Patent ID: 6975480

Claim:
In a magnetic disk drive having a head disk assembly (HDA) and a sampled servo controller, the HDA including a rotating magnetic disk having distributed position information in a plurality of uniformly spaced-apart servo wedges, a rotary actuator that pivots relative to a base and that carries a transducer head that periodically reads the position information from the servo wedges, a voice coil motor (VCM) circuit that includes a voice coil motor that is coupled to the rotary actuator, the VCM circuit responding to a control effort signal, the sampled servo controller for periodically adjusting the control effort signal during a track-following operation based on the position information, a method for determining repeatable runout cancellation values comprising the steps of: reading the position information related to a predetermined track during track following for more than one disk rotation; averaging the position information, for each servo wedge, obtained during the disk rotations to generate an averaged servo position value for each servo wedge; and filtering the averaged servo position values with a filter for compensating effects of the sampled servo controller, which tend to distort the averaged servo position values, to generate the repeatable runout cancellation values for the predetermined track, wherein filtering the plurality of averaged servo position values comprises: performing a Fourier transform on the averaged servo position values to generate transformed frequency values; multiplying the transformed frequency values by filter values representing an inverse error rejection function, for compensating effects of the sampled servo controller, to generate filtered frequency values; and performing an inverse Fourier transform on the filtered frequency values to generate the repeatable runout cancellation values.