LVDT with merits and demerits

LVDT (Linear Variable Differential Transformer) is a passive inductive transducer that converts the linear displacement of a core into a proportional AC voltage, using a differential transformer.

Construction and working

   |=====S1=====|===Primary===|=====S2=====|   <- tubular former
          [====== movable soft-iron core ======]  --> displacement
   output eo = e1 - e2  (S1 and S2 in series opposition)

• One primary winding in the centre and two identical secondary windings, one on each side, are wound on a single tubular former. A movable soft-iron core slides inside it.
• The secondaries are connected in series opposition. The primary is excited by AC (1 to 10 kHz, 3 to 15 V).
• Core at centre (null): e1 = e2, so the output is 0.
• Core moves towards S1: the flux linkage of S1 increases, so e1 > e2; the output is in phase with the primary voltage and proportional to the displacement.
• Core moves towards S2: e2 > e1; the output is shifted by 180°. The phase gives the direction, and the magnitude gives the displacement.

Merits

1. Wide range: ±1.25 mm to ±250 mm.
2. No friction or contact, so no wear; very fine (practically infinite) resolution.
3. High sensitivity (up to about 40 V/mm) and good linearity.
4. Rugged and works in harsh environments; low hysteresis and good repeatability.
5. Direct electrical output, so little amplification is needed; low power consumption.
6. Gives the direction as well as the magnitude of the displacement.

Demerits

1. A relatively large displacement is needed for an appreciable differential output.
2. Sensitive to stray magnetic fields, so it needs shielding.
3. Needs AC excitation, and the output needs a demodulator (phase-sensitive detector) to give the direction.
4. Dynamic response is limited by the core mass and by the carrier frequency (measurable frequency about one tenth of it).
5. Temperature affects the performance, and the residual voltage at null never becomes exactly zero (due to harmonics and stray capacitance).
6. Heavier and more expensive than some other displacement transducers.