Head and disk tester with a thermal drift-compensated closed-loop positioning system

• US 6,023,145 A
• Filed: 04/21/1999
• Issued: 02/08/2000
• Est. Priority Date: 04/21/1999
• Status: Expired due to Term

• Current Assignee: Guzik Technical Enterprises Incorporated
• Original Assignee: Guzik Technical Enterprises Incorporated
• Inventors: Varlakhanov, Alexander, Bokchtein, Ilia, Guzik, Nahum, Stein, Anatoli, Karaaslan, Ufuk
• Primary Examiner(s): Ro, Bentsu
• Application Number:
  US09/295,912
• Time in Patent Office: 293 Days
• Field of Search: 318/565, 318/632, 318/634, 318/652, 318/671, 318/685, 360/31, 360/75, 360/78.12, 360/78.13, 360/105, 369/43, 369/53, 369/176, 369/215, 369/219, 369/223
• US Class Current: 318/652
• CPC Class Codes: G11B 5/455 Arrangements for functional testing of heads; Measuring arrangements for heads
• CPC Class Codes: G11B 5/5552 using fine positioning means for track acquisition separate from the coarse (e.g. track changing) positioning means
• CPC Class Codes: G11B 5/5565 system adaptation for compensation of variations of physical parameters, e.g. temperature
• CPC Class Codes: G11B 5/596 for track following on disks G11B5/5526, G11B5/5552, G11B5/5565, G11B5/5582 take precedence

Abstract

The head/disk tester of the invention has a housing 43 that houses spindle 44 that rotatingly supports a magnetic disk 31. The housing supports a moveable carriage 30 that, in turn, supports a magnetic head 32. Positioning means 39 and 41 are used to move the carriage and the magnetic head across the magnetic disk. These positioning means include stepper motors that realize coarse positioning of the magnetic head, and a piezo actuator 37 that is used for fine positioning. Linear encoders 40 and 42 located at both sides of the carriage provide feedback information to a closed-loop positioning system that controls the piezo actuator. A set of special signals ("servo bursts") pre-written at a given track of the magnetic disk is used as an additional source of feedback information for the same closed-loop positioning system. This positioning system includes a servo analyzer 45 that reads and processes servo burst signals from the magnetic disk, and a position controller 49 that controls the piezo actuator. The controller contains two control loops: a proportional-integral-derivative (PID) loop and an additional servo burst loop. During any movement of the magnetic head to a prescribed command position, the servo burst feedback is turned off, and feedback from linear encoders is used by the PID loop to move the head. When the magnetic head reaches the prescribed command position, servo burst loop is turned on; it changes the command position of the PID loop in a way to keep the ratio of pre-written burst signals constant. As a result, the position of the magnetic head with respect to the data track remains the same for as long as necessary, even in unstable temperature conditions.

Claims

1. A head/disk tester having a thermal drift-compensated closed-loop positioning system for positioning a magnetic head at a desired offset with respect to a data track on a magnetic disk disposed about a disk axis, said data track extending along a circular track center line (TCL) on said magnetic disk disposed about said disk axis, comprising:

A. a base;

B. a spindle attached to said base, said spindle being adapted for rotatingly supporting a magnetic disk about said disk axis;

C. a movable carriage supported by said base, said carriage having an assembly for supporting a magnetic head in a region overlying said magnetic disk, said magnetic head having a read element and a write element;

D. positioning means for moving said carriage with said magnetic head with respect to said magnetic disk in a direction transverse to said disk axis;

E. a first feedback means for determining the position of said magnetic head with respect to said data track of said magnetic disk in stable temperature conditions;

F. a second feedback means for determining the position of said magnetic head with respect to said data track of said magnetic disk in varying temperature conditions, said second feedback means being responsive to servo burst signals on said magnetic disk at predetermined positions radially offset from said track center line; and

G. a closed-loop positioner responsive to said first feedback means and said second feedback means to control said positioning means whereby said magnetic head is substantially at said desired offset from said track center line.

2. The head/disk tester of claim 1, wherein said positioning means includes:

a plurality of motors for effecting open-loop positioning of said magnetic head to a nominal location with respect to said track center line, and at least one piezo actuator for effecting closed-loop positioning of said head.

3. The head/disk tester of claim 1, wherein said first feedback means includes at least one linear encoder for sensing position of said carriage with respect to said base along the direction of movement of said carriage.

4. The head/disk tester of claim 1, wherein said second feedback means includes a detector that detects the amplitudes of said servo burst signals prewritten on said magnetic disk.

5. The head/disk tester of claim 4, wherein said second feedback means further includes an averager that samples of the amplitudes of said detected servo burst signals and determines an average of said prewritten burst signals.

