Neodymium-Iron-
Boron Magnets:
Flexibility in Sensor Design
“Neo” magnets are well worth considering for new or existing sensor applications where manufacturing and materials cost is an issue and high field strength is desirable.
David Miller and
Peter Campbell,
Magnequench Technology Center
The conventional view of permanent magnet materials commonly used in sensors is that if you want low cost you should use ceramic ferrite, and if you want good temperature stability you should use samarium-cobalt. One important but often overlooked factor is that sensor magnets are generally small, so their cost is dominated by processing considerations rather than by the price of materials. So, for example, a bonded magnet might just as well use neodymium-iron-boron (Neo) as a higher energy alternative to ferrite. And if you want a fully dense magnet, Neo can be manufactured less expensively than can samarium-cobalt. Neo actually offers a nicely balanced combination of attributes for sensor design, featuring a much higher magnetic energy and better temperature stability than ferrite magnets, and it is inherently less expensive than samarium-cobalt. In short, Neo is well worth considering for any new or existing sensor application.
Manufacturing Costs
Figure 1. Neodymium-iron-boron magnets, whether bonded or fully dense, can be formed directly to their final shape by means of simple processes that do not require machining or grinding. This “net shape” processing delivers the tight tolerances that many applications demand but without the expense of secondary operations. Some common magnet shapes that can be directly molded or pressed are shown.
Ferrite is the magnet material most frequently chosen for sensor applications, simply because the raw material is by far the least expensive. Indeed, there are few magnetic sensors where cost is not an issue, but raw material prices are quite often not significantly reflected to the bottom line. Rather, it is the manufacturing processes required to produce and integrate a magnet into a sensor that dominate the overall cost and are more likely to determine the material that will be used.
Neo is produced by a wider variety of methods than are used for ferrite or samarium-cobalt magnets. Injection molding produces magnets with some of the most diluted properties, while sintering or hot pressing yields the strongest fully dense magnets. Today’s commercial compositions are all based on the Nd2Fe14B alloy and are fairly mature in their development. Consequently, most of the innovative applications of Neo into sensors and other products rely on the processing route, which has generated a whole range of material properties between the two extremes described below (see the sidebar, “It’s All in the Processing”).
Injection Molded. Injection molding the magnet directly into an assembly reduces the number of process steps and the need for adhesives or fasteners, so a switch from ceramic ferrite to bonded Neo may actually reduce the overall expense of the magnetic subassembly. Furthermore, an injection-molded Neo magnet will be stronger than a ceramic ferrite magnet of similar shape. This property provides the designer with a magnetic advantage and brings more options to the table. At the very least, it is possible to make a simple material substitution without redesign of the entire magnetic circuit. Although the operation of a magnetic circuit designed for ceramic ferrite may be altered after changing to injection-molded Neo, the circuit’s performance should now exceed its former specification. Preferably, though, the designer will choose to exploit the extra strength of the Neo material, either to enhance the sensor’s performance or to shrink the magnet and hence the overall package size. If absolutely necessary, it is also possible to reduce the proportion of magnetic powder in some of the new injection-molded materials so that the magnetic characteristics closely match those of the magnet being replaced.
Injection-molded Neo (MQIM) is an isotropic material having no preferred direction of magnetization. This property gives great flexibility both in the shape of magnet that can be molded and in its magnetic orientation. MQIM’s magnetic properties are superior to those of anisotropic ceramic ferrite, whose magnetic orientation complicates the production process and limits its practical shapes. And MQIM has about half again better temperature stability than ceramic ferrite. Compression-molded Neo (MQ1) is also isotropic and offers benefits similar to those of MQIM, although the use of an epoxy binder gives it greater densification and hence higher magnetic properties. Both forms of bonded Neo allow parts to be molded directly to their final shape and tolerance, eliminating the need for finish grinding. Hot-pressed Neo (MQ2) takes the advantages of the isotropic magnet to their limit, using no binder but relying instead on the hot-flow properties of the Neo alloy itself to form a fully dense magnet. The ultimate magnetic properties in any permanent magnet are achieved when it is oriented in a preferred direction during the production process. In sintered Neo this is done by compaction and heat treatment in a magnetic field, but finish grinding is usually required and complex shapes are hard to achieve. The alignment in hot-pressed Neo is achieved by deforming an MQ2 part into MQ3 (without the need for an applied field), thus molding the magnet directly to its final shape and tolerance. MQIM, MQ1, MQ2, and MQ3 are trademarks of Magnequench Technology Center.
It’s All in the Processing
When selecting magnetic materials, a designer will look first at the demagnetization characteristics (see Figure 2), perhaps next at the material’s reversible temperature coefficient, and finally investigate the processing method that can best suit the application.
Figure 2. Demagnetization characteristics for the common types of permanent magnets are shown. Neo materials include injection-molded MQIM, compression-molded MQ1, hot-pressed isotropic MQ2, and hot-deformed anisotropic MQ3.
