Testing the Homogeneity of Magnets for Rotary Position Sensors

Inhomogeneity of the magnetic field around the magnet used in magnetic angle sensors affects the accuracy of the rotary position sensor. The setup for testing the homogeneity of the field of small rotating permanent magnets must meet several conditions, including the following: 1) two-axis magnetic field probe used for the test must be placed very accurately at the centerline of the rotation axis of the magnet; 2) offsets of probe measurement channels must be canceled; 3) magnetic sensitivities of the measurement channels must be equal; and 4) sensitivity vectors of the measurement channels should be mutually orthogonal. In this paper, we describe a procedure that leads to fulfilment of these conditions. To align the probe position with respect to the magnet rotation axis, we roughly scan the magnet, establish a misalignment error function, and then calculate the position of the minimum of this function. The position of the minimum corresponds to the rotation axis. Then, the center of the sensitive spot of the probe is precisely aligned with the axis of rotation, and the corrections of the offsets, amplitude mismatch and shift are done. These corrections are performed using the Fourier analysis of the probe signals. After the probe alignment and the corrections, the homogeneity error is reduced for a factor of 6-7, and the residual error is of the order of the noise of the applied Hall probe.