LVDT
Excites a primary coil with an AC carrier (1–10 kHz at a few volts), measures the differential AC output between two opposing secondary coils, and reports that difference (in-phase amplitude) as linear position.
Three coils share a common axis: one primary in the centre, two secondaries flanking it. A ferromagnetic core (no contact with the coils) slides through the bore. With the core centred, mutual inductance to each secondary is equal and the differential output is zero. Move the core: one secondary couples more, the other less, and the differential AC amplitude grows linearly with displacement. A synchronous demodulator referenced to the primary's phase recovers a DC voltage proportional to position.
In plain terms
Like a children's seesaw between two friends — slide the fulcrum (the core) and weight tips toward one side; the LVDT reads the imbalance as position.
Why designers use it
- Frictionless — the core never touches the coils, so cycle life is essentially unlimited.
- Robust — no moving electrical contacts; survives harsh environments (corrosive fluids, radiation, vibration).
- Theoretically infinite resolution — limited only by demodulator noise; sub-nm motion is measurable.
- Reliable across temperature — symmetric coil structure cancels common-mode drift.
Best for
- Aircraft actuators
- Materials testing
- Seismic
Key specifications
- Operating limits: Check the exact manufacturer's datasheet (A family name does not establish voltage, current, temperature or timing limits.)
- Pin assignment: Match the complete part and package code (A similar name or function does not guarantee the same wiring.)
- Mechanical fit: Use the exact package drawing (Check pad layout, dimensions and viewing direction before building.)
When not to use it
- Where AC carrier excitation is impractical or noisy — pull DC-output Hall or potentiometer alternatives.
- For long-stroke (>500 mm) applications — the LVDT's coil structure becomes impractical; magnetostrictive or laser interferometers are better.
Common mistakes
- Forgetting to phase-match the demodulator to the primary excitation — quadrature error makes the output read 5 % low and saturates near full scale.
- Mounting the core off-axis — radial coupling adds a non-linear error term that no calibration can remove.
Where you will find it
- A Boeing 787's flight-control surface position uses a TE Connectivity LVDT (Schaevitz heritage) in each spoiler actuator: the LVDT's frictionless construction is qualified to DO-160 vibration, and its core position feeds the flight-control computer at 200 Hz.
- An MTS Landmark fatigue-testing machine uses a Trans-Tek 0240-series LVDT for actuator stroke feedback: the LVDT's 2 µm resolution at ±100 mm range lets the machine cycle aluminium aerospace coupons through 10 million cycles with strain repeatability that wouldn't survive a potentiometer's wear.
- A SLB (Schlumberger) SHELLPRO downhole tool measures formation-pressure-induced casing strain through a Macro Sensors HSAR LVDT housed inside the tool: the sensor reads ±5 mm of casing motion at ±0.1 % accuracy through 175 °C and 20,000 psi.
A short history
An LVDT (Linear Variable Differential Transformer) is a position sensor that converts displacement into an electrical signal without contact between its moving core and coil assembly. A primary coil excited with a sine wave voltage induces voltages in two secondary coils; as the core moves, their difference changes with position while phase indicates direction. This contactless operation suits long life, reliability applications in harsh environments.