# Patent

## Title
Data storage device measuring reader/writer offset by reading spiral track and concentric servo sectors

## Abstract

A data storage device is disclosed comprising a disk comprising a spiral track, and a head actuated over the disk, wherein the head comprises a read element offset radially from a write element by a reader/writer offset. The spiral track is first read to write a plurality of concentric servo sectors on the disk that define at least one concentric servo track on the disk. The spiral track is second read and the concentric servo sectors are read to measure the reader/writer offset.

## Background
BACKGROUND
When manufacturing a data storage device such as a disk drive, concentric servo sectors 20-2N are written to a disk 4 which define a plurality of radially-spaced, concentric servo tracks 6 as shown in the prior art disk format of FIG. 1. A plurality of concentric data tracks are defined relative to the servo tracks 6, wherein the data tracks may have the same or a different radial density (tracks per inch (TPI)) than the servo tracks 6. Each servo sector (e.g., servo sector 24) comprises a preamble 8 for synchronizing gain control and timing recovery, a sync mark 10 for synchronizing to a data field 12 comprising coarse head positioning information such as a track number, and servo bursts 14 which provide fine head positioning information. The coarse head position information is processed to position a head over a target data track during a seek operation, and the servo bursts 14 are processed to maintain the head over a centerline of the target data track while writing or reading data during a tracking operation.
In the past, external servo writers have been used to write the concentric servo sectors 20-2N to the disk surface during manufacturing. External servo writers employ extremely accurate head positioning mechanics, such as a laser interferometer, to ensure the concentric servo sectors 20-2N are written at the proper radial location from the outer diameter of the disk to the inner diameter of the disk. However, external servo writers are expensive and require a clean room environment so that a head positioning pin can be inserted into the head disk assembly (HDA) without contaminating the disk. Thus, external servo writers have become an expensive bottleneck in the disk drive manufacturing process.
The prior art has suggested various “self-servo” writing methods wherein the internal electronics of the disk drive are used to write the concentric servo sectors independent of an external servo writer. For example, U.S. Pat. No. 5,668,679 teaches a disk drive which performs a self-servo writing operation by writing a plurality of spiral servo tracks to the disk which are then processed to write the concentric servo sectors along a circular path. Each spiral servo track is written to the disk as a high frequency signal (with missing bits), wherein the position error signal (PES) for tracking is generated relative to time shifts in the detected location of the spiral servo tracks. The read signal is rectified and low pass filtered to generate a triangular envelope signal representing a spiral servo track crossing, wherein the location of the spiral servo track is detected by detecting a peak in the triangular envelope signal relative to a clock synchronized to the rotation of the disk.



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 data storage device in the form of a disk drive according to an embodiment comprising a head actuated over a disk.
FIG. 2B shows an embodiment wherein the head comprises a read element offset radially from a write element by a reader/writer offset.
FIG. 2C is a flow diagram according to an embodiment wherein a spiral track and concentric servo sectors are read in order to measure the reader/writer offset.
FIG. 2D illustrates an example embodiment wherein a spiral track and concentric servo sectors are read in order to measure the reader/writer offset.
FIGS. 3A and 3B show an embodiment wherein concentric servo sectors are written to the disk while servoing the head over the disk based on a plurality of spiral tracks.
FIG. 4A shows an embodiment wherein the reader/writer offset is measured at a number of discrete radial locations, the data points are curve fitted to a suitable polynomial, and a reader/writer offset error is generated at each of the different radial locations based on a difference between the measured reader/writer offset and a nominal reader/writer offset generated based on the polynomial.
FIG. 4B shows an embodiment wherein after rewriting the concentric servo sectors based on the spiral tracks and the reader/writer offset errors, the reader/writer offset errors are reduced.
FIG. 5A is a flow diagram according to an embodiment wherein the concentric servo sectors are rewritten for at least part of the disk based on the spiral tracks and the reader/writer offset errors.
FIG. 5B is a flow diagram according to an embodiment wherein the reader/writer offset is measured at a number of discrete radial locations, the data points are curve fitted to a suitable polynomial, and a reader/writer offset error is generated at each of the different radial locations based on a difference between the measured reader/writer offset and a nominal reader/writer offset generated based on the polynomial.



