Rejecting oscillatory, non-synchronous mechanical disturbances in hard disk drives

This paper describes a "textbook" technique for rejecting narrow-band distu rbances to a hard-disk-drive servo loop, by including a model of the distur bance in the drive's state-space controller, and then canceling it. Both nu merical simulations and experimental evidence are presented which show sign ificant reductions in tracking errors due to the targeted disturbance in st eady-state operation, Simulations of the transient behavior of the augmente d control loop, however, show some undesirable characteristics.