# Patent

## Title
Disk drive applying feed-forward compensation when writing consecutive data tracks

## Abstract

A disk drive is disclosed comprising a disk comprising a plurality of servo tracks defined by servo sectors, a head actuated over the disk, and control circuitry comprising a servo control system operable to servo the head over the disk. A plurality of data tracks are defined relative to the servo tracks, and a first data track is accessed while servoing the head over the first data track based on a position error signal Xn−1(k) generated at each servo sector of the first data track. Feed-forward compensation values are generated based on Xn−1(k), and a second, consecutive data track is accessed while servoing the head over the second data track based on a position error signal Xn(k) generated at each servo sector of the second data track and based on the feed-forward compensation values.

## Background
BACKGROUND
Disk drives comprise a disk and a head connected to a distal end of an actuator arm which is rotated about a pivot by a voice coil motor (VCM) to position the head radially over the disk. The disk comprises a plurality of radially spaced, concentric tracks for recording user data sectors and servo sectors. The servo sectors comprise head positioning information (e.g., a track address) which is read by the head and processed by a servo control system to control the actuator arm as it seeks from track to track.
FIG. 1 shows a prior art disk format 2 as comprising a number of servo tracks 4 defined by servo sectors 60-6N recorded around the circumference of each servo track. Each servo sector 6i comprises a preamble 8 for storing a periodic pattern, which allows proper gain adjustment and timing synchronization of the read signal, and a sync mark 10 for storing a special pattern used to symbol synchronize to a servo data field 12. The servo data field 12 stores coarse head positioning information, such as a servo track address, used to position the head over a target data track during a seek operation. Each servo sector 6i further comprises groups of servo bursts 14 (e.g., N and Q servo bursts), which are recorded with a predetermined phase relative to one another and relative to the servo track centerlines. The phase based servo bursts 14 provide fine head position information used for centerline tracking while accessing a data track during write/read operations. A position error signal (PES) is generated by reading the servo bursts 14, wherein the PES represents a measured position of the head relative to a centerline of a target servo track. A servo controller processes the PES to generate a control signal applied to a head actuator (e.g., a voice coil motor) in order to actuate the head radially over the disk in a direction that reduces the PES.
The data sectors are accessed indirectly using logical block addresses (LBAs) mapped to physical block addresses (PBAs) representing the physical location of each data sector. This indirect accessing facilitates mapping out defective data sectors during manufacturing as well as while the disk drive is deployed in the field. Access commands (read/write) received from the host include LBAs which the disk drive maps to corresponding PBAs using any suitable mapping technique.
The LBA to PBA mapping may also facilitate log structured writes wherein at least part of the disk is written as a circular buffer. For example, the circular buffer may be written from an outer diameter track toward an inner diameter track, and then circle back to the outer diameter track. Data is written to the head of the circular buffer such that the LBAs of new write commands are mapped to the PBAs of the corresponding data sectors. When the same LBA is written by the host, the data is written to a new PBA at the head of the circular buffer and the old PBA is marked invalid so that it may be overwritten. During a garbage collection operation, valid PBAs previously written in the circular buffer may be relocated to the head of the circular buffer so that the old PBAs may be overwritten. In one embodiment, the tracks are written in a shingled manner such that a previously written track is partially overwritten, thereby increasing the overall capacity of the disk drive.



BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 shows a prior art disk format comprising a plurality of servo tracks defined by servo sectors.
FIG. 2A shows a disk drive according to an embodiment comprising control circuitry including a servo control system operable to actuate a head over a disk.
FIG. 2B is a flow diagram according to an embodiment wherein feed-forward compensation values are generated based on a position error signal generated when servoing on a first data track, wherein the feed-forward compensation values are used to servo the head over a second data track.
FIG. 2C shows an embodiment wherein the second servo track is consecutive with the first servo track.
FIG. 3 illustrates how a reference offset for servoing the head over the second data track is generated based on the position error signal generated while servoing over the first data track according to an embodiment.
FIG. 4 shows a servo control system according to an embodiment wherein the feed-forward compensation values are generated by convolving the reference offset with an inverse of a sensitivity function of the servo control system.
FIG. 5 illustrates an embodiment wherein a write operation to the second data track is aborted when a position error signal of the second data track exceeds an unsafe threshold based on a position error signal of the first data track.
FIG. 6 shows a servo control system according to an embodiment wherein a predicted position error signal is generated.
FIG. 7 illustrates an embodiment wherein a write operation to the second data track is aborted when the predicted position error signal exceeds an unsafe threshold based on a position error signal of the first data track.



