Choosing magnets for sensors

Although sensing is a rapidly growing field that taps into every kind of technology, many sensors depend on the flux of an adjacent permanent magnet to do their job, says Andrew Myers of Magnequench.

Mention sensors and magnets to an engineer, and it's likely that AlNiCo will be the first type that comes to mind.

Widespread use in familiar applications such as ABS reluctance sensors and the reed switch type for detecting position have undoubtedly given the material a high profile, reinforced more recently by the crash sensors based on the inertia principle that perform vital safety functions in cars.

Generically speaking, AlNiCo magnets are often favoured for measuring analogue signals, or when slight differences in switching thresholds are necessary.

In addition, the material's low, reversible temperature coefficient (which means that there is very little loss of strength when a heated magnet cools down) makes AlNiCo suitable where signal processing does not require additional compensation.

The temperature coefficient of intrinsic coercivity (HcJ) is also very low, ranging from +0.03 to -0.07%/K depending on alloy composition and heat treatment.

As a rule this temperature dependence can be neglected, as it does not cause any change in flux density in the magnet's operating range.

Equally well established - especially for semiconductor sensors - are plastic-bonded hard ferrite magnets.

They are a frequent choice for sensing devices in motor vehicles, electrical equipment, and many other contexts where contact-free measurement is required - usually with a multi-pole configuration so that switching functions can be handled by a Hall sensor or IC.

The fact that magnetisation is usually part of the manufacturing process gives this type clear advantages in terms of cost and reproducibility.

And when high-temperature thermoplastics are used in the formulation the range of effective performance extends from -40 to +200C (and that upper limit is essential for automotive applications such as ABS and transmission systems).

From the mechanical point of view, plastic-bonded hard ferrites offer even more advantages over sintered magnets.

Insert technology makes it possible to integrate shafts, bushings and many other elements to perform secondary mechanical functions; interference fits and centred fits are possible.

Sintered hard ferrite is capable of the same flexibility with respect to temperature - and it remains fully functional in demanding technical contexts where heat and corrosion would cause other types to lose magnetic strength irreparably.

Injection moulding is often the assembly method chosen, in which case tolerances, antirotation elements and possible material stresses have to be taken into account early on in the design process.

If the magnet manufacturer is involved at this point, it can manipulate the material's characteristics to improve the sensitivity and accuracy of the sensor itself; and both parameters can be further refined by the choice of magnetisation method.

Alongside its extremely high magnetic properties and excellent corrosion resistance, samarium cobalt offers the greatest stability over a range of temperatures, which is important for many sensors.

Alloys in this family have energy products ranging from 10 to 32MGOe (79-253kJ/m3).

In general these alloys have grade-dependent peak operating temperatures ranging from 200 to 350C.

In fact its reversible temperature coefficient of induction (-0.030%/K for Sm2Co17) is less than half that of NdFeB; and although AlNiCo's is lower, it has a significantly lower magnetic output, as explained above.

Samarium is also the densest of the available materials, and therefore especially appropriate in circumstances where a high magnetic field is required.

Typical applications include monitoring the integrity of pressure vessels used in high-temperature manufacturing processes.

Still a relative newcomer is plastic-bonded neodymium-iron-boron, which is notable for its versatility and high magnetic quality, and whose potential for new sensor applications is still largely untapped.

The maximum energy product ranges from approximately 32 to 86kJ/m3.

It can be produced in a wide variety of shapes; it allows a wide choice of magnetisation techniques largely independent of manufacturing method and magnet geometry; and its isotropic nature and good machinability create favourable conditions for function testing at the design stage.

For applications where elevated temperatures are likely to be encountered, a compression-moulded grade is now available for continuous operation up to 150C, with brief peaks as high as 50 degrees above that.

As with all the materials described here, involving the magnet supplier at the "drawing board" stage always pays dividends.

As well as saving money by rationalising material volumes, it acts as a double-check that the choice of material is the best for the job; and it may even improve the design by alerting the designer to other functions that the magnet can handle.

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