Installing Magnetostrictive Displacement Sensors in Practice: Position Magnet, Zero Point, Noise Immunity

A magnetostrictive displacement sensor is itself maintenance-free, but how well it is installed decides whether it stays that way. Of ten abnormal readings on site, seven or eight come from mounting and cabling, not from a failed sensor. This article covers the practical points that are most often overlooked.

Position-magnet mounting: concentricity and clearance are critical

The position magnet (permanent magnet) rides on the outside of the measuring rod and senses position by magnetic field. It and the rod must be coaxial, with uniform clearance. If the magnet is mounted off-centre on the piston rod, or rubs the measuring rod, the signal will fluctuate; in severe cases the echo is lost altogether.

• There must be a sensible clearance between the magnet inner diameter and the rod outer diameter — neither so loose that it wobbles radially, nor so tight that it wears the rod;
• The magnet must be firmly fixed; on machines with high vibration, add an anti-loosening arrangement so the magnet cannot shift axially;
• The magnet must not travel beyond the end of the rod's effective measuring range, or the portion beyond cannot be read.

Zero calibration: establish the datum before you trust the reading

After installation, the first task is to calibrate zero. Most controllers support "set current position as zero" or an offset compensation. Recommended practice:

• Set zero at the machine mechanical zero (for example with the cylinder fully retracted);
• Cross-check the sensor reading against a mechanical scale and confirm the full-scale correspondence;
• Record the zero offset so that a later change of type or reinstallation has a documented baseline.

Cabling and screening: seven-tenths of interference starts here

Signal cables dislike running face-to-face with power cables. Rules of thumb:

• Run signal and power cables in separate trunking, at least 20 cm apart; where they must cross, cross at right angles;
• Screen earthing depends on cable type: single-end earth analog signal cables (usually at the control-cabinet side) to avoid earth loops from potential difference; fieldbus cables (CANopen / Profibus / EtherCAT) should have 360° bonding at both ends when the two ends are reliably at the same potential, so that high-frequency interference has a path to dissipate;
• Use twisted pair for bus cable, and tighten connectors — a loose contact invites more interference than a long run.

Special points for in-cylinder mounting

For sensors built into a cylinder, such as the Series 17 hydraulic-cylinder integrated type and the Series 16 hydraulic profile type, also note:

• Confirm that the cylinder bore diameter matches the sensor outer diameter; the bore must match the series: Series 16 cap Ø18G7 / rod Ø10 / piston bore ≥12.7 mm; Series 17 cap M18×1.5 / through-hole Ø13 mm / piston bore ≥12.7 mm;
• Oil cleanliness must be adequate; debris wearing the rod shortens life;
• The pressure rating (350/530 bar (Series 16), 350/600 bar (Series 17), 300/600 bar (Series 19 in-cylinder)) must cover system peak pressure; do not size on working pressure alone.