Self-calibration and mirror center offset elimination of a multi-beam laser tracking system

New methods for the self-calibration of a laser tracking coordinate measuring machine are reported in this paper. Proper calibration of a laser tracking system is essential prior to using such a device for coordinate measuring. The task is nontrivial in the absence of other measurement systems of comparable accuracy. One has to rely on a self-calibration strategy. Two calibration methods, one based on a four-tracker system and the other based on three trackers combined with precision planes to constrain the target motion, are proposed. Iterative optimization algorithms are developed. A basic assumption for these calibration methods is that there is no tracking mirror center offset. Error analysis presented in this paper reveals that measurement errors due to mirror center offsets are significant. Therefore before calibrating the laser tracking system, the mirrors must be adjusted so that the laser beams hit the mirror centers. The adjustment is a tedious manual task. In the FAU laser tracking machine, an adjustment procedure which employs a four quadrant detector has been devised for the elimination of tracking mirror center offsets. Simulation and experimental results are presented to demonstrate the effectiveness of the suggested methods.