Title: Demodulation compensation for spiral servo tracks in hard disk drives
Abstract: A hard disk drive, and method and computer program therefor, comprising a hard disk having one or more spiral servo tracks written thereon, wherein each of the servo tracks comprises servo data comprising one or more synchronization symbols; a head adapted to produce a waveform as the head moves across the servo track; a sampler adapted to obtain a plurality of samples of the waveform, wherein the plurality of samples of the waveform comprises one of the synchronization symbols; and a processor adapted to determine an estimated location of a peak amplitude of the waveform with respect to the plurality of samples of the waveform, determine a sampling phase defined by a number of the samples between one of the synchronization symbols and a predetermined one of the samples of the waveform, determine a compensation amount based on a compensation curve and the sampling phase, determine a refined estimated location of the peak amplitude of the waveform based on the estimated location of the peak amplitude of the waveform and the compensation amount, and determine the location of the center of the servo track based on the refined estimated location of the peak amplitude of the waveform.
Patent Number: 7,002,761 Issued on 02/21/2006 to Sutardja, et al.
| Inventors:
|
Sutardja; Pantas (San Jose, CA);
Han; Ke (Fremont, CA)
|
| Assignee:
|
Marvell International Ltd. (Hamilton, BM)
|
| Appl. No.:
|
644166 |
| Filed:
|
August 20, 2003 |
| Current U.S. Class: |
360/29; 360/39 |
| Current Intern'l Class: |
G11B 20/06 (20060101) |
| Field of Search: |
360/46,26,27,28,53,51,48,17,75,770.8,770.6
369/442.6,592.1
714/746
|
References Cited [Referenced By]
U.S. Patent Documents
| 5526200 | Jun., 1996 | Yada.
| |
| 5574706 | Nov., 1996 | Verboom et al.
| |
| 5615065 | Mar., 1997 | Cheung.
| |
| 5668679 | Sep., 1997 | Swearingen et al.
| |
| 5892632 | Apr., 1999 | Behrens et al.
| |
| 5961658 | Oct., 1999 | Reed et al.
| |
| 6304407 | Oct., 2001 | Baker et al.
| |
| 6324030 | Nov., 2001 | Cheung et al.
| |
| 6344942 | Feb., 2002 | Yarmchuk.
| |
| 6366418 | Apr., 2002 | McEwen et al.
| |
| 6392834 | May., 2002 | Ellis.
| |
| 6429989 | Aug., 2002 | Schultz et al.
| |
| 6519107 | Feb., 2003 | Ehrlich et al.
| |
| 6529460 | Mar., 2003 | Belser.
| |
| 6674593 | Jan., 2004 | Jolly et al.
| |
| 6738205 | May., 2004 | Moran et al.
| |
| 6924960 | Aug., 2005 | Melkote et al.
| |
| 2002/0135927 | Sep., 2002 | Yatsu.
| |
Primary Examiner: Hudspeth; David
Assistant Examiner: Tzeng; Fred F.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application claims the benefit of U.S. Provisional Patent Application Ser.
No. 60/458,358 entitled "Demodulation Compensation For Spiral Servo," filed Mar.
27, 2003, the disclosure thereof incorporated by reference herein in its entirety.
Claims
What is claimed is:
1. A hard disk drive comprising:
a hard disk having one or more spiral servo tracks written thereon, wherein each
of the servo tracks comprises servo data comprising one or more synchronization symbols;
a head adapted to produce a waveform as the head moves across the servo track;
a sampler adapted to obtain a plurality of samples of the waveform, wherein the
plurality of samples of the waveform comprises one of the synchronization symbols; and
a processor adapted to
determine an estimated location of a peak amplitude of the waveform with respect
to the plurality of samples of the waveform,
determine a sampling phase defined by a number of the samples between one of
the synchronization symbols and a predetermined one of the samples of the waveform,
determine a compensation amount based on a compensation curve and the sampling phase,
determine a refined estimated location of the peak amplitude of the waveform
based on the estimated location of the peak amplitude of the waveform and the compensation
amount, and
determine the location of the center of the servo track based on the refined
estimated location of the peak amplitude of the waveform.
2. The apparatus of claim 1, wherein the processor is further adapted to:
determine an offset defined by a number of the samples of the waveform between
the estimated location of the peak amplitude of the waveform and a second predetermined
one of the samples; and
select as the compensation curve one of a plurality of candidate compensation
curves based on the offset.
