REPETITIVE CONTROL METHODS WHEN THE DISTURBANCE PERIOD IS NOT AN INTEGER MULTIPLE OF THE SAMPLE TIME

摘要

Repetitive control creates methods of canceling the effects of periodic disturbances on the output of feedback control systems. Fine pointing instruments on spacecraft are often subject to periodic disturbances from rotating wheels within the spacecraft, and repetitive control offers one option for improving the performance of active isolation systems for such equipment. Typical repetitive control assumes that the period of the disturbance can be represented as an integer multiple of the sample time of the digital control system. In practice, it will not in general be an integer multiple. It is shown here how this can seriously degrade performance. Interpolation schemes are studied to address the situation. Methods are developed to ensure stability of the learning process over a range of speeds when interpolation is used. It is seen that the process of stabilization is often done at the expense of the final error level reached. However, examples are given in which the interpolation process stabilizes an otherwise unstable repetitive control law.