LVDT & Displacement Transducers

Sensor

The Linear Variable Differential Transformer: a movable iron core between one primary and two opposed secondaries turns position into a phase-referenced AC voltage. Frictionless, infinite resolution, and the single most-asked transducer in Instrumentation exams.

What it looks like

A typical sensor — exact shape, colour and markings vary between manufacturers.

Types & variants

AC LVDT (external signal conditioning)DC-LVDT (built-in oscillator/demodulator)RVDT (rotary version, angular displacement)Spring-loaded gauge-head LVDTs (dimension checking)Draw-wire / potentiometric / capacitive displacement sensors (alternatives)

Key specs

Stroke range

±mm

±0.5 mm gauging heads to ±300 mm long-stroke versions; linearity is specified over this range.

Sensitivity

mV/V/mm

Output per volt of excitation per mm of displacement — the number used in every numerical.

Excitation

V / kHz

AC types: 1–10 V RMS at 1–10 kHz. Output amplitude gives distance; phase (0°/180°) gives direction — why demodulation is 'phase-sensitive'.

Linearity

% FS

0.1–0.5% of full scale typical — excellent, and with zero friction there's no wear or hysteresis from contact.

Null voltage

Residual output at centre (secondaries never cancel perfectly) — small but exam-favourite imperfection.

Markings

Body label: stroke, sensitivity, excitation. Leads: primary pair, two secondary pairs (wired series-opposing). DC-LVDTs simplify to supply + signal wires (0–10 V or 4–20 mA out).

Standard values

Lab trainer LVDTs: ±10 mm or ±25 mm with a micrometer jig for plotting output vs displacement. Industrial gauge heads: ±1 mm spring-loaded. RVDTs: ±30–60° for actuator feedback.

How to choose

1) Stroke with 20% margin — linearity dies past the rated range. 2) DC-LVDT unless you're building the conditioning yourself (the AC version is the educational one). 3) Harsh environments (vibration, oil, temperature) are exactly where LVDTs beat optical/capacitive rivals — aerospace and turbines use them for a reason. 4) For rotation, RVDT or encoder; for very long strokes, draw-wire or magnetostrictive.

Pinout & package

Cylindrical stainless body with a free-sliding threaded core on a rod. 6 wires (AC type): primary P1-P2, secondaries S1-S3 in series opposition. Output Vo = (Vs1 − Vs2): zero at null, amplitude ∝ displacement, phase flips through centre.

Example circuits

Lab experiment: micrometer drive vs output voltage plot, finding sensitivity and null
Phase-sensitive detector turning the AC output into a signed DC position signal
Thickness gauging: spring-loaded LVDT riding sheet metal on a production line
Turbine valve position feedback loop using a DC-LVDT's 4–20 mA output

Common failures

Broken lead wires at the moving-cable end, lost/wrong core (each LVDT is calibrated with ITS core), core rod bent and rubbing (friction where there should be none), moisture in windings, and conditioning drift misread as sensor error.

How to test

Winding resistance: primary and both secondaries per datasheet, secondaries matching each other closely. Functional: excite the primary at spec, watch each secondary rise/fall as the core slides, and confirm near-zero at null. A scope shows the 180° phase flip through centre beautifully.

Substitutes

Potentiometric sliders (cheap, but friction and wear), capacitive/eddy-current gauges (short range, non-contact), magnetostrictive rods (long stroke), optical linear encoders (digital, highest resolution). None match the LVDT's ruggedness-plus-infinite-resolution combination.

Where to buy

Robu Amazon DigiKey Mouser