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.