DETAILED DESCRIPTION
FIG. 2A shows a disk drive according to an embodiment comprising a disk 16 comprising a plurality of servo tracks 18 defined by servo sectors 200-20N, a head 22 actuated over the disk 16, and control circuitry 24 comprising a servo control system operable to servo the head 22 over the disk 16. The control circuitry 24 is operable to execute the flow diagram of FIG. 2B, wherein a plurality of data tracks are defined relative to the servo tracks (block 26), and a first data track is accessed while servoing the head over the first data track based on a position error signal Xn−1(k) generated at each servo sector of the first data track (block 28). Feed-forward compensation values are generated based on Xn−1(k) (block 30), and a second, consecutive data track is accessed (block 34) while servoing the head over the second data track based on a position error signal Xn(k) generated at each servo sector of the second data track and based on the first feed-forward compensation values (block 32).
In the embodiment of FIG. 2A, the control circuitry 24 processes a read signal 36 emanating from the head 22 to demodulate the servo sectors 200-20N and generate a position error signal (PES) representing an error between the actual position of the head and a target position relative to a target track. The control circuitry 24 filters the PES using a suitable compensation filter to generate a control signal 38 applied to a voice coil motor (VCM) 40 which rotates an actuator arm 42 about a pivot in order to actuate the head 22 radially over the disk 16 in a direction that reduces the PES. The servo sectors 200-20N may comprise any suitable head position information, such as a track address for coarse positioning and servo bursts for fine positioning. The servo bursts may comprise any suitable pattern, such as an amplitude based servo pattern or a phase based servo pattern.
FIG. 2C illustrates an embodiment wherein the data tracks are written in a consecutive manner, such as in a log structured system where data received from a host is written to consecutive data tracks using a dynamic LBA to PBA mapping system. In one embodiment, the consecutive data tracks may be written in an overlapping (shingled) manner in order to increase a radial density of the data tracks. As shown in the embodiment of FIG. 2C, feed-forward compensation values are generated based on the servo sectors of a previous data track, where the feed-forward compensation values are used to servo the head while accessing a next data track (e.g., while writing data to the next data track). In this manner, the head may be servoed over the next data track based on a non-circular target reference so as to reduce the chance of aborting a write operation due to an off-track condition.
An example of this embodiment is illustrated in FIG. 3 wherein a position error signal Xn−1(k) is generated at each servo sector of a first data track. If the position error signal Xn−1(k) were to exceed a first threshold Tw, the write operation would have been aborted. When the position error signal Xn−1(k) exceeds a second threshold To lower than the first threshold Tw, a corresponding non-zero reference offset Rn(k) is generated for the second data track as illustrated in FIG. 3. That is, the reference offset Rn(k) is generated based on:




  
    
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where To+ represents a threshold toward the first data track, To− represents a threshold toward a next data track after the second data track, and Δmax+ and Δmax−, a maximum of |Rn(k)|.

FIG. 4 shows a servo control system according to an embodiment for servoing the head while writing data to the second data track. A position error signal Xn(k) is generated by subtracting a measured position of the head (as determined from reading the kth servo sector) from the reference offset Rn(k) shown in FIG. 3. Feed-forward compensation values FFn(k) are generated for the second data track by convolving the reference offset Rn(k) with an inverse of a sensitivity function S−1 of the servo control system. The feed-forward compensation value FFn(k) at the kth servo sector is added to the position error signal Xn(k) to generate an error signal 44 that is filtered by a suitable compensator 46 to generate the control signal 38 applied to the VCM 40 (or other suitable actuator). The non-zero reference offset Rn(k) and the corresponding feed-forward compensation values FFn(k) cause the head to deviate in a direction that corresponds to the deviations that occurred while writing data to the first data track as illustrated in FIG. 3, thereby reducing the chance of aborting the write operation due to an off-track write condition.
Referring again to FIG. 3, an off-track write condition occurs when the position error signal exceeds an unsafe threshold Tw that corresponds to a minimum allowed spacing between adjacent data tracks (track squeeze). If the position error signal Xn−1(k) of a previous data track deviates toward the next data track, it reduces the track squeeze margin allowed when writing to the next data track. In one embodiment, the unsafe threshold Tw for aborting a write operation may be configured to assume the worst case condition for the position error signal generated for a previous and current data track. This is illustrated in FIG. 5 which shows a rectangle designated by unsafe threshold Tw′ representing the worst case condition for the position error signal. Accordingly, if the position error signal Xn−1(k) of a previous data track or the position error signal Xn(k) of a current data track exceeds the unsafe threshold Tw′, the write operation is aborted. However, since the worst case condition for the position error signal does not always occur, a write operation may be aborted unnecessarily when employing the worst case unsafe threshold Tw′.
According, in one embodiment a variable unsafe threshold Tw is used to abort the write operation to the second data track that is based on the position error signal Xn−1(k) of the first data track. In this embodiment, the unsafe threshold Tw is based on:

