Calibration of the spherical pointing motor

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Abstract
We have built a new miniature pan-tilt actuator, the spherical pointing motor (SPM). The SPM is an absolute positioning device, designed to orient a small camera sensor in two degrees of rotational freedom. The basic idea is to orient a permanent magnet to the magnetic field induced by three orthogonal coils by applying the appropriate ratio of currents to the coils. The function describing the relation between the coil currents and the resultant motor position can be calculated, but it is not very accurate as the actual coils do not exactly satisfy the assumptions made in these calculations. The motor must be calibrated to find the coil currents accurately. This paper describes a procedure for automatic calibration of the SPM. It is based on image feedback from a camera returning space-variant images, mounted on the rotor of the motor. It assumes that a calibrated image sensor and lens are used, i.e., that it is known how many degrees each pixel subtends. It also assumes that the camera rotates about its focal point. The calibration algorithm uses a scene of black dots on a white background. For each motor position that is to be calibrated, the algorithm moves the motor approximately to that position using the calculated currents. The algorithm analyzes the image, and uses the position of the relevant dot to calculate the actual position of the motor. It then associates this position with the coil currents and stores it in a look-up-table. Finally, we interpolate between calibrated points to move to other positions.

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