6. The head/disk tester of claim 4, further comprising means for prewriting said servo burst signals at a plurality of positions along a track of said magnetic disk, and for detecting the amplitudes of said prewritten burst signals.

7. The head/disk tester of claim 4, further comprising means for pre-writing said servo burst signals at a plurality of offsets from the center of a track of said magnetic disk, and for detecting the amplitudes of said prewritten burst signals.

8. The head/disk tester of claim 1, wherein said closed-loop positioner includes a proportional-integral-derivative (PID) control loop, a divider that determines ratios of averaged amplitudes of said servo burst signals, a memory that stores said ratios corresponding to prescribed head positions, and an additional servo loop that modifies command position of said PID control loop according to the changes of said ratios.

9. The head/disk tester of claim 8, wherein said additional servo loop is inoperative during a movement of said magnetic head to a prescribed location, and operative after said movement is completed.

10. A method of positioning of a magnetic head/disk tester, wherein at least two servo burst signals are prewritten to a magnetic disk with a magnetic head at a plurality of offsets from the center line of a data track on said magnetic disk before the beginning of a test of said magnetic head, said method comprising the steps of:

A. moving said magnetic head from one location over said magnetic disk to another location with a closed loop positioner that uses a first feedback source, wherein said first feedback source reflects the position of said magnetic head with respect to said magnetic disk, said position being in a wide range including a substantial portion of the whole width of said magnetic disk; and

B. adjusting the position of said magnetic head at a prescribed location over said magnetic disk using an additional closed loop responsive to a second feedback source, said second feedback source being prewritten set of servo burst signals stored on said magnetic disk at locations laterally offset from said centerline, wherein said second feedback source reflects the local position of said magnetic head with respect to said magnetic disk in a relatively narrow vicinity of said data track of said magnetic disk.

11. The method of claim 10, wherein a ratio of servo burst signals is used to determine the position of the said magnetic head with respect to said magnetic disk.

12. The method of claim 11, wherein said servo burst signals are prewritten at a plurality of positions along a track of said magnetic disk.

13. The method of claim 11, wherein a plurality of servo burst signals are prewritten at different offsets from the center of a track of said magnetic disk, thereby effecting a relatively wide operative range of said second feedback source.

14. The method of claim 13, including the further step of choosing the bursts to be used with said second feedback source using the substeps of:

i. selecting three burst signals, B₁ ≦

B₂ ≦

B₃, with maximum amplitudes;

ii. calculating ratios B₁ /B₂ and B₂ /B₃ ;

iii. selecting the two bursts that correspond to the ratio that is the closest to 1.

15. The method of claim 11, wherein narrow band-pass filtering and/or averaging effects relatively high noise immunity for said second feedback source.

16. The method of claim 11, wherein said burst ratios are stored in a memory and are subsequently read from said memory, and used to control movement of said magnetic head to a predetermined prior location.

17. System for generating and storing signals for positioning a magnetic read/write head with respect to a data track on a magnetic disk, said data track having width T and extending along a circular track center line disposed about a disk axis, comprising:

A. a base,

B. a disk support assembly affixed to said base for rotatably supporting said magnetic disk about said disk axis,

C. a head support assembly affixed to said base, including a magnetic read/write head, said read/write head having a write width T, and including an associated positioning assembly for selectively positioning said head to a nominal locations with respect to said track center line,

D. a controller for successively positioning said read/write head to one or more positions characterized by an associated desired offset with respect to said track center line, including;

i. means for writing in a time succession as said disk rotates two or more servo bursts over a relatively short time, wherein each of said bursts is laterally offset from said track center line by a predetermined distance, and

ii. means for reading said servo bursts at each of said offsets, and generating and storing signals representative of each read burst associated with each of said offsets.

18. System according to claim 17, further comprising:

means responsive to an applied desired offset signal to retrieve said stored signal associated with said offset, and applying said retrieved signals to said controller to position said magnetic read/write head in a closed loop manner in accordance with said retrieved signals.

19. Method for positioning a magnetic read/write head with respect to a data track on a magnetic disk, said read/write head having a write width T, and said data track having width T and extending along a circular track center line disposed about a disk axis, comprising the steps of:

A. writing in a time succession as said disk rotates, two or more servo bursts on said disk over a relatively short period of time, wherein each of said bursts is written at locations laterally offset from said track center line by a predetermined distance,

B. positioning said read/write head to a plurality of desired offsets with respect to said track center line, and at each of said offsets, reading said servo bursts in succession and determining an amplitude value representative of said bursts at each of said desired offsets,

C. storing said amplitude values in connection with said respective ones of desired offsets, and

D. in a closed loop manner, using the respective ones of said stored values to reposition said magnetic read/write head to selected ones at said desired offsets.