Magnetostrictive vs. Encoder: How to Choose for Linear Position Detection

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

Choosing magnetostrictive or an encoder for linear position detection is not a matter of “which is more accurate”, but of two items: whether the measuring chain goes through a mechanical conversion and whether homing is required. A rotary encoder measures angle; to obtain linear position it must convert through a leadscrew, rack-and-pinion or sprocket. Backlash, pitch error, elastic deflection and slip are all added into the measurement result. Magnetostrictive measures the absolute position of the magnet ring directly along the motion axis; there is no mechanical conversion in the measuring chain. A linear scale also measures directly, but its tolerance of oil, dust and shock is limited. This article compares the two on four dimensions: measuring chain, position reference, environmental endurance and installation. The overall selection framework is in Magnetostrictive vs. LVDT vs. potentiometer.

Measuring-chain difference: direct measurement versus converted measurement

When a rotary encoder is used to measure linear position, what is actually measured is the angle of the motor shaft or leadscrew shaft; linear position is inferred from “angle × pitch / reduction ratio”. Error sources on that chain include: coupling torsional backlash, accumulated leadscrew pitch error, nut reversal backlash, rack meshing backlash, thermal expansion of transmission parts, and slip and elastic lag under hydraulic or belt drive. These errors are especially obvious at the instant of reversal, appearing as reversal dead band.

A magnetostrictive displacement sensor fixes the magnet ring on the moving part (piston, sliding carriage, cross-beam) and measures the distance of that point from the fixed end, independent of the drive method. A hydraulic cylinder, which has no usable rotary shaft, cannot take a rotary encoder directly and can only add a mechanical conversion such as a rack — which is why hydraulic position control commonly uses magnetostrictive sensing. Industry background is in Why choose magnetostrictive sensing for hydraulic cylinder position control.

Position reference: absolute output versus incremental counting

Magnetostrictive outputs absolute position: position is set by the physical location of the magnet ring. After power-down and power-up, the electronics head measures travel time once more and obtains the correct reading; homing is not required, nor is a reference-point switch. An incremental encoder records a pulse accumulation; the reference is lost on power-down, and homing must be executed at power-up. An absolute encoder can solve absolute position within one revolution, but multi-turn counting still depends on a battery or a mechanical gearbox, and a check is needed after a long shutdown. The full explanation of this mechanism difference is in Absolute versus incremental position: why magnetostrictive does not lose zero on power-down.

The practical engineering effect: when a multi-axis machine recovers from power-down, an absolute-position scheme can independently confirm the current position of each axis at once, without homing axis by axis in an interlocked sequence. That saves a complete motion beat and also avoids the risk of mechanical interference during homing.

Environmental endurance and protection

An encoder’s code disc (optical type) and bearings are precision parts that fear oil mist, dust, condensate and shock; a magnetic encoder has better dirt tolerance, but limited immunity to strong magnetic-field interference. A magnetostrictive sensing rod is a stainless-steel tube structure; the electronics head can be made to IP65 / IP67 / IP68 / IP69K in several grades. The Series 13 mobile hydraulic type is rated 25 g vibration, 100 g shock, suiting construction machinery, metallurgy and other applications where vibration and wash-down coexist. What protection classes actually mean in testing is in IP65/67/68/69K are not a numbers game.

Also note that magnetostrictive sensing itself depends on a magnetic field. If high-current busbars, electromagnets or permanent-magnet chucks are near the mounting position, interference and magnet-ring demagnetisation risk must be assessed; related content is in EMC electromagnetic compatibility: why a strong magnetic field interferes with measurement.

Comparison of the two schemes

Selection-criterion checklist

• Does the actuator have a usable rotary shaft: hydraulic cylinders, pneumatic cylinders and linear motors do not — a rotary-encoder scheme is ruled out directly;
• Does the transmission chain have backlash or elasticity: reversal dead band of rack, chain, belt or long-leadscrew drives cannot be removed by the encoder itself; the linear quantity must be measured directly;
• Can homing be accepted: the homing motion of large presses, gates and multi-axis synchronised equipment is costly and risky; an absolute-position scheme should be chosen;
• Site environment: where there is oil mist, water wash-down, dust or shock, prefer magnetostrictive; where it is clean and sub-micrometre is needed, consider a linear scale;
• Interface and refresh rate: when matching a servo cycle is required, confirm the update rate. Magnetostrictive update rate varies with measuring range and falls on a long range; matching methods are in How to match response time / refresh rate to the control cycle;
• Stroke and installation space: how to calculate stroke allowance, dead zones and installation size is in Selection calculation in practice: how to work out stroke, installation space and output.

On interface selection, if the original system already has an SSI channel, the Series 192 SSI digital interface can substitute an encoder SSI input; if it is an EtherCAT servo architecture, the Series 197 EtherCAT supports distributed-clock (DC) synchronisation, so that it can share one real-time bus with the servo axes.