[min(Tw+,W+Xn−1(k)),Tw−]

where Tw+; represents a maximum threshold toward the first data track, Tw−; represents a maximum threshold toward a next data track following the second data track, and W is a predetermined constant based on a squeeze limit between the first and second data tracks. As can be seen in FIG. 5, the maximum threshold Tw+ toward the first data track decreases as the position error signal Xn−1(k) for the first data track increases toward the second data track (i.e., becomes more negative). This variable unsafe threshold Tw+ provides an increased margin as compared to unsafe threshold Tw− which may allow a write operation to continue without exceeding the track squeeze limit. In the embodiment of FIG. 5, the magnitude of the maximum threshold Tw− is configured to be less than the magnitude of Tw+ in order to provide margin for the next track.

In one embodiment, when deciding whether to abort a write operation while writing data to the second data track, a position error signal {circumflex over (X)}n(k) is generated as shown in FIG. 4 by adding Xn(k) to Rn(k). This adjusted position error signal {circumflex over (X)}n(k) is then compared to the unsafe thresholds as shown in FIG. 5 to determine whether to abort the write operation when writing to the second data track. That is, the write operation is aborted when the position error signal {circumflex over (X)}n(k) relative to the center of the second data track exceeds the unsafe threshold.
In one embodiment, the adjusted position error signal {circumflex over (X)}n(k) is also used to generate the feed-forward compensation values for the next data track (a third data track following the second data track). That is, the non-zero reference offset Rn+1(k) is generated for the third data track based on when {circumflex over (X)}n(k) of the second data track exceeds the threshold To as shown in the example of FIG. 3.
FIG. 6 shows a servo control system according to an embodiment wherein a predictor 48 process the position error signal {circumflex over (X)}n(k) to generate a predicted position error signal {circumflex over (X)}n(k+1) for a next servo sector (servo sector k+1). The predictor 48 may implement any suitable prediction algorithm, wherein in one embodiment the predicted position error signal {circumflex over (X)}n(k+1) is generated based on:

{circumflex over (X)}n(k+1)=2·{circumflex over (X)}n(k)−{circumflex over (X)}n(k−1).

FIG. 7 shows an embodiment wherein a variable unsafe threshold Tp is generated based on the position error signal Xn−1(k+1) generated for the first servo track at servo sector k+1 similar to the unsafe threshold Tw described above with reference to FIG. 5. That is, the unsafe threshold Tp may be generated based on:

[min(Tp+,W+Xn−1(k+1)),Tp−]

and the write operation aborted if the predicted position error signal {circumflex over (X)}n(k+1) exceeds the unsafe threshold Tp. In one embodiment, the unsafe threshold Tp of FIG. 7 may be less than the unsafe threshold Tw of FIG. 5 to compensate for the error in predicting the position error at the next servo sector.

