LVDT Fundamentals

The figure below shows a cross-sectional view of an LVDT. Two secondary windings are located on either side of the primary winding. The core causes the magnetic field generated by the primary to be coupled to the secondaries. When the core is at center as shown, the voltage induced in each secondary is equal and the LVDT output (for the series-opposed connection shown in this case) is zero because both voltages cancel.
Move the core to the left and the first secondary is more strongly coupled to the primary than the second secondary. The greater voltage of the first secondary causes an output voltage which is in phase with the primary voltage.
Likewise, move the core to the right and the second secondary is more strongly coupled to the primary than the first secondary. The greater voltage of the second secondary causes an output voltage to be out-of-phase with the primary voltage.

LVDT signal conditioners generate a sine wave for the primary and synchronously demodulate the secondary output signal, so that the DC voltage that results is proportional to core displacement. The sign of the DC voltage indicates whether the displacement is to the left or right.