The Linear Variable Differential Transformer (LVDT) is the most broadly used variable-inductance transducer in industry. It is an electro-mechanical device designed to produce an AC voltage output proportional to the relative displacement of the transformer and the armature, as illustrated in the figure below.

	Input:	Power input is a 3 to 15 V (rms) sine wave with a frequency between 60 to 20,000 Hz (the two most common signals are 3 V, 2.5 kHz and 6.3 V, 60 Hz).
	Stroke:	Full-range stroke ranges from ±125 µm to ±75 mm (±0.005 to ±3 in).
	Sensitivity:	Sensitivity usually ranges from 0.6 to 30 mV per 25 µm (0.001 in) under normal excitation of 3 to 6 V. Generally, the higher the frequency the higher the sensitivity.
	Nonlinearity:	Inherent nonlinearity of standard units is on the order of 0.5% of full scale.

•	Pros:
	-	Relative low cost due to its popularity.
	-	Solid and robust, capable of working in a wide variety of environments.
	-	No friction resistance, since the iron core does not contact the transformer coils, resulting in an infinite (very long) service life.
	-	High signal to noise ratio and low output impedance.
	-	Negligible hysteresis.
	-	Infinitesimal resolution (theoretically). In reality, displacement resolution is limited by the resolution of the amplifiers and voltage meters used to process the output signal.
	-	Short response time, only limited by the inertia of the iron core and the rise time of the amplifiers.
	-	No permanent damage to the LVDT if measurements exceed the designed range.
•	Cons:
	-	The core must contact directly or indirectly with the measured surface which is not always possible or desirable. However, a non-contact thickness gage can be achieved by including a pneumatic servo to maintain the air gap between the nozzle and the work piece.
	-	Dynamic measurements are limited to no more than 1/10 of the LVDT resonant frequency. In most cases, this results in a 2 kHz frequency cap.