Any suitable control circuitry may be employed to implement the flow diagrams in the above embodiments, such as any suitable integrated circuit or circuits. For example, the control circuitry may be implemented within a read channel integrated circuit, or in a component separate from the read channel, such as a disk controller, or certain operations described above may be performed by a read channel and others by a disk controller. In one embodiment, the read channel and disk controller are implemented as separate integrated circuits, and in an alternative embodiment they are fabricated into a single integrated circuit or system on a chip (SOC). In addition, the control circuitry may include a suitable preamp circuit implemented as a separate integrated circuit, integrated into the read channel or disk controller circuit, or integrated into a SOC.
In one embodiment, the control circuitry comprises a microprocessor executing instructions, the instructions being operable to cause the microprocessor to perform the flow diagrams described herein. The instructions may be stored in any computer-readable medium. In one embodiment, they may be stored on a non-volatile semiconductor memory external to the microprocessor, or integrated with the microprocessor in a SOC. In another embodiment, the instructions are stored on the disk and read into a volatile semiconductor memory when the disk drive is powered on. In yet another embodiment, the control circuitry comprises suitable logic circuitry, such as state machine circuitry.
The various features and processes described above may be used independently of one another, or may be combined in various ways. All possible combinations and subcombinations are intended to fall within the scope of this disclosure. In addition, certain method, event or process blocks may be omitted in some implementations. The methods and processes described herein are also not limited to any particular sequence, and the blocks or states relating thereto can be performed in other sequences that are appropriate. For example, described tasks or events may be performed in an order other than that specifically disclosed, or multiple may be combined in a single block or state. The example tasks or events may be performed in serial, in parallel, or in some other manner. Tasks or events may be added to or removed from the disclosed example embodiments. The example systems and components described herein may be configured differently than described. For example, elements may be added to, removed from, or rearranged compared to the disclosed example embodiments.
While certain example embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions disclosed herein. Thus, nothing in the foregoing description is intended to imply that any particular feature, characteristic, step, module, or block is necessary or indispensable. Indeed, the novel methods and systems described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the methods and systems described herein may be made without departing from the spirit of the inventions disclosed herein.

## Claims

1. A disk drive comprising:
a disk comprising a plurality of servo tracks defined by servo sectors;
a head actuated over the disk; and
control circuitry comprising a servo control system operable to servo the head over the disk, the control circuitry operable to:
define a plurality of data tracks relative to the servo tracks, wherein each data track comprises a plurality of data sectors;
access a first data track while servoing the head over the first data track based on a position error signal Xn−1(k) generated at each servo sector of the first data track;
generate first feed-forward compensation values based on Xn−1(k); and
access a second data track while servoing the head over the second data track based on a position error signal Xn(k) generated at each servo sector of the second data track and based on the first feed-forward compensation values,
wherein:
the second data track is consecutive with the first data track;
the access of the first data track accesses at least one data sector in the first data track; and
the access of the second data track accesses at least one data sector in the second data track.

2. The disk drive as recited in claim 1, wherein the control circuitry is operable to generate the first feed-forward compensation values based on a reference offset Rn(k) generated based on




  
    
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where:
To+ represents a threshold toward the first data track;
To− represents a threshold toward a next data track after the second data track;
Δmax+ and Δmax− bound a maximum of |Rn(k)|.

3. The disk drive as recited in claim 2, wherein the control circuitry is operable to generate the first feed-forward compensation based on:


Rn(Xn−1)*S−1 

wherein S−1 represents an inverse of a sensitivity function of the servo control system.

4. The disk drive as recited in claim 2, wherein the control circuitry is operable to generate the position error signal Xn(k) based on a nominal centerline of the second data track offset by the reference offset Rn(k).

5. The disk drive as recited in claim 4, wherein the control circuitry is further operable to:
generate a position error signal {circumflex over (X)}n(k) by adding Xn(k) to Rn(k);
generate a first unsafe threshold based on Xn−1(k); and
abort the access of the second data track based on the first unsafe threshold.

6. The disk drive as recited in claim 5, wherein the control circuitry is operable to abort the access of the second data track when {circumflex over (X)}n(k) exceeds the first unsafe threshold.

7. The disk drive as recited in claim 6, wherein the first unsafe threshold is based on:


[min(Tw+,W+Xn−1(k)),Tw−]

where:
Tw+ represents a maximum threshold for {circumflex over (X)}n(k) toward the first data track;
Tw− represents a maximum threshold for {circumflex over (X)}n(k) toward a next data track following the second data track; and
W is a predetermined constant based on a squeeze limit between the first and second data tracks.

8. The disk drive as recited in claim 5, wherein the control circuitry is operable to:
generate a predicted position error signal {circumflex over (X)}n(k+1) based on {circumflex over (X)}n(k);
generate a second unsafe threshold based on Xn−1(k+1); and
abort the access of the second data track based on the second unsafe threshold.

9. The disk drive as recited in claim 8, wherein the control circuitry is operable to abort the access of the second data track when {circumflex over (X)}n(k+1) exceeds the second unsafe threshold.