DETAILED DESCRIPTION
FIG. 2A shows a data storage device in the form of a disk drive according to an embodiment comprising a disk 16 comprising a spiral track 180, and a head 20 actuated over the disk 16, wherein the head 20 comprises a read element 22A offset radially from a write element 22B by a reader/writer offset 24 (FIG. 2B). The disk drive further comprises control circuitry 26 configured to execute the flow diagram of FIG. 2C, wherein the spiral track is first read to write a plurality of concentric servo sectors on the disk that define at least one concentric servo track on the disk (block 28). The spiral track is second read and the concentric servo sectors are read to measure the reader/writer offset (block 30).
In the embodiment of FIG. 2A, the control circuitry 26 processes a read signal 32 emanating from the head 20 to demodulate the spiral track 180 and generate a position error signal (PES) representing an error between the actual position of the head and a target position. The control circuitry 26 filters the PES using a suitable compensation filter to generate a control signal 34 applied to a voice coil motor (VCM) 36 which rotates an actuator arm 38 about a pivot in order to actuate the head 20 radially over the disk 16 in a direction that reduces the PES. In an embodiment shown in FIGS. 3A and 3B, the disk 16 may comprise a plurality of spiral tracks 180-18N which may be demodulated by the control circuitry 26 in order to servo the head 20 radially over the disk 16 while writing a plurality of concentric servo sectors 400-40N that define a plurality of concentric servo tracks. The embodiment of FIG. 3A shows each spiral track 18, may be written over a partial revolution of the disk 16, whereas in an alternative embodiment, each spiral track 18, may be written over multiple revolutions of the disk 16 as shown in FIG. 2A.
Referring again to FIG. 2B, the read element 22A may be offset radially from the write element 22B, and therefore when writing to the disk 16 and/or when reading from the disk 16 a “jog” value is added to the servo system to account for the reader/writer offset 24. Since the jog value may change across the radius of the disk due to the skew angle of the head 20 as illustrated in FIG. 2B, a jog profile may be calibrated that spans the radius of the disk surface. The jog profile is then used to generate a jog value corresponding to the radial location of the head when accessing a target data track on the disk surface.
In one embodiment illustrated in FIG. 2D, the control circuitry 26 measures the reader/writer offset 24 by reading the spiral track 180 and the concentric servo sectors (e.g., servo sector 400) written by servoing off of the spiral track 180. The measurement may be made during the process of servo writing the concentric servo sectors 400-40N or after writing the concentric servo sectors 400-40N. In the example of FIG. 2D, the read element 22A1 may be positioned at radial location RLs by reading the spiral track 180 while writing concentric servo sector 400 using the write element 22B1 positioned at radial location RLc. When the read element 22A2 reads the center of concentric servo sector 400, the read element 22A2 is positioned at radial location RLc, and therefore the reader/writer offset 24 may be measured as the difference between RLs and RLc.
In one embodiment, the radial location RLs generated by reading the spiral track 180 may be written into the concentric servo sector 400. That is, when servoing the read element 22A1 at the center of a servo track defined by the spiral track 180, the corresponding radial location RLs is written as the center of a corresponding servo track (defined by servo sector 400) at radial location RLc. In this manner, when the read element 22A2 is positioned at radial location RLc, the reader/writer offset 24 may be measured as the difference between the radial location as defined by reading the spiral track 180 at radial location RLc and the radial location as defined by reading the concentric servo sector 400 at radial location RLc.
If the shape of the spiral tracks 180-18N (e.g., slope) varies across the disk surface from an optimal shape, the error will propagate through to the concentric servo sectors 400-40N causing a variable track squeeze of the concentric servo tracks. The error in the shape of the spiral tracks 180-18N may also cause an error when measuring the reader/writer offset 24. Referring to FIG. 4A, the fixed geometry of the head 20 means the actual reader/writer offset 24 will vary across the radius of the disk along a continuous curve 42 which may be represented as a polynomial. The reader/writer offset 24 (represented as black dots) measured at different radial locations will deviate from the curve 42 due to a reader/writer offset error caused by the imperfect spiral tracks 180-18N. In one embodiment, the reader/writer offset error may be measured at each radial location by curve fitting the reader/writer offset measurements to a polynomial curve 42 such as shown in FIG. 4A, and then computing the difference between each reader/writer offset measurement and the curve 42.
