Draw Wire, Potentiometric or Magnetostrictive: Choosing Position Feedback

Once you need to know where something is rather than whether it has arrived, you are buying a transducer instead of a switch, and the cost step is significant. It is worth being sure you need it, and then worth choosing the right one.

If two end of stroke positions would do, fit two proximity switches and stop reading here.

Potentiometric linear transducer

A wiper runs along a resistive track. The output is a voltage proportional to position, usually conditioned to 0 to 10 V or 4 to 20 mA.

Strengths: genuinely absolute, so position is known the instant power is applied with no homing move. Simple to interface. Excellent resolution. The lowest cost per millimetre of the three for short and medium strokes.

Limits: there is mechanical contact, so there is a wear life expressed in cycles. On a slow moving axis that is effectively unlimited; on a fast cycling press it is a maintenance interval.

Specify it for: strokes up to around a metre, moderate cycle rates, machine axes, valve position, gate position, hydraulic cylinder feedback where cycle counts are moderate.

Our Novotechnik linear transducers cover this class, with the selection logic in linear transducer selection.

Magnetostrictive

A waveguide runs the length of the stroke and a permanent magnet travels along it on the moving part. A current pulse down the waveguide interacts with the magnet’s field and generates a return pulse; the time of flight gives position.

Strengths: no contact between the magnet and the waveguide, so no wear mechanism and essentially unlimited cycle life. Absolute. Long strokes, several metres, without losing accuracy. The rod version can be installed inside a hydraulic cylinder through a gun drilled piston rod, giving sealed feedback with nothing exposed.

Limits: the most expensive of the three. The resolution depends on the timing electronics. It needs a magnet properly positioned on the moving element, which is a mechanical design task.

Specify it for: long strokes, high cycle counts, servo hydraulic control, press position control, anything where a wear life is unacceptable or the transducer must be inside the cylinder.

Draw wire

A stainless wire unwinds from a spring loaded drum; the drum drives a rotary sensor. The output is proportional to the extracted wire length.

Strengths: very long strokes, several metres to tens of metres, from a compact housing. The mechanical installation is the simplest of the three because only the wire end attaches to the moving part, so it tolerates misalignment and non linear paths. Low cost per metre of stroke.

Limits: the wire is exposed and can be snagged, cut or fouled. Accuracy is lower than the other two because it depends on consistent wire tension and drum winding. Fast reversals can let the wire go slack and lose a turn.

Specify it for: long travel where accuracy requirements are moderate. Crane hook height, platform and lift position, telescopic boom extension, gate and barrier position, test rigs where a transducer has to be rigged temporarily.

Where the rotary element is the sensor, our rotary sensors and OEM kits and multi turn rotary sensors are the relevant parts.

Comparison

	Potentiometric	Magnetostrictive	Draw wire
Typical stroke	Up to about 1 m	Up to several m	Several m and beyond
Contact wear	Yes, cycle limited	None	Wire and drum
Accuracy	High	Highest	Moderate
Relative cost	Lowest	Highest	Low per metre
Install effort	Needs alignment	Needs magnet design	Simplest
In cylinder	No	Yes	No

Output and interface

Whichever you choose, settle the output early. Analogue 0 to 10 V or 4 to 20 mA is the common case and the easiest to interface; prefer 4 to 20 mA over any distance or in electrical noise, because current loops reject noise and a broken wire reads as zero current rather than as zero position. See analogue outputs explained.

Where the controller expects a bus, CANopen position sensors are available and remove the analogue conversion entirely.

And if you were considering an LVDT

LVDTs are excellent at very short strokes with very high resolution, typically in test and metrology rather than machine control. For a machine axis they are usually the wrong tool. See LVDT alternatives in India.