Aside from selecting whether or not to use a diametric cylinder for an on-axis measurement or a ring magnet for either off-axis or in-plane measurements, various other magnet properties will influence the magnetic field being observed.

Material Grade

The material grade for any magnet directly influences the B-H relationship of that magnet. It is useful to understand the relationship of the resulting B-field for any magnet as a result of an applied external H-field source.

Figure 2-3 Example B-H Curve

When the applied H-field used to magnetize a permanent magnet is removed, the residual B-field value is referred to as remanence, or B_r. This value is measured in a closed magnetic circuit, and is not equivalent to the surface field of the magnet. Surface field is measured in an open magnetic circuit using a magnetometer, and will vary with the magnet geometry. B_r will be constant for any size magnet of the same grade, while the surface field produced by a magnet will depend on the relationship between the surface area and pole length of that magnet.

The various difference grades of any magnetic material will have a different B_r values, which determine the resulting magnetic field strength for that magnet. Consider the relationship between B-field and distance of various grades and materials of magnets with equal dimensions shown in Figure 2-5. With the sensor placed along the axis of polarization of the magnet, it is possible to adjust range to find the peak B-field value which will be observed during rotation. For this case and all following plots, a magnet which has a diameter of 6 mm and a thickness of 3 mm will be used.

Figure 2-4 Airgap Range of Magnet and Sensor

Figure 2-5 Peak Field Magnitude vs. Airgap Distance for Various Magnet Materials

Size

As mentioned in Material Grade, the size of the magnet will influence the surface magnetic flux density, and resulting measured field for any given magnet. Consider the relationship of B-field vs. Distance for various N52 grade neodymium magnets whose height and radius are equal shown in Figure 2-6.

Figure 2-6 Peak Field Magnitude vs. Airgap Distance for Various Magnet Radius

Notice that the surface field is equivalent for each magnet, and the observed field at any air gap distance is greater for a larger magnet radius. What is also interesting to note, is that the ratio of the field observed at any distance to the radius of the magnet remains constant. That is, the field observed at 2 mm using the 1.5 mm radius magnet is equivalent to the field at 8 mm using the 6 mm magnet radius.

Temperature Drift

The various magnetic materials available all have differing responses to variations in temperature. For all magnets, the magnetic field strength will reduce by some value up until the Curie temperature, where the atomic dipoles within the magnet can no longer retain magnetic alignment. The typical value for various magnet materials is shown in Table 2-1

Ultimately for most applications, the goal will be to use the smallest magnet available that can produce a measurable input for the sensor. For any system, the ability to mount the magnet to a rotating shaft, cost and availability might be the dominant factors in magnet selection. Neodymium magnets (NdFeB) tend to offer the highest magnetic field strength while ferrite magnets tend to be the lowest cost.