An LVDT measures linear position by turning displacement into an AC voltage — no sliding contacts, no wear, effectively infinite resolution. It's the sensor of choice when you need micron-level accuracy on something that will move billions of times.

The construction. Three coils wound on a hollow bore: one primary in the middle, two secondaries flanking it, wound in opposite phase (series-opposing). A ferromagnetic core — the "armature" — slides freely inside the bore, mechanically attached to whatever you want to measure. The core never touches the coils.

How it works. You excite the primary with an AC signal, typically 1–10 kHz at a few volts. The core couples flux from the primary into both secondaries. When the core is centered, the two secondary voltages are equal and opposite — they sum to zero. This is the null point. Move the core one way and one secondary sees more flux, the other sees less; the differential output grows linearly with displacement. Direction is encoded in the phase of the output relative to the excitation — 0° for one direction, 180° for the other.

The signal chain. Raw output is AC and needs a synchronous demodulator to extract a DC voltage proportional to signed displacement. Most modern LVDTs ship as "DC-in, DC-out" units with the oscillator and demodulator built into the housing — you feed 24 V and get back ±10 V representing position.

Why engineers reach for them.

• Frictionless. No brushes, wipers, or bearings on the sensing element — the core floats. Life is limited by whatever guides the core externally, not the sensor itself.
• Infinite resolution. Output is analog and continuous. Real resolution is set by the signal conditioning electronics — typically sub-micron.
• Survives hell. Sealed stainless housings tolerate submersion, radiation (nuclear plants), and 200°C+ ambient. No semiconductors near the sensing element.
• Absolute. Unlike an incremental encoder, it always knows where the core is at power-on.

Real-world example. Hydraulic press ram positioning. The press cycles millions of times, sees oil mist and heat, and needs to know ram position to 0.001". An LVDT with its core threaded into the ram, coils bolted to the frame, and no rubbing contact will outlive the press. Compare to a linear pot, which would wear a dead spot within a year.

Rule of thumb. LVDT linear range is roughly ±25% of the total coil length. A 4-inch-long LVDT gives you about ±1 inch of usable stroke. Beyond that, linearity degrades sharply. Always spec a device with 20–30% stroke margin so you never operate near the ends of the linear region.

Watch out for. Nearby ferromagnetic material or strong DC magnetic fields will distort the flux paths and shift the null point. Mount LVDTs on non-magnetic brackets (aluminum, 300-series stainless) and keep them clear of solenoids and motors.