The reader/writer offset errors shown in FIG. 4A may be used in any suitable manner. In one embodiment, the reader/writer offset errors may be used to screen out disk surfaces when the errors are excessive and/or to improve the process of writing the spiral tracks 180-18N to the disk 16. For example, in one embodiment the spiral tracks 180-18N may be erased and then rewritten to the disk 16 if the reader/writer offset errors are excessive. In another embodiment described below, the reader/writer offset errors may be used to rewrite the concentric servo sectors 400-40N over at least part of the disk radius in a manner that reduces the reader/writer offset errors and corresponding variance in the track squeeze of the concentric servo tracks. That is, the reader/writer offset errors may be used to adjust the radial location generated by reading the spiral tracks 180-18N, thereby adjusting the radial location of the concentric servo tracks. When the reader/writer offset is again measured after rewriting the concentric servo sectors over at least part of the disk radius, the reader/writer offset measurement errors will decrease as illustrated in FIG. 4B. That is, each reader/writer offset measurement will be closer to the curve 42 that represents the actual reader/writer offset.
FIG. 5A shows a flow diagram executed by the control circuitry 26 according to an embodiment wherein the spiral track 180 is read to write a plurality of concentric servo sectors 400-40N on the disk 16 that define at least one concentric servo track on the disk 16 at a first radial location (block 44). The reader/writer offset 24 is measured (block 46), and when rereading the spiral track the measured reader/writer offset is used to rewrite the at least one concentric servo track on the disk at a second radial location different from the first radial location (block 48). For example, as described above with reference to FIGS. 4A and 4B, the measured reader/writer offset may be processed to generate a reader/writer offset error which is then used to rewrite at least one of the concentric servo tracks at a different radial location.
FIG. 5B is a flow diagram according to an example embodiment wherein a plurality of spiral tracks 180-18N are read in order to write a plurality of concentric servo sectors 400-40N on the disk 16 that define at least one concentric servo track on the disk (block 50). The reader/writer offset is then measured at a plurality of radial locations (block 52) such as shown in FIG. 3A. The data points representing the measured reader/writer offsets are curve fitted to a polynomial (block 54) such as the curve 42 shown in FIG. 3A. A reader/writer offset error is generated at the plurality of radial locations (block 56), such as by computing a difference between the measured reader/writer offset and the polynomial curve. The spiral tracks 180-18N are read again and the reader/writer offset error used to rewrite at least part of the concentric servo tracks (block 58), thereby reducing the read/writer offset error as shown in FIG. 3B.
Any suitable technique may be used to measure the reader/writer offset measured at block 46 of FIG. 5A or block 52 of FIG. 5B, such as the technique of reading both the spiral track 180 and the concentric servo sectors 400-40N as described above with reference to FIG. 2D. In an alternative embodiment, the reader/writer offset may be measured by writing a test pattern to the disk while servoing the read element 22A based on the concentric servo sectors 400-40N. The read element 22A may then be scanned radially over the disk and the resulting read signal evaluated to determine where the test pattern was written relative to the reader/writer offset.
In one embodiment, the process of rewriting the concentric servo sectors 400-40N to reduce the variance in the track squeeze of the concentric servo tracks may be executed over a number of iterations. At each iteration, the measured reader/writer offset error is accumulated so that after each rewrite of at least part of the concentric servo tracks, the reader/writer offset error decreases. In one embodiment, the iterations may terminate when the average (and/or maximum) reader/writer offset error falls below a threshold.
In one embodiment, the reader/writer offset errors may be upsampled using any suitable technique, such as by interpolating between the measured reader/writer offsets or interpolating between the reader/writer offset errors such as shown in FIG. 4A. In this manner, a reader/writer offset error may be generated for each of the rewritten concentric servo tracks.
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.
In various embodiments, a disk drive may include a magnetic disk drive, an optical disk drive, etc. In addition, while the above examples concern a disk drive, the various embodiments are not limited to a disk drive and can be applied to other data storage devices and systems, such as magnetic tape drives, solid state drives, hybrid drives, etc. In addition, some embodiments may include electronic devices such as computing devices, data server devices, media content storage devices, etc. that comprise the storage media and/or control circuitry as described above.
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 embodiments disclosed herein.

