Two Positions from One Waveguide: Multi-Magnet / Multi-Position Measurement

Disclaimer: Germanjet supplies only magnetostrictive linear displacement/position sensors; descriptions of other displacement-measurement technologies below are for technical comparison only.

A standard magnetostrictive displacement sensor uses one position magnet to read one position. By placing several permanent magnets (position magnets) on the same waveguide, a single sensor can measure several independent positions at once, cutting hardware cost and cabling where multi-position feedback is needed. This is one of magnetostriction's structural advantages over most single-point displacement transducers.

How multi-magnet measurement works

The current pulse from the electronics head travels along the waveguide; each time it meets a magnet's axial field, a torsional-wave echo is launched at the superposition. The electronics receives the echoes in time order; the flight time of each echo corresponds to the distance of that magnet from the fixed end. That is one excitation, several positions, with the magnets independent of one another.

How many magnets one rod can carry

Limited by the echo time window and resolving ability, common models support 2–4 independent magnets (the exact number depends on model and measuring range). The key is that magnets must keep a minimum spacing — if two magnets are too close, their echoes overlap in time and the electronics cannot separate them. In the Series 19 analogue (190 / 191) dual-magnet minimum spacing is 76 mm; closer than that, the two echoes cannot be separated. Do not lay out to the old 78 mm figure. For other series, take the minimum spacing from that series catalogue and confirm against the ordered model, leaving a margin for installation error.

Typical applications

• Dual-carriage / dual-fixture mechanisms: two moving parts on the same linear guide are positioned independently, sharing one sensor;
• Gantry beam, both sides in sync: the two carriages are read separately for synchronisation correction;
• Injection molding machines: screw position and platen position can be fed back together, simplifying the construction.

The common prerequisite is that all measured parts move along the same linear axis, and that the sensor rod can span the whole measuring interval. Mechanisms that do not meet this cannot use multi-magnet measurement.

Unsuitable: multi-stage (telescopic) cylinders

A common misunderstanding needs to be cleared up: multi-stage hydraulic cylinders (telescopic cylinders) cannot use magnetostrictive displacement sensors, whether single- or multi-magnet. The reasons are structural:

• The stages nest and extend; there is no through-cavity spanning the full stroke inside the cylinder, so there is nowhere to fit the rod;
• The pistons move in bores of different diameters and cannot share one waveguide, nor can magnets be fixed on every stage piston;
• Even if only the last stage is measured, the rod length cannot change as the cylinder telescopes.

Stroke feedback on multi-stage cylinders should use an externally mounted magnetostrictive displacement sensor, or measure the last stage alone. At the selection stage, first confirm whether the cylinder is single- or multi-stage, to avoid choosing the wrong scheme.

Single-magnet vs. multi-magnet

Configuration points

• Bus mapping: bus types map each magnet position separately through the object dictionary (for example CANopen multi-magnet reading);
• Independent zeros: the zero offset of each magnet must be set separately;
• Spacing check: lay out to the model's minimum spacing so that echoes do not overlap.

The underlying principle of multi-magnet measurement is in the magnetostrictive measuring principle; CANopen multi-magnet configuration is in CANopen multi-magnet reading configuration; in-cylinder layout is covered in in-cylinder drilling requirements. Overall output-type selection is in analog vs. fieldbus.