3. The apparatus of claim 1, wherein the sampler is further adapted to:
obtain a window of the samples of the waveform from the hard disk, wherein the
window comprises n frames each comprising m of the samples of the waveform, and
wherein the m samples of the waveform within at least one of the n frames comprises
the one of the synchronization symbols.
4. The apparatus of claim 3, wherein the processor is further adapted to:
determine the estimated location of a peak amplitude of the waveform with respect
to the window of the samples.
5. The apparatus of claim 3, wherein the processor is further adapted to:
determine a sampling phase defined by a number of the samples between the one
of the synchronization symbols and the start of the one of the n frames comprising
the one of the synchronization symbols.
6. The apparatus of claim 1, further comprising:
a head controller adapted to cause the head to write further servo data to the
hard disk at a location based on the determined location of the center of the servo track.
7. The apparatus of claim 1, wherein the processor is further adapted to:
compute a demodulation value of the samples within each frame;
integrate the demodulation values;
select the integrated demodulation value having a value that is one-half of the
value of the largest integrated demodulation value;
select the sample corresponding to the selected integrated demodulation value; and
determine the estimated location of a peak amplitude of the waveform based on
the selected sample.
8. A hard disk drive comprising:
hard disk means for storing data, the hard disk means having one or more spiral
servo tracks written thereon, wherein each of the servo tracks comprises servo
data comprising one or more synchronization symbols;
head means for producing a waveform as the head moves across the servo track;
sampler means for obtaining a plurality of samples of the waveform, wherein the
plurality of samples of the waveform comprises one of the synchronization symbols; and
processor means for
determining an estimated location of a peak amplitude of the waveform with respect
to the plurality of samples of the waveform,
determining a sampling phase defined by a number of the samples between one of
the synchronization symbols and a predetermined one of the samples of the waveform,
determining a compensation amount based on a compensation curve and the sampling phase,
determining a refined estimated location of the peak amplitude of the waveform
based on the estimated location of the peak amplitude of the waveform and the compensation
amount, and
determining the location of the center of the servo track based on the refined
estimated location of the peak amplitude of the waveform.
9. The apparatus of claim 8, wherein the processor means:
determines an offset defined by a number of the samples of the waveform between
the estimated location of the peak amplitude of the waveform and a second predetermined
one of the samples; and
selects as the compensation curve one of a plurality of candidate compensation
curves based on the offset.
10. The apparatus of claim 8, wherein the sampler means:
obtains a window of the samples of the waveform from the hard disk, wherein the
window comprises n frames each comprising m of the samples of the waveform, and
wherein the m samples of the waveform within at least one of the n frames comprises
the one of the synchronization symbols.
11. The apparatus of claim 10, wherein the processor means:
determines the estimated location of a peak amplitude of the waveform with respect
to the window of the samples.
12. The apparatus of claim 10, wherein the processor means:
determines a sampling phase defined by a number of the samples between the one
of the synchronization symbols and the start of the one of the n frames comprising
the one of the synchronization symbols.
13. The apparatus of claim 8, further comprising:
head controller means for causing the head to write further servo data to the
hard disk at a location based on the determined location of the center of the servo track.
14. The apparatus of claim 8, wherein the processor means:
computes a demodulation value of the samples within each frame;
integrates the demodulation values;
selects the integrated demodulation value having a value that is one-half of
the value of the largest integrated demodulation value;
selects the sample corresponding to the selected integrated demodulation value; and
determines the estimated location of a peak amplitude of the waveform based on
the selected sample.
15. An apparatus for determining the location of the center of a servo track
embodied in a recording medium, wherein the servo track comprises servo data comprising
one or more synchronization symbols, the apparatus comprising:
a sampler adapted to obtain a plurality of samples of a waveform produced as
a head moves across the servo track, wherein the plurality of samples of the waveform
comprises one of the synchronization symbols; and
a processor adapted to
determine an estimated location of a peak amplitude of the waveform with respect
to the plurality of samples of the waveform,
determine a sampling phase defined by a number of the samples between one of
the synchronization symbols and a predetermined one of the samples of the waveform,
determine a compensation amount based on a compensation curve and the sampling phase,
determine a refined estimated location of the peak amplitude of the waveform
based on the estimated location of the peak amplitude of the waveform and the compensation
amount, and
determine the location of the center of the servo track based on the refined
estimated location of the peak amplitude of the waveform.
16. The apparatus of claim 15, wherein the processor is further adapted to:
determine an offset defined by a number of the samples of the waveform between
the estimated location of the peak amplitude of the waveform and a second predetermined
one of the samples; and
select as the compensation curve one of a plurality of candidate compensation
curves based on the offset.
