Patent ID: 6914740

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 wedges; (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; measuring a wedge-to-wedge time representing a time period between each servo wedge; calibrating a reference time period representing an accumulation of a predetermined number of wedge-to-wedge times while the BEMF speed error is substantially zero; switching to a wedge spindle speed control mode; measuring a wedge time period representing an accumulation of a predetermined number of wedge-to-wedge times; generating a wedge speed error representing a difference between the reference time period and the wedge time period; and maintaining the disk at the operating speed by updating the spindle control current in response to the wedge speed error.