Robust servo for disk-shift compensation in rotating storage system

Abstract

A method and apparatus, in a direct access storage device including a head positioned for interaction with a data storage medium mounted on a rotating spindle, an actuator for positioning the head, and a servo control loop for positioning the actuator. The improvement comprises: a narrow band gain enhancing filter for connection in the servo control loop. The filter has a response frequency related to the rotational frequency of the spindle. The filter has programmable initial states. Also provided is a switch for switching in the narrow band filter as the head approaches a target position on the data storage medium. Initial states of the filter are determined and supplied to program the filter. At least one additional filter having a peak response frequency at a harmonic of the frequency of rotation of the spindle, may be provided. Initial states of the additional filters are determined and supplied to program the additional filter. The filter may include successive delay circuits for inputs to the filter, which circuits provide delayed outputs; multipliers for multiplying selected delayed outputs by a constant; and a summer for summing the delay outputs after the delayed outputs are multiplied by the multipliers. The initial states may be stored as phase, frequency and amplitude of a sine function.