## Claims

1. A data storage device comprising:
a disk comprising a spiral track;
a head actuated over the disk, the head comprising a read element offset radially from a write element by a reader/writer offset; and
control circuitry configured to:
first read the spiral track to write a plurality of concentric servo sectors on the disk that define at least one concentric servo track on the disk;
generate a first radial location measurement based on reading the spiral track;
generate a second radial location measurement based on reading the concentric servo sectors; and
measure the reader/writer offset based on a difference between the first and second radial location measurements.

2. The data storage device as recited in claim 1, wherein the control circuitry is further configured to:
read the spiral track to write the plurality of concentric servo sectors on the disk to define a plurality of concentric servo tracks on the disk; and
read the spiral track and the concentric servo sectors at a plurality of different radial locations to generate the reader/writer offset measurement at each radial location.

3. The data storage device as recited in claim 2, wherein the control circuitry is further configured to:
curve fit the reader/writer offset measurements to a polynomial; and
generate a reader/writer offset error at each of the different radial locations based on a difference between the measured reader/writer offset and a nominal reader/writer offset generated based on the polynomial.

4. The data storage device as recited in claim 3, wherein the control circuitry is further configured to read the spiral track and use the reader/writer offset error to rewrite at least one of the concentric servo tracks at a different radial location in order to compensate for the reader/writer offset error.

5. A data storage device comprising:
a disk comprising a spiral track;
a head actuated over the disk, the head comprising a read element offset radially from a write element by a reader/writer offset; and
control circuitry configured to:
first read the spiral track to write a plurality of concentric servo sectors on the disk that define at least one concentric servo track on the disk at a first radial location;
measure the reader/writer offset; and
second read the spiral track and use the measured reader/writer offset to rewrite at least one of the concentric servo tracks at a second radial location different from the first radial location.

6. The data storage device as recited in claim 5, wherein rewriting the concentric servo track compensates for an error in the measured reader/writer offset.

7. The data storage device as recited in claim 6, wherein the control circuitry is further configured to third read the spiral track and the concentric servo sectors to measure the reader/writer offset.

8. A method of operating a data storage device, the method comprising:
using a head to first read a spiral track on a disk to write a plurality of concentric servo sectors on the disk that define at least one concentric servo track on the disk;
generating a first radial location measurement based on reading the spiral track;
generating a second radial location measurement based on reading the concentric servo sectors; and
measuring a reader/writer offset between a read element of the head and a write element of the head based on a difference between the first and second radial location measurements.

9. The method as recited in claim 8, further comprising:
reading the spiral track to write the plurality of concentric servo sectors on the disk to define a plurality of concentric servo tracks on the disk; and
reading the spiral track and the concentric servo sectors at a plurality of different radial locations to generate the reader/writer offset measurement at each radial location.

10. The method as recited in claim 9, further comprising:
curve fitting the reader/writer offset measurements to a polynomial; and
generating a reader/writer offset error at each of the different radial locations based on a difference between the measured reader/writer offset and a nominal reader/writer offset generated based on the polynomial.

11. The method as recited in claim 10, further comprising reading the spiral track and using the reader/writer offset error to rewrite at least one of the concentric servo tracks at a different radial location in order to compensate for the reader/writer offset error.

12. A method of operating a data storage device, the method comprising:
using a head to first read a spiral track on a disk to write a plurality of concentric servo sectors on the disk that define at least one concentric servo track on the disk at a first radial location;
measuring a reader/writer offset between a read element of the head and a write element of the head; and
using the head to second read the spiral track and using the measured reader/writer offset to rewrite at least one of the concentric servo tracks at a second radial location different from the first radial location.

13. The method as recited in claim 12, wherein rewriting the concentric servo track compensates for an error in the measured reader/writer offset.

14. The method as recited in claim 13, further comprising third reading the spiral track and the concentric servo sectors to measure the reader/writer offset.