Side-by-side comparison
| Parameter | LVDT | RVDT |
|---|---|---|
| Full Name | Linear Variable Differential Transformer | Rotary Variable Differential Transformer |
| Measured Quantity | Linear displacement (mm to cm range) | Angular displacement (typically ±40° to ±60°) |
| Core Motion | Iron core moves axially inside a coil assembly | Specially shaped iron core rotates inside coils |
| Output | Differential AC voltage proportional to linear displacement | Differential AC voltage proportional to angular displacement |
| Linearity Range | Excellent linearity over full stroke (±0.1% FSO typical) | Excellent linearity within ±40°; non-linear beyond ±60° |
| Excitation | 1–10 kHz AC, 1–10 V RMS from signal conditioner | Same — 1–10 kHz AC from dedicated RVDT signal conditioner |
| Null Position | Core at center of coil assembly — differential output = 0 | Core at center — differential output = 0 |
| Application | Hydraulic actuator position, material testing machines, micrometer probes | Aircraft control surface angle, throttle position, valve stem angle |
Key differences
LVDT and RVDT share an identical operating principle: a primary coil excited at 2.5 kHz drives two secondary coils; the differential output voltage V_A – V_B is proportional to core displacement. At the null (center) position, both secondaries have equal and opposite voltages giving zero output. Move the core 5 mm in an LVDT — the output might be 2.5 V (sensitivity 0.5 V/mm). Rotate an RVDT 30° — the output is proportional to sinusoidal function of angle. LVDT sensors from Honeywell or TE Connectivity operate over strokes from ±1 mm to ±250 mm; RVDTs are typically limited to ±40° to ±60° before nonlinearity exceeds 0.5%.
When to use LVDT
Use an LVDT when you need to measure precise linear displacement without mechanical contact — for example, a 100 mm stroke LVDT on a UTM (Universal Testing Machine) to measure material elongation to 0.01 mm accuracy.
When to use RVDT
Use an RVDT when angular displacement of a rotating element must be measured without contact — for example, an RVDT on the throttle shaft of a gas turbine engine measuring ±30° of shaft rotation for the fuel control unit.
Recommendation
In exam MCQs, identify the type of displacement first: linear means LVDT, rotary means RVDT. Both give AC differential output, both are frictionless, and both require a signal conditioner (demodulator) to convert the AC output to a DC voltage proportional to displacement.
Exam tip: Examiners ask why the LVDT output at the null position is zero — explain that the two secondary voltages V_A and V_B are equal in magnitude but opposite in phase at the null; the differential connection gives V_A – V_B = 0 V at center.
Interview tip: Interviewers at aerospace or process control companies ask about the advantage of LVDT over a potentiometer for position sensing — state that LVDTs have infinite resolution (no wiper contact), no mechanical wear, and are immune to contamination, whereas potentiometers wear at the wiper contact and degrade over time.