How Does an LVDT Work? Working Principle, Construction and Applications

What Does LVDT Stand For?

LVDT stands for Linear Variable Differential Transformer. It is an electromechanical sensor that converts straight-line displacement into an electrical signal proportional to that displacement. Because nothing inside the sensor physically rubs or wears, an LVDT is one of the few position sensors that can hold its calibration for years in an environment full of oil, coolant, vibration and temperature cycling.

If you only need the definition, that's it. If you need to know why it behaves the way it does — and where it stops being the right choice — read on.

LVDT Construction: Four Parts, No Contact

An LVDT has four essential parts:

1. A primary winding in the centre of the bore, driven by an AC excitation voltage (typically 1–10 kHz, 1–7 Vrms).
2. Two secondary windings, one on each side of the primary, wound symmetrically and connected in series-opposing.
3. A movable core of magnetically permeable material (usually a nickel-iron alloy), attached to whatever you are measuring.
4. A non-magnetic housing — stainless steel in industrial versions — that shields the windings.

There is no electrical contact between the core and the coils. The core floats inside the bore, separated by a small air gap. That single design decision explains almost every advantage the LVDT has.

LVDT Working Principle, Step by Step

1. AC excitation is applied to the primary. It generates an alternating magnetic field that couples into both secondaries.
2. When the core sits exactly at the null position (centre), both secondaries receive the same induced voltage. Because they are wired in series-opposing, the outputs cancel. Net output = 0 V.
3. Move the core toward the upper secondary and that coil couples more flux, producing a larger voltage. The lower coil sees less. The difference appears as a net AC output that grows with displacement.
4. Move the core the other way and the phase of the output inverts by 180°. The amplitude still tells you how far you moved; the phase tells you which direction.
5. A demodulator (signal conditioner) converts that amplitude-and-phase pair into a clean DC voltage or a 4–20 mA current that a PLC can read.

Two properties fall directly out of this geometry:

• Infinite resolution. Output is a continuous analogue function of core position, not a count of steps. Resolution is limited by your electronics, not by the sensor.
• Direction sensing through phase, not through a second channel. One sensor gives you both magnitude and sign.

AC LVDT vs DC LVDT: Where the Electronics Live

Our Macro Sensors CD375 is a classic AC unit: ±0.025 to ±1 inch range, ±0.25 % FS linearity, IP61, output in mV/V. At the other end, the DC750-SE750 is a DC LVDT covering 0.05–20 inch with a 0–10 VDC output and ±0.25 % FS linearity, IP62 — you bolt it on and wire it to an analogue input.

The Specs That Actually Decide Your Selection

Linearity (% FS). This is the accuracy number that matters. ±0.25 % FS on a 50 mm sensor means roughly ±0.125 mm worst-case deviation from a best-fit straight line. Every LVDT in our range holds ±0.25 % FS or better; the DLBB digital probe improves to ±0.1 % of reading.

Range and stroke. Measuring a short travel inside a fixture? The PTL-TSD covers 0–40 mm, the PTL-ARTSD 0–50 mm. Need a longer stroke with the same body style? The PTL-TSD-HR spans ±1.27 to ±254 mm, with an AC mV/V output for high-temperature zones.

Environment. An LVDT's weak point is never the coil — it's the cable entry and the connector. The PTL-ARTSD is rated IP68 with a 0–10 V / 4–20 mA output, which is why it turns up on hydraulic presses and outdoor gates rather than only on the test bench.

Output compatibility. This is where most projects lose a week. If your PLC card takes 4–20 mA, do not buy a 0–5 V unit and add a converter later. The PTL-TSK-T080 speaks Modbus RS-485 directly at 0–15 mm range — no analogue card, no scaling drift, and every sensor on the bus is individually addressable.

Where LVDTs Win — and Where They Don't

Choose an LVDT when:

• the measurement runs 24/7 for years and you cannot afford wear;
• the environment has oil mist, coolant, steam or washdown;
• you need sub-micron repeatability on a short stroke;
• you want a sensor with no mechanical life limit.

Look elsewhere when:

• the stroke is longer than about 300 mm — a draw wire sensor is cheaper and far smaller (see our draw wire selection guide);
• the target rotates continuously — you need a rotary sensor, not a linear one;
• you need absolute digital position over a long axis inside a hydraulic cylinder — magnetostrictive is the better fit (see hydraulic cylinder position sensors);
• unit cost dominates and the duty cycle is light — a string potentiometer will do the job for a fraction of the price (see LVDT vs potentiometer).

Mounting and Wiring Mistakes We See Repeatedly

1. Over-clamping the core rod. The core must float. If you bolt it rigidly to a target that also moves sideways, you side-load the bore and wear it out.
2. Running the excitation cable beside a VFD output. Induced noise lands directly on the mV-level signal. Use shielded twisted pair, ground the shield at the conditioner end only.
3. Ignoring the dead band at each end of the stroke. Usable stroke is shorter than the physical bore. Size the sensor so your travel stays inside the linear region.
4. Assuming any conditioner works. AC LVDTs need a carrier frequency matched to the winding; a mismatched conditioner produces a temperature-drifting, noisy signal.

Frequently Asked Questions

What is the full form of LVDT? Linear Variable Differential Transformer.

How accurate is an LVDT? Industrial units are typically ±0.25 % of full stroke. On a 50 mm sensor that is about ±0.125 mm. Precision laboratory versions reach ±0.05 %, and digital probes such as the DLBB quote ±0.1 % of reading.

Does an LVDT measure direction? Yes. The output phase shifts 180° on either side of the null, so a phase-sensitive demodulator resolves both magnitude and direction from a single winding set.

Can an LVDT work underwater? Yes, if the housing and cable entry are sealed to IP68. The PTL-ARTSD is rated IP68, which covers continuous immersion under defined conditions — but check the cable gland, not just the sensor body.

Why is my LVDT reading drifting? Usually thermal. Either the conditioner is not temperature-compensated, or the sensor is mounted such that the housing expands with the machine. Re-zero at operating temperature, not at start-up.

Need Help Specifying One?

Our range covers 149 position sensor models — AC and DC LVDT from ±0.025 inch to ±254 mm, plus draw wire, rotary, tilt and linear velocity. Tell us your stroke, output, environment and mounting constraints, and we will point you to the right family.

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