Thursday, November 26, 2015

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What is a Linear Variable Differential Transformer (LVDT)?

The Linear Variable Differential Transformer (LVDT) is a position sensing device that provides an AC output voltage proportional to the displacement of its core passing through its windings. LVDTs provide linear output for small displacements where the core remains within the primary coils. The exact distance is a function of the geometry of the LVDT.

Theory of Operation


An LVDT is much like any other transformer in that it consists of a primary coil, secondary coils, and a magnetic core. An alternating current, known as the carrier signal, is produced in the primary coil. The changing current in the primary coil produces a varying magnetic field around the core. This magnetic field induces an alternating (AC) voltage in the secondary coilsthat are in proximity to the core. As with any transformer, the voltage of the induced signal in the secondary coil is linearly related to the number of coils. The basic transformer relation is:


Vout is the voltage at the output,
Vin is the voltage at the input,
Nout is the number of windings of the output coil, and
Nin is the number of windings of the input coil.

As the core is displaced, the number of coils in the secondary coil exposed to the coil changes linearly. Therefore the amplitude of the induced signal varies linearly with displacement.


The LVDT indicates direction of displacement by having the two secondary coils whose outputs are balanced against one another. The secondary coils in an LVDT are connected in the opposite sense (one clockwise, the other counter clockwise). Thus when the same varying magnetic field is applied to both secondary coils, their output voltages have the same amplitude but differ in sign. The outputs from the two secondary coils are summed together, usually by simply connecting the secondary coils together at a common center point. At an equilibrium position (generally zero displacement) a zero output signal is produced.

The induced AC signal is then demodulated so that a DC voltage that is sensitive to the amplitude and phase of the AC signal is produced.

Carrier Demodulator

In practice, LVDTs are used with carrier demodulator modules that provide the carrier signal (the AC signal to the primary coil) and converts (or demodulates) the induced AC signal to a DC signal.

The phase sensitive demodulator is an AC to DC converter that produces a DC voltage (typically between 0 and 10 volts) proportional to the magnitude of the LVDT output and sensitive to the phase of the output signal relative to the input (carrier) signal.

When the core is displaced to one side of the primary, the LVDT output is in phase with the input and the demodulator produces a positive signal. When the core is displaced to the other side, the LVDT output is 180 degrees out of phase with the signal to the primary. The demodulator output is then a negative voltage proportional to the displacement. (from university of california, berkeley)
Comments (1)Add Comment
LVDT comment on excitation voltage
written by R. Stuart Bondurant, November 11, 2015
In the Theory of Operation section, the second sentence ends with "... is produced in the primary coil...."

This is incorrect. The excitation voltage is produced external to the LVDT. This is the primary voltage to the LVDT. As with any transformer, the transformer itself does not produce the primary voltage, but serves to transform this primary voltage to the secondary coil.

I suggest the following wording for this sentence:

An alternating current, known as the carrier signal, is used to excite the primary coil.

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