How it works
The LVDT has one primary coil excited by 1–10 kHz AC (say, 5 Vrms at 5 kHz) and two secondary coils wound in series-opposition. When the core is centred, mutual inductance to both secondaries is equal, so their induced voltages cancel and Vout = 0 V. Moving the core by +x increases coupling to S1 and decreases it to S2, producing a net output V1 − V2 that is proportional to displacement and in phase with the primary. Moving to −x gives the same magnitude but opposite phase. A phase-sensitive demodulator distinguishes direction by comparing output phase with the primary reference.
Key points to remember
LVDT characteristics: infinite resolution (no contact, no friction), excellent repeatability (< 0.1% FS), linearity within ±0.25% over the calibrated range (typically ±5 mm to ±250 mm depending on model). Operating frequency must be at least 10× the highest frequency of the measured displacement to avoid dynamic errors. The null voltage is never exactly zero due to residual quadrature voltage — this is a favourite examiner concept. Temperature affects sensitivity because coil resistance changes; LVDT signal conditioners like AD598 provide ratiometric output that cancels excitation amplitude variation.
Exam tip
The examiner always asks why LVDT output is never exactly zero at null position — explain that residual quadrature voltage (90° out of phase with primary) is due to winding capacitance and imperfect magnetic symmetry.