Method of generating a position error signal, method of writing a data track, and method and apparatus for testing a head

1. 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.

2. A method according to claim 1 , wherein step (b) includes detecting the position of at least the servo null radially nearest the head.

3. A method according to claim 1 , wherein the position error is averaged over successive revolutions of the disk to generate the position error signal.

4. A method according to claim 1 , wherein the desired radial position of the head is selected so as to coincide with a servo null.

5. A method according to claim 1 , wherein the track is concentric with the disk, and the plurality of servo nulls are circumferentially spaced and have different radial positions at each circumferential position.

6. A method according to claim 1 , wherein the servo nulls are evenly spaced on the disk.

7. A method according to claim 1 , wherein the positions of the servo nulls extend, successively in a single radial direction on going round the track.

8. A method according to claim 1 , wherein the radial positions of the servo nulls vary linearly with circumferential position.

9. A method according to claim 1 , wherein the positions of the servo nulls define at least part of at least one spiral of servo nulls on the disk.

10. A method according to claim 9 , wherein the pitch of the spiral is the width of the track.

11. A method according to claim 1 , wherein the servo null locus is concentric with the disk.

12. A method according to claim 11 , wherein the track defines a sine wave.

13. A method according to claim 12 , wherein the sine wave has a wavelength equal to the track circumference.

14. A method according to claim 1 , wherein the disk is initially free of servo bursts, the method comprising performing, before step (a), the step of writing said servo bursts to the disk.

15. A method of testing a read/write head, the method comprising:

moving the head to a desired position relative to a track of a disk according to the method of claim 1 ;

and,

testing the head.

16. Apparatus for testing a read/write head, the apparatus comprising:

a disk having a track, 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;

a positioner for positioning a said head over a radial position on the disk;

a processor arranged to:

(a) receive a desired radial head position relative to the track;

(b) determine 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;

(c) detect the position of at least one servo null with the head;

(d) determine from at least one detected servo null position the position error of the head relative to the target null position; and,

(e) generate a position error signal in accordance with the position error; and,

a feedback controller arranged to receive said position error signal as a feedback input, and to cause said positioner to position said head so as to reduce said position error signal to zero,

wherein the processor is arranged in step (b) to detect the position of a plurality of servo nulls, and in step (c) to interpolate between said plurality of null positions in order to find said position error, said interpolation being calculated in accordance with the known position relationship.