Linear Position Transducers: Potentiometric vs LVDT vs Magnetostrictive

Choose a potentiometric transducer when you want absolute position, a simple ratiometric output and low cost. Choose LVDT when you need a sealed, contactless element over a short stroke in a harsh or high-vibration environment. Choose magnetostrictive when the stroke is long, the duty is continuous, and wear is unacceptable — typically inside hydraulic cylinders.

How each technology works

Potentiometric

A wiper travels along a resistive track — in a quality industrial transducer, conductive plastic rather than wirewound. The output is a voltage divider: apply an excitation voltage across the track and read the wiper. Because the output is a fixed ratio of the excitation, drift in the supply cancels out, which is why potentiometric sensors are described as ratiometric and why they tolerate modest power supplies so well.

The wiper is a physical contact, so there is a wear mechanism. Modern conductive-plastic elements are rated for very large numbers of movements, and in most industrial duties the transducer outlives the machine. High-frequency dithering in one small part of the track is the case where wear genuinely bites.

LVDT

A linear variable differential transformer has one primary and two secondary windings, with a ferromagnetic core free to move inside. The primary is excited with AC; the two secondaries are wired in opposition. With the core centred their outputs cancel. As it moves, one secondary dominates, and the amplitude and phase of the difference give position.

The core does not touch the windings, so there is nothing to wear. The winding assembly can be fully sealed and even hermetically welded, and the whole device is essentially insensitive to shock and vibration. Resolution is limited only by the electronics, which is why LVDTs remain standard in test rigs and aerospace.

Magnetostrictive

A current pulse is launched down a magnetostrictive waveguide. A permanent magnet, carried on the moving element, sits somewhere along that waveguide. Where the pulse’s field meets the magnet’s field, a torsional strain wave is generated and travels back to a pickup at the head. Measure the time of flight, multiply by the known wave velocity, and you have absolute position.

Nothing touches. The magnet can be separated from the waveguide by a non-magnetic barrier, which is why these sensors can be installed inside a hydraulic cylinder through a gun-drilled rod, with the rod itself acting as the pressure boundary.

Direct comparison

	Potentiometric	LVDT	Magnetostrictive
Contact	Wiper on track	None	None
Typical stroke	10 mm – 1 m+	1 mm – 500 mm	50 mm – several metres
Absolute output	Yes	Yes	Yes
Output type	Ratiometric voltage; conditioned 0–10 V / 4–20 mA versions available	AC differential, or DC with built-in conditioning	Analogue, SSI, CANopen, IO-Link and others
Wear mechanism	Wiper and track	Bearings only, if fitted	None in the sensing path
Vibration tolerance	Good	Excellent	Very good
Suits hydraulic cylinder installation	Rarely	Occasionally	Yes — the standard solution
Relative cost	Low	Medium to high	High
Electronics required	Minimal	Oscillator and demodulator	Integrated, always

Choosing by application

Application	Usual choice	Reason
Injection moulding — screw and clamp position	Potentiometric or magnetostrictive	Long stroke, continuous cycling; magnetostrictive where duty is extreme
Hydraulic cylinder feedback	Magnetostrictive	Mounts inside the cylinder; no seal to fail on a moving rod
Valve and actuator position	Potentiometric	Short stroke, low duty, cost matters
Materials test rig	LVDT	Resolution and shock tolerance outweigh cost
Aerospace control surface	LVDT	Sealed, contactless, proven in the sector
Packaging machine format adjustment	Potentiometric	Infrequent movement, absolute readout on power-up
Steel mill roll gap	Magnetostrictive or LVDT	Environment and duty rule out contact devices
Machine tool axis	Neither — use a linear encoder	Micron-level accuracy over long travel

Rotary equivalents

The same logic applies to angular measurement. A rotary single-turn sensor covers up to 360° and suits throttle, damper and valve positions. Where the shaft turns through many revolutions — a screw jack, a cable drum, a steering column — a rotary multi-turn sensor keeps an absolute count across the whole travel, so position is known immediately on power-up with no homing move.

Contactless rotary sensors using Hall-effect or inductive principles have largely displaced potentiometric rotary units in high-cycle duties, for exactly the wear reasons described above.

Specification checklist

1. Stroke and mounting length. The installed length always exceeds the electrical stroke. Confirm the mechanical envelope before selecting.
2. Linearity you actually need. Independent linearity is usually quoted as a percentage of full stroke; over a long stroke a good percentage is still a large absolute error. Ask for the number in millimetres.
3. Duty cycle. Movements per day, and whether the motion concentrates in one part of the stroke. This decides whether a contact device is acceptable.
4. Output interface. Match the controller: ratiometric voltage, 0–10 V, 4–20 mA, SSI, CANopen or IO-Link. Retrofits usually dictate this.
5. Environment. Temperature, vibration, ingress and any chemical exposure — see our guide to IP ratings and what each test proves.
6. Coupling. A rigid coupling between machine and transducer transmits misalignment straight into the element and shortens its life. Use the specified ball joint or flexible coupling.

Accent Controls is the sole authorised distributor in India and SAARC for Novotechnik of Germany and Contelec of Switzerland, alongside the proximity and photoelectric sensors we manufacture in Mumbai. That means we can specify contact and contactless position measurement from the same conversation — see our overview of position sensor solutions.

Frequently asked questions

What is the difference between a linear transducer and a linear encoder?

A transducer of the types described here gives an absolute analogue or digital position over its stroke, typically at millimetre to sub-millimetre resolution, and needs no homing on power-up. A linear encoder — usually optical or magnetic scale — targets much finer resolution over machine-tool travels and is often incremental, requiring a reference move after power-up. They solve different problems at different price points.

Do potentiometric transducers wear out?

The wiper and track are a contact pair, so yes in principle. In practice a good conductive-plastic element is rated for a very large number of movements and outlasts the machine in most industrial duties. The exception is high-frequency dithering concentrated in one small region of the stroke, which wears a localised flat and shows up as noise at that position — a contactless technology is the right answer there.

Can a magnetostrictive sensor be fitted inside a hydraulic cylinder?

Yes, and it is the standard solution for cylinder position feedback. The waveguide is installed in a gun-drilled bore in the piston rod, with the position magnet on the piston. Nothing penetrates the pressure boundary and there is no dynamic seal on a moving sensor element, which is exactly why the arrangement is so reliable.

Which is most accurate?

LVDTs generally offer the best resolution and repeatability over short strokes, because resolution is limited by the electronics rather than by any physical division. Over long strokes magnetostrictive devices are usually the most accurate in absolute terms. But for most industrial applications repeatability and long-term stability matter more than headline accuracy — specify what the machine needs rather than the best number available.

Do I need a linear transducer or would proximity switches do?

If you only need to know that the axis has reached a small number of fixed positions, proximity switches are simpler, cheaper and more robust — see our inductive proximity switch range. A transducer is warranted when you need continuous position, when the target positions change with the product format, or when the controller closes a loop on position.