10. The disk drive as recited in claim 9, wherein the second unsafe threshold is based on:


[min(Tp+,W+Xn−1(k+1)),Tp−]


where:
Tp+ represents a maximum threshold for {circumflex over (X)}n(k+1) toward the first data track;
Tp− represents a maximum threshold for {circumflex over (X)}n(k+1) toward a next data track following the second data track; and
W is a predetermined constant based on a squeeze limit between the first and second data tracks.

11. The disk drive as recited in claim 4, wherein the control circuitry is operable to:
generate a position error signal {circumflex over (X)}n(k) by adding Xn(k) to Rn(k);
generate second feed-forward compensation values based on {circumflex over (X)}n(k); and
access a third data track while servoing the head over the second data track based on a position error signal Xn+1(k) generated at each servo sector of the third data track and based on the second feed-forward compensation values,
wherein the third data track is consecutive with the second data track.

12. A method of operating a disk drive comprising a disk comprising a plurality of servo tracks defined by servo sectors, a head actuated over the disk, and control circuitry comprising a servo control system operable to servo the head over the disk, the method comprising:
defining a plurality of data tracks relative to the servo tracks, wherein each data track comprises a plurality of data sectors;
accessing a first data track while servoing the head over the first data track based on a position error signal Xn−1(k) generated at each servo sector of the first data track;
generating first feed-forward compensation values based on Xn−1(k); and
accessing a second data track while servoing the head over the second data track based on a position error signal Xn(k) generated at each servo sector of the second data track and based on the first feed-forward compensation values,
wherein:
the second data track is consecutive with the first data track;
the access of the first data track accesses at least one data sector in the first data track; and
the access of the second data track accesses at least one data sector in the second data track.

13. The method as recited in claim 12, further comprising generating the first feed-forward compensation values based on a reference offset Rn(k) generated based on




  
    
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where:
To+ represents a threshold toward the first data track;
To− represents a threshold toward a next data track after the second data track;
Δmax+ and Δmax− bound a maximum of |Rn(k)|.

14. The method as recited in claim 13, further comprising generating the first feed-forward compensation based on:


Rn(Xn−1)*S−1 

wherein S−1 represents an inverse of a sensitivity function of the servo control system.

15. The method as recited in claim 13, further comprising generating the position error signal Xn(k) based on a nominal centerline of the second data track offset by the reference offset Rn(k).

16. The method as recited in claim 15, further comprising:
generating a position error signal {circumflex over (X)}n(k) by adding Xn(k) to Rn(k);
generating a first unsafe threshold based on Xn−1(k); and
aborting the access of the second data track based on the first unsafe threshold.

17. The method as recited in claim 16, further comprising aborting the access of the second data track when {circumflex over (X)}n(k) exceeds the first unsafe threshold.

18. The method as recited in claim 17, wherein the first unsafe threshold is based on:


[min(Tw+,W+Xn−1(k)),Tw−]

where:
Tw+ represents a maximum threshold for {circumflex over (X)}n(k) toward the first data track;
Tw− represents a maximum threshold for {circumflex over (X)}n(k) toward a next data track following the second data track; and
W is a predetermined constant based on a squeeze limit between the first and second data tracks.

19. The method as recited in claim 16, further comprising:
generating a predicted position error signal {circumflex over (X)}n(k+1) based on {circumflex over (X)}n(k);
generating a second unsafe threshold based on Xn−1(k+1); and
aborting the access of the second data track based on the second unsafe threshold.

20. The method as recited in claim 19, further comprising aborting the access of the second data track when {circumflex over (X)}n(k+1) exceeds the second unsafe threshold.

21. The method as recited in claim 20, wherein the second unsafe threshold is based on:


[min(Tp+,W+Xn−1(k)),Tp−]

where:
Tp+ represents a maximum threshold for {circumflex over (X)}n(k+1) toward the first data track;
Tp− represents a maximum threshold for {circumflex over (X)}n(k+1) toward a next data track following the second data track; and
W is a predetermined constant based on a squeeze limit between the first and second data tracks.

22. The method as recited in claim 15, further comprising:
generating a position error signal {circumflex over (X)}n(k) by adding Xn(k) to Rn(k);
generating second feed-forward compensation values based on {circumflex over (X)}n(k); and
accessing a third data track while servoing the head over the second data track based on a position error signal Xn+1(k) generated at each servo sector of the third data track and based on the second feed-forward compensation values,
wherein the third data track is consecutive with the second data track.