Disk drive estimating a sinusoidal error in a wedge time period due to eccentricity in disk rotation

A method of estimating a sinusoidal error in a wedge time period (WTP) in a disk drive due to eccentricity in a disk rotating, the disk drive comprising the disk having a plurality of tracks, wherein each track comprises a plurality of data sectors and a plurality of servo sectors forming N servo wedges, wherein the WTP occurs between each servo wedge, the disk drive further comprising a head actuated over the disk, the method comprising the steps of:

• (a) positioning the head over a selected one of the tracks;
• (b) calculating an estimated WTP according to:
• EST_WTP=RTP+â*cos(2πk/N)+{circumflex over (b)}*sin(2πk/N)
• wherein RTP is a reference time period corresponding to a nominal WTP, k is an index representing one of the servo wedges, and {â,{circumflex over (b)}} are adjustable coefficients;
• (c) detecting an actual WTP by detecting an interval between the head passing over a first and second servo wedge;
• (d) estimating a wedge time error e(k) as the difference between the estimated WTP and the detected actual WTP;
• (e) adjusting the coefficients {â,{circumflex over (b)}} for generating the estimated WTP according to:
• â(k+1)=â(k)−G*e(k)*cos(2πk/N)
• {circumflex over (b)}(k+1)={circumflex over (b)}(k)−G*e(k)*sin(2πk/N)
• wherein G is a predetermined gain;
• and
• (g) repeating steps (b)–(e) at least once.