2 Diametric Magnet Approach

Let's first consider the approach with the diametric magnet axis at the shifter stick fulcrum. For a point centered under the magnet when the magnet is centered on the axis of rotation like in Figure 2-2, the magnetic flux density (B-fields) look similar to Figure 2-1. Taking the arctan of these fields we can get a linear slope of values directly related to lever position, shown as the angle in Figure 2-1. Assuming there is a flexibility in placement of our device, we can sweep the sensor offset from the magnet in the z-axis, like in Figure 2-3, to determine the bounds of where the device is too close or too far from the magnet. Neglecting the space occupied by the device or magnet, the device is too close when the b-field saturates the sensor for more than two singular angle values over a 360° magnet rotation. The device is too far from the magnet, when more than two singular angle values are beneath the device noise floor over a 360° magnet rotation. Figure 2-3 shows that an N42, 12.7-mm diameter, 3.175-mm thick magnet does not fall outside the sensing bounds of the TMAG5170 for any z-offset from the magnet origin within -8 mm and -2.5 mm.

Other notable metrics to gauge prior to proceeding with a specific design include the impact of magnet diameter and thickness on error when the device is offset in the xy plane from the ideal location, such as captured in Figure 2-4 and Figure 2-5. Whatever tolerance can be expected with assembly in fabrication is good to use for this analysis. This paper assumes ±1.5-mm offset. For Figure 2-4 the sensor z-offset was fixed at 7.5 mm. For the Figure 2-5, the diameter was fixed at 12 mm and the sensor z-offset was adjusted such that the air gap remained constant. The air gap is the distance between the magnet surface and the sensor plane.

Figure 2-4 indicates that smaller diameters are less forgiving in error for the same offset. Based on a large group of simulation data not shown, offsets less than 10% of the magnet diameter length frequently appear to provide less than 1° error. As for magnet thickness, Figure 2-5 suggests that only a slight change in angle error is observed for different thicknesses.

Figure 2-4 and Figure 2-5 are based on measurements from a single sensor. For automotive applications, there is often a desire to have redundancy to satisfy safety requirements. As redundancy requires multiple devices, and multiple devices cannot physically occupy the same space, at least one if not both sensors will measure fields different from the lone sensor above. Also with mechanical manufacturing and assembly tolerances, there can be increased discrepancies between sensor measurements. The deviation from the ideal behavior and the discrepancy between sensors is dependent on relative sensor placement. Two common sensor placements are side by side and stacked as shown in Figure 2-6. Stacked die is the arrangement found in the TMAG5170D-Q1 and those die are typically vertically separated by 0.123 mm. For the side-by-side die, around 1 mm of separation horizontally is not uncommon.