Valve position, actuator stroke, wellhead component travel — a lot of surface equipment needs to know exactly how far something has moved, reliably, for years, without the sensor itself wearing out. The Linear Variable Differential Transformer (LVDT) is the usual answer, and it earns that position by having no sliding electrical contact at all.
Construction: three coils and a moving core
An LVDT is built around a hollow, non-magnetic tube wound with three coils: a central primary coil, and two secondary coils wound symmetrically on either side of it. A free-moving core made of ferromagnetic material sits inside the tube, attached by a non-magnetic rod to whatever is being measured — a valve stem, an actuator shaft, a piston. Critically, the core doesn’t touch the coils. There’s no wiper, no sliding contact, and therefore nothing inside the sensing element to wear out.
How the position becomes a signal
The primary coil is driven with an AC excitation signal, which induces a voltage in each secondary coil by transformer action — exactly like a small transformer with a moveable core instead of a fixed one. When the core sits exactly in the centre, it couples equally to both secondary coils, and their voltages cancel each other out: the net output is zero. This centre point is the LVDT’s “null position.”
As the core moves off-centre toward one secondary coil, it couples more strongly to that coil and more weakly to the other. The two voltages no longer cancel, and a net output appears whose amplitude is proportional to how far the core has moved, and whose phase (matching or opposing the excitation signal) indicates which direction it moved in. A phase-sensitive demodulator circuit reads both amplitude and phase together and outputs a single signed voltage or current proportional to displacement — positive one way, negative the other, zero at centre.
Why LVDTs hold up in harsh environments
Because the core never contacts the coils, there’s no friction and no contact wear — the sensing element itself can outlast almost everything else in the assembly. The core and housing can be sealed independently, so the electrical windings can be fully isolated from the process environment while the core still moves freely, which is why LVDTs turn up in position feedback for valve actuators, wellhead component travel, and other duty where a potentiometer’s wiper would fail long before the equipment around it does.
What to specify
- Stroke length — matched to the actual travel of the application, not oversized “to be safe,” which costs resolution.
- Linearity — how closely the output tracks true position across the full stroke.
- Housing and sealing — ingress protection and materials suited to the process and area classification.
- Output type — raw AC, or a version with built-in signal conditioning giving DC voltage or 4–20 mA directly.
See how this fits into what we supply on the What We Supply page, or get in touch with the application and we’ll tell you honestly what fits.