Magnetostrictive vs. Draw-Wire Displacement Sensors: Cylinder-Integrated or External Wire?
When engineers specify linear displacement measurement, magnetostrictive displacement sensors and draw-wire displacement sensors (cable-extension transducers) often come up together. Both measure linear displacement, but one is built around "cylinder-integrated, high accuracy, non-contact" and the other around "long stroke, external, low cost". This article compares the two schemes on five dimensions — principle, accuracy, life, installation and range — so that the choice can be settled in one pass.
Operating principle: one measures "time", the other measures "cable length"
A magnetostrictive displacement sensor contains a waveguide and a following position magnet. The electronics head sends a current pulse along the waveguide; a torsion wave is generated where the pulse and the magnet field superimpose; timing (distance = speed of sound × time) gives the magnet position. The magnet and the waveguide are non-contact; there is no mechanical wear.
A draw-wire displacement sensor winds a steel cable on a drum, with one end of the cable fixed to the measured object. As the object moves, the cable turns the drum, and an encoder or potentiometer converts the angle of rotation into linear displacement. It is mechanical contact measurement, transmitting motion through the cable, a spring and the drum.
The core differences: one table
| Item | Magnetostrictive displacement sensor | Draw-wire displacement sensor |
|---|---|---|
| Measuring method | Non-contact (position magnet + waveguide) | Contact (cable + drum) |
| Accuracy / resolution | Resolution down to 1 μm; high repeatability | Resolution typically of the order of 0.1–1 mm |
| Life | No wear, maintenance-free, long life | Cable, spring and potentiometer/encoder wear |
| Mounting position | Cylinder-integrated (16/17/19H), external rod (19P) | Must be external; the cable must be routed |
| Range | Cylinder-integrated 75–475 mm; external up to 7600 mm | Commonly 1–15 m, and can be longer |
| Environment | IP67/IP68, 25 g vibration, oil- and pressure-resistant | Limited by the cable and sealing; lower protection |
| Cost | Higher | Lower |
Cylinder-integrated applications: magnetostrictive is the only reasonable choice
Piston position inside a hydraulic cylinder can almost only be measured with a cylinder-integrated magnetostrictive displacement sensor — the rod inserts directly into the cylinder, pressure-rated 350–600 bar, with the position magnet moving with the piston. Germanjet Series 16 with a flat electronics housing, Series 17 hydraulic-cylinder integrated and 19H cylinder-integrated are all designed for this duty. A draw-wire sensor cannot enter a high-pressure cylinder; it can only pull a cable from outside, and neither accuracy nor life is an advantage.
Long-stroke external applications: both schemes work — it depends on the accuracy demand
On long-stroke external measurement — machine slides, lifting mechanisms, gate opening and similar — both schemes are feasible. If the requirement is μm-level accuracy, long-term stability and little maintenance, choose an external magnetostrictive type (for example 19P extra-long stroke, 19F flexible long stroke). If the accuracy demand is modest, the budget is limited and the mounting space is open, a draw-wire scheme is more economical. On mobile machinery, Series 13 (IP68/IP69K, 25 g vibration) is better suited to severe duty than an ordinary draw-wire unit.
Selection in one sentence
Inside a hydraulic cylinder, high accuracy, long life, severe environment → magnetostrictive displacement sensor; long-stroke external, cost-sensitive, modest accuracy → draw-wire displacement sensor. The draw-wire scheme is here for comparison only — Germanjet supplies magnetostrictive displacement sensors. For cylinder-integrated, high-accuracy or severe-duty measurement, contact us with stroke and output.
