Patent ID: 8243382

Claim:
A method of positioning a head relative to a disk in a test apparatus, the method comprising: commanding the head to a desired radial position relative to a track of a disk, wherein the track has a plurality of servo bursts defining a plurality of servo nulls for the track, the servo nulls being positioned such that there are servo nulls at more than four different radial positions relative to the track, the servo nulls defining a predetermined locus having a known position relationship with the track, the locus extending across the radial extent of the track; generating a position error signal for the difference between the actual radial position and the desired radial position of the head by: (a) determining a target null position on the null locus corresponding to said desired radial position of the head relative to the track in accordance with said known position relationship; (b) detecting the position of at least one servo null with the head; (c) determining from said at least one detected servo null position the position error of the head relative to the target null position; and, (d) generating a position error signal in accordance with said position error; and, controlling the position of the head with a feedback controller arranged to reduce the position error signal substantially to zero; and, writing and/or reading test data to the disk with the head, wherein step (b) includes detecting the position of a plurality of servo nulls and step (c) includes interpolating between said plurality of null positions in order to find said position error, said interpolation being calculated in accordance with the known position relationship.