Hot Pressed. When an application requires a very durable magnet, “net-shape” hot pressing directly to its final shape can reduce manufacturing cost. Many sensors require a high magnetic field, which must be provided by a fully dense rare-earth magnet. If a sintered samarium-cobalt or Neo material is selected, manufacturing will include a process step for grinding the magnets to their final desired shape and tolerance. Alternatively, Neo can be hot pressed directly to an “application ready” state, without any extra shaping operations. This proc ess, now commonly used for Neo, can produce a magnet with a more complex shape but less expensively than the sintering and grinding techniques required for other fully dense materials. In applications where sintered samarium-cobalt is being used, and where there is also some leeway in the application’s temperature stability, hot-pressed Neo may reduce both the processing and the material cost.
As is the case with injection-molded Neo, the benefits from pressing the magnet to its final form become greater as the magnet’s shape becomes more complex (see Figure 1, page 94). This is particularly true of isotropic Neo, in which no magnetic orientation is predetermined during pressing. Relatively complex air gap field distributions can be provided by means of the magnet’s mold.
Material Costs
As previously noted, materials costs can be reduced by switching from samarium-cobalt to Neo, as long as the application’s temperature stability requirement can be satisfied. Furthermore, Neo’s higher magnetic field strength can be used to provide a sensor with greater immunity to temperature variations. There are certainly some sensors in which creative magnetic circuit design can solve temperature issues by reducing the requirement for linearity in the magnet material and hence its cost.
TABLE 1
Sensor element
Hall effect
Magnetoresistor
Giant magnetoresistor
Applied field
250 gauss
50 gauss
50 gauss
Field direction
normal
transverse
transverse
5 V signal
50 mV
40 mV
220 mV
There are a few applications where linearity over a wide temperature range is of the utmost importance, but it is still possible to achieve a cost-effective materials solution. It is now commonplace to use compensation circuitry to correct for temperature variations caused by the magnet material. The circuitry may be designed into an ASIC or come packaged with the flux sensor chip, and these circuits can be programmed to offset the temperature change in any permanent magnet material that may be selected. So while it is likely that an application may use samarium-cobalt together with compensation to optimize thermal stability, this circuitry can be used with Neo as well to achieve the same linearity while offering a materials cost reduction. When a new sensor application requiring a high degree of linearity is still in the design stage, the designer should consider that compensation circuitry may be needed to provide the desired linearity, so whether the magnet material itself has good temperature stability may not be too important. For applications that require better linear stability than the least expensive magnet materials can provide, but where cost considerations are paramount, Neo may offer respectable sensor linearity at a reasonable material cost.
Increased Magnetic Field
Figure 3. Three common angular position sensors use magnets in different ways (magnets are shown in black and elements in green). An “interrupter” vane (right) periodically diverts flux from the sensor element as it rotates. A toothed gear wheel (top left) implements a variable air gap to modulate sensor flux. And a multipole ring (lower left) has alternating magnetic poles to produce changing flux as it rotates.
As a rule, Neo can be used wherever high field is an advantage. Since Neo is inherently the highest energy permanent magnet material on the commercial market, it will provide the strongest air gap field whatever the processing technique used to make it. Noncontact magnetic sensors often achieve improved reliability by sacrificing magnetic circuit efficiency in favor of the low cost of a simple structure, putting an even greater burden on the magnet.
The three most common magnetic angular position sensing schemes are shown in Figure 3. For the lowest resolution, the “interrupter” vane periodically diverts the magnet’s field from the sensing element. For medium resolution, a toothed wheel causes a periodic pulsation in the magnet’s field through the element. The highest resolution is achieved with an isotropic magnet molded around a wheel whose periphery is magnetized with a multipole array.
Rotating machinery applications regularly take advantage of the higher fields offered by Neo, with increased air gaps that can in turn permit greater clearance between a stator and a rotor. The larger air gap may lead to relaxed machining tolerances, or thicker overmolding on subassemblies may be an option. The advantages apply to magnetic commutation rings and encoder rings, which because of their larger size are more strongly driven by material cost. The field required at the sensor will depend on both the magnetic circuit configuration and the characteristics of the element itself (see Table 1).
In some sensor applications, circuitry is necessary to amplify the field signal either because there are large air gaps in the device or because the sensor element has a low response. Because the higher energy of Neo can provide a greater flux signal, less amplification is needed. The benefits are a reduction in the amplifier’s input power, lower distortion, and enhanced overall performance of the sensor.
Summary
Neodymium-iron-boron magnets offer the highest energy available in today’s commercial magnets. Although the basic alloy is Nd2Fe14B, there are variations on it that can tailor the material to suit particular operating conditions. There are also processing techniques that can be selected for sensing and other requirements. Particularly in its isotropic forms, Neo can be molded or pressed into complex geometries and magnetized with elaborate field patterns, offering significant opportunities for value-added magnet assemblies.
David Miller is Senior Applied Scientist and Peter Campbell, Ph.D., is Director of Applied Technology, Magnequench Technology Center, 9000 Development Dr., PO Box 14827, Research Triangle Park, NC 27709; 919-993-5500, fax 919-993-5501, dmiller@mqii.com or peter@magnetweb.com.
