Abstract: A system and method of measuring a rotational motion of a code wheel measures a rotational movement of the code wheel including an error component due to a non-rotational movement of the code wheel; measures the non-rotational movement of the code wheel; and produces an error-corrected measurement of the rotational movement of the code wheel by using the measured non-rotational movement of the code wheel to cancel the error component of the measured rotational movement of the code wheel. A signal can be produced indicating a need for maintenance when the non-rotational movement of the code wheel exceeds a threshold.

Abstract: A pointing device having an elongated body with a transparent end adapted for movement over a surface is disclosed. An illumination subsystem illuminates the surface in an area adjacent to the transparent end, and an imaging subsystem forms images of a portion of the surface in that area. The present invention also includes a reference mark system that defines a direction that is independent of rotations of the elongated body. A controller periodically compares two of the images to determine the distance and direction that the transparent end moved between the two images. The controller utilizes the direction defined in the reference mark system to reduce the computational workload. In one embodiment, the reference mark system includes a disk that is free to rotate about an axis through the disk and a sensor, and the position of a reference mark that is displaced from the axis is measured.