Method and apparatus for adaptive feedforward cancellation

In a disc drive, a servo system for positioning a head over a track, the servo system comprising:

• a controller capable of generating a control signal based in part on a position error signal generated by the head;
• an actuator-head assembly comprising an actuator and the head and capable of moving the head based at least in part on the control signal from the controller and capable of generating the position error signal based on the position of the head over a track;
• an adaptive feedforward cancellation component capable of receiving a filtered position error signal for a current sector and capable of producing a cancellation signal for a next sector based in part on the filtered position error signal, the cancellation signal being provided at an output of the cancellation component and comprising at least one tap weight multiplied by at least one trigonometric function;
• and
• a filter receiving the position error signal at its input and producing the filtered position error signal that is provided to the adaptive feedforward cancellation component, the transfer function of the filter approaching the inverse of the transfer function measured from the output of the adaptive feedforward cancellation component to the input of the filter.

A disc drive for storing data in tracks along a disc, the disc drive comprising:

• a servo loop for positioning a head relative to a track;
• an adaptive feedforward cancellation component coupled to the servo loop for canceling repeatable runout positioning errors based on a filtered position error signal;
• and
• a filter capable of filtering a position error signal using a transfer function that is substantially the inverse of a transfer function of a portion of the servo loop to output the filtered position error signal.