3 Axial Magnet Approach

Having assessed the Diametric approach, lets now consider the approach with the axial magnet spaced some radial distance from the shifter stick fulcrum. The general b-field profile for an axial magnet rotating past a hall sensor in approach, like the one featured in Figure 1-1, is shown in Figure 3-1. Based on such field behavior, we can expect that taking the arctan of Bz and Bx yields a linear output similar to Figure 3-2. For best results, one of the field values must be scaled such that the global max for each curve is the same prior to processing.

With intuition of the behavior above, an initial attempt at design can be pursued. Similar to the axial approach, an initial head-on sweep of the axial magnet along the z-axis of the device can be performed to determine if there are any restrictions on vertical offset from the device. For this example, involving a Ferrite, C11 grade, 9.52 mm diameter, 3.18 mm thick magnet, a typical 4 mm offset along the z-axis is sufficiently strong when centered over the device to neither saturate a TMAG5170 output nor provide a field indistinguishable from noise. Hence, the next step is to determine what flexibility there is with offset radial from the shifter stick fulcrum. Figure 3-4 and Figure 3-5, illustrate the field Bz and Bx field characteristics for the magnet and device spaced radially from the fulcrum. These figures show that as the radial distance increases, the curve behavior appears to change more rapidly. Operating within the region prior to the device slowly asymptotically approaching 0 mT, is advised. Therefore, increasing radial offset, reduces the measurable range of shifter stick rotation.

These plots suggest, Figure 3-5 in particular, that for 40 mm radial offset, a range of about 15° is possible. This is the angle from the starting position shown in Figure 3-3. Due to symmetry, this corresponds to ±15°, which is a total distance of 30°. The subsequent sensor comparison proceeds from this radial spacing constraint.