Linear variable-differential transformer (LVDT) technology’s capability to accurately measure linear movement is used to provide a position to electrical output for pressure, acceleration and force measurements. For example, in a pressure sensor, the sensing element is a diaphragm that moves a rod or cylindrical core inside the coils of a differential transformer to change the output voltage by changing the inductances of the coils in equal but opposite amounts.Source: Automotive Electronics Handbook, Chapter 2
An alternating current (AC) voltage source (typically in the kilohertz range) energizing the primary winding, results in induced voltages in each of the secondary windings. Since the windings are connected in series opposing, the equal but opposite output of each winding tends to cancel except for a small residual value called the null voltage. For a pressure sensor, the pressure applied to the diaphragm causes the core to move from its null position, so the coupling between the primary and the secondaries is no longer equal. The resulting output varies linearly and has a phase change of 180° from one side of the null position to the other.
Since the core and coil structures are not in physical contact, essentially frictionless movement results.
Signal conditioning to achieve a useful output includes an oscillator for the supply voltage, circuitry to transform the constant voltage to a constant current, an amplifier with a high input impedance for the output, a synchronous demodulator and a filter designed for quasistatic or dynamic measurements.
In addition to AC operated units, DC LVDTs powered by uni- or bi-polar DC voltages are also available that have their signal conditioning built into them from companies such as TE Connectivity.