In this paper, an adaptive digital algorithm for rejecting periodic disturbances is proposed. Modified from the adaptive tracking controller [4], the controller is constructed in a “plug-in” manner, i.e., it can be added to an existed feedback control system without altering the original closed-loop configuration. It is shown that the controller can reject disturbances at selected frequencies independently. Furthermore, since the controller only deals with phase and gain of the error signal, no structural information about the plant is required. The controller is implemented on a disk drive system for track following. The result shows that by rejecting the disturbance up to four times of its fundamental frequency, the tracking error is reduced substantially.

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