When selecting a magnet for a linear mover, consider temperature, magnetic material, magnet grade, magnet geometry, and general mechanical constraints. These system variables all have an impact on the overall function and reliability of the system.

As an example study, an axially magnetized cylinder magnet with a remanence (B_r ) of 850 mT was simulated traveling above a sensor with an incrementally increased air gap.

The magnet had an outer diameter of 16mm and a thickness of 6mm. Each horizontal line represents the magnitude of the vertical component (B_z) of the observable magnetic flux density. Plotted together, a 3D heat map is produced that shows how this one field component varies in the region below the magnet. In this case, the horizontal displacement is swept from –40mm to +40mm, and the vertical air gap between the sensor and magnet ranges from 2mm to 15mm.

In the case of TMAG5170, the device offers two variants, each with three user programmable input ranges that vary from ±25 mT to ±300 mT. Any inputs above the maximum range are not useful. Alternatively, the TMAG5170D-Q1 is a dual-die version of the TMAG5170 that can be used if redundancy in the system is required.

When examining these results closely, when the air gap distance between the sensor element and magnet is very small, there is distortion near the peak of the bell-shaped input curve as the curve begins to flatten. At very close ranges, bad peaking results from the corner effects of the magnet body concentrating on the magnetic field.

For comparison, consider how a change in the magnet geometry can impact overall function. For instance, if the magnet had an outer diameter of 8mm while all other parameters were the same, you can note several different changes in behavior.

One noted change is that the useful width of the input range dramatically reduced. The Section 2.2 section explains that the maximum sensing range is typically at least 2x the diameter of the magnet.

Additionally, the corner effects observed at very close proximity are dramatically reduced when using a narrower magnet. The peak amplitude drops quickly when the air gap range increases. Intuitively, the sensing range for a smaller magnet is not as large as the sensing range of the original. What makes things even more difficult is that reducing the signal-to-noise ratio (SNR) increases uncertainty in any measurement. Therefore, target a peak input value nearly the full scale input range of the sensor selected.

Assuming then that the larger diameter is more desirable for the increased sensing range, another study of the impact of the magnet thickness can be informative. Consider next, the impact of increasing the thickness from 6mm to 12mm.

Similar to reducing the diameter of the magnet, increasing the thickness reduced the cornering effect, although visible distortion still occurs at very close proximity. From here, we can deduce that the ratio of the thickness to the diameter can impact the closest sensing range possible for this motion. Unsurprisingly, the peak B-field magnitude also increased when a magnet with larger mass was used.

Beyond just the considerations for magnet size, the material and grade of the magnet can impact field strength and cost to build the system. For example, magnetic materials can weaken as temperatures rise. Table 2-1 shows the typical values for this behavior.

If the working environment experiences large temperature variations, consider selecting a Samarium Cobalt (SmCo) magnet to reduce the effects of temperature drift.

Consider the approximate working air gap range for the sensor. The magnetic flux density observed by the sensor is inversely proportional to the square of the distance. That is, as the range increases, expect to see exponential decay in the strength of the field.

For any magnetic material, there are often several different grades of magnets which are usually distinguished by the B_r of the material. This value is well defined for any particular material, regardless of the size of the magnet. As B_r decreases, the B-field for any specific shape magnet can weaken. Neodymium type magnets, such as N35 and N52, tend to be the strongest commercially available option and inexpensive ferrite materials such as FRM-12 tend to be the weakest.