Patent ID: 6972540

Claim:
A disk drive comprising: (a) a disk comprising a plurality of tracks, wherein: each track comprises a plurality of data sectors and a plurality of servo sectors forming N servo wedges; and a wedge time period (WTP) occurs between each servo wedge; (b) a head actuated over the disk; (c) a spindle motor for rotating the disk at an operating speed in response to a spindle control current, the spindle motor comprising a plurality of windings which generate a back electromotive force (BEMF) voltage; (d) a BEMF detector for generating a BEMF signal by comparing the BEMF voltage to a threshold; and (e) a disk controller for: measuring a BEMF speed error responsive to the BEMF signal during a BEMF spindle speed control mode; updating the spindle control current in response to the BEMF speed error to drive the disk at the operating speed; calibrating a reference time period (RTP); estimating a sinusoidal error in the WTP due to eccentricity in the disk rotating to generate an eccentricity compensation value; switching to a wedge spindle speed control mode; detecting an actual WTP by detecting an interval between the head passing over a first and second servo wedge; generating a wedge speed error in response to the RTP, the detected actual WTP, and the eccentricity compensation value; and maintaining the disk at the operating speed by updating the spindle control current in response to the wedge speed error.