Repetitive control is formulated and analyzed in the discrete-time domain. Sufficiency conditions for the asymptotic convergence of a class of repetitive controllers are given. The “plug-in” repetitive controller is introduced and applied to track-following in a disk-file actuator system. Inter-sample ripples in the tracking error were present when the “plug-in” repetitive controller was installed. The performance is enhanced, however, when the zero-holding device is followed by a low-pass filter or replaced by a delayed first-order hold.

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This paper considers the use of external models for the cancellation of disturbances that can be regarded as output of stable (in the sense of Lyapunov) autonomous systems. The external model is set up outside the feedback loop so that the design of the feedback controller is accomplished without favoring particular disturbances. External model controllers can, therefore, be add-on devices to existing control systems. An application to track-following in a disk-file actuator system is presented.

There is a need for reliable and low-cost shocksensing in magnetic hard-disk drives to maintain data integrity when a drive is subjected to external shocks. This paper describes a novel method that is both reliable and lowcost, using only the existing head. preamp and data channel.