilohertz (kHz) is a unit of frequency equal to 1,000 hertz (1,000 cycles per second). ...

As the core moves, these mutual inductances change, causing the voltages induced in the secondaries to change. The coils are connected in reverse series, so that the output voltage is the difference (hence "differential") between the two secondary voltages. When the core is in its central position, equidistant between the two secondaries, equal but opposite voltages are induced in these two coils, so the output voltage is zero.

When the core is displaced in one direction, the voltage in one coil increases as the other decreases, causing the output voltage to increase from zero to a maximum. This voltage is in phase with the primary voltage. When the core moves in the other direction, the output voltage also increases from zero to a maximum, but its phase is opposite to that of the primary. The magnitude of the output voltage 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 voltage 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. In its most general definition, a servomechanism, usually just shortened to servo, is a device used to provide mechanical control at a distance. ...