Magnetostrictive linear position sensor (rod and profile types)

01

Size the sensing rod from stroke, null and dead zone

Design World's guide to magnetostrictive sensors explains that the sensor measures the distance between a position magnet and the head end of the sensing rod, and that strokes run from as short as 25 mm to as long as 10 m. The rod is longer than the stroke: a minimum distance called the null must be kept between the head end and the magnet, and an unusable dead zone at the rod tip. In its example the null is 12 mm and the dead zone 82 mm, so a two-metre travel needs a rod of L = 0.012 m + 2 m + 0.082 m = 2.094 m. [1]

The same guide describes two cylinder-mounting options, standard and two-piece, both with rods and flanges that seal against cylinder pressure, and a profile housing in which the sensing rod is enclosed in an aluminium extrusion bolted to the machine frame. In the profile version the magnet can be a bar or floating magnet passing near the top of the extrusion, or captive in a shuttle riding on a rail in the extrusion; a clevis-mounted rod and cylinder version moves the magnet with a metal rod. [1]

The magnet can be read through a barrier of non-magnetic material, such as a cylinder wall, and some sensor models accept up to 30 position magnets on one rod, so one sensor can read more than one carriage on the same axis; the guide's example is a paper slitter with one magnet per knife. [1]

Editorial note: null and dead zone lengths depend on the model and mounting; ask the supplier for both figures for the housing you order before fixing the axis layout.

Send the travel to be measured, available length for the rod or profile, cylinder or frame mounting, number of magnets, magnet type (ring, floating, captive), output interface and update rate, supply voltage, temperature, shock and vibration, and enclosure needs.

02

Understand the waveguide timing behind the reading

A Chinese patent on a high-measuring-frequency magnetostrictive sensor, CN 104457809 A, describes the Wiedemann effect: an interrogation pulse sent along the waveguide wire meets the field of the magnet ring and forms a torsional stress wave, whose return is timed; multiplying the time difference by the wave's propagation speed in the waveguide material, about 2,830 m/s, gives the absolute distance from the reference point. [2] Editorial calculation: at 2,830 m/s the wave covers 1 m in about 0.35 ms.

The same patent states that because the returning torsional wave is slow compared with the excitation pulse, the measuring frequency of a single measuring unit is limited; it proposes n ≥ 2 measuring units taking turns so that the highest measuring frequency becomes k times that of the original design, and singles out sensors with ranges over 1 m as where the gain is largest. [2]

SICK Atech GmbH's patent US 11,713,981 B2, for an evaluation unit, describes the magnetostrictive element as a thin rod, wire or tube of a ferromagnetic material such as iron, nickel or cobalt, from a few centimetres to metres long, with a ring-shaped permanent position magnet around it. It uses a reference signal time of flight so that length changes or speed-of-sound changes in the element caused by temperature can be compensated. [3]

Design World lists 24 Vdc as the standard supply for industrial sensors, with 15 Vdc on older systems and a 9–28.8 Vdc extended option; analog outputs of 0–20 mA, 4–20 mA or −10 to 10 V; and SSI, CANbus, DeviceNet, Profibus, EtherCAT or PowerLink digital interfaces, with SSI and EtherCAT cited for a 10 kHz update rate. [1]

03

Compare housing types

The housings below come from the sources reviewed; each fixes how the magnet and rod are mounted.

04

Waveguide, magnet and electronics head

The sensor is built around a ferromagnetic waveguide; the parts below are those the sources describe.

05

Quote the sensor on stroke, housing and interface

The order fields map onto the stroke, null, dead-zone and interface data above; values are cited where a source gives them.

06

Send the stroke, mounting and interface together

Paste it into your enquiry below or into an email to a maker, then replace every bracket.

Stroke: [travel; space for null and dead zone]
Mounting: [cylinder rod; profile; clevis]
Magnets: [type; number]
Output: [analog range or bus; update rate]
Supply: [voltage]
Environment: [temperature; shock; vibration; enclosure]
Cable: [connector; length]

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