17. The apparatus of claim 15, wherein the sampler is further adapted to:
obtain a window of the samples of the waveform from the recording medium, wherein
the window comprises n frames each comprising m of the samples of the waveform,
and wherein the m samples of the waveform within at least one of the n frames comprises
the one of the synchronization symbols.
18. The apparatus of claim 17, wherein the processor is further adapted to:
determine the estimated location of a peak amplitude of the waveform with respect
to the window of the samples.
19. The apparatus of claim 17, wherein the processor is further adapted to:
determine a sampling phase defined by a number of the samples between the one
of the synchronization symbols and the start of the one of the n frames comprising
the one of the synchronization symbols.
20. The apparatus of claim 15, further comprising:
a head controller adapted to cause the head to write further servo data to the
recording medium at a location based on the determined location of the center of
the servo track.
21. The apparatus of claim 15, wherein the processor is further adapted to:
compute a demodulation value of the samples within each frame;
integrate the demodulation values;
select the integrated demodulation value having a value that is one-half of the
value of the largest integrated demodulation value;
select the sample corresponding to the selected integrated demodulation value; and
determine the estimated location of a peak amplitude of the waveform based on
the selected sample.
22. An apparatus for determining the location of the center of a servo track
embodied in a recording medium, wherein the servo track comprises servo data comprising
one or more synchronization symbols, the apparatus comprising:
sampler means for obtaining a plurality of samples of a waveform produced as
a head moves across the servo track, wherein the plurality of samples of the waveform
comprises one of the synchronization symbols; and
processor means for
determining an estimated location of a peak amplitude of the waveform with respect
to the plurality of samples of the waveform,
determining a sampling phase defined by a number of the samples between one of
the synchronization symbols and a predetermined one of the samples of the waveform,
determining a compensation amount based on a compensation curve and the sampling phase,
determining a refined estimated location of the peak amplitude of the waveform
based on the estimated location of the peak amplitude of the waveform and the compensation
amount, and
determining the location of the center of the servo track based on the refined
estimated location of the peak amplitude of the waveform.
23. The apparatus of claim 22, wherein the processor means:
determines an offset defined by a number of the samples of the waveform between
the estimated location of the peak amplitude of the waveform and a second predetermined
one of the samples; and
selects as the compensation curve one of a plurality of candidate compensation
curves based on the offset.
24. The apparatus of claim 22, wherein the sampler means:
obtains a window of the samples of the waveform from the recording medium, wherein
the window comprises n frames each comprising m of the samples of the waveform,
and wherein the m samples of the waveform within at least one of the n frames comprises
the one of the synchronization symbols.
25. The apparatus of claim 24, wherein the processor means:
determines the estimated location of a peak amplitude of the waveform with respect
to the window of the samples.
26. The apparatus of claim 24, wherein the processor means:
determines a sampling phase defined by a number of the samples between the one
of the synchronization symbols and the start of the one of the n frames comprising
the one of the synchronization symbols.
27. The apparatus of claim 22, further comprising:
head controller means for causing the head to write further servo data to the
recording medium at a location based on the determined location of the center of
the servo track.
28. The apparatus of claim 22, wherein the processor means:
computes a demodulation value of the samples within each frame;
integrates the demodulation values;
selects the integrated demodulation value having a value that is one-half of
the value of the largest integrated demodulation value;
selects the sample corresponding to the selected integrated demodulation value; and
determines the estimated location of a peak amplitude of the waveform based on
the selected sample.
29. A method for determining the location of the center of a servo track embodied
in a recording medium, wherein the servo track comprises servo data comprising
one or more synchronization symbols, and wherein a waveform is produced in a head
as the head moves across the servo track, the method comprising:
obtaining a plurality of samples of the waveform, wherein the plurality of samples
of the waveform comprises one of the synchronization symbols;
determining an estimated location of a peak amplitude of the waveform with respect
to the plurality of samples of the waveform;
determining a sampling phase defined by a number of the samples between one of
the synchronization symbols and a predetermined one of the samples of the waveform;
determining a compensation amount based on a compensation curve and the sampling phase;
determining a refined estimated location of the peak amplitude of the waveform
based on the estimated location of the peak amplitude of the waveform and the compensation
amount; and
determining the location of the center of the servo track based on the refined
estimated location of the peak amplitude of the waveform.
30. The method of claim 29, further comprising:
determining an offset defined by a number of the samples of the waveform between
the estimated location of the peak amplitude of the waveform and a second predetermined
one of the samples; and
selecting as the compensation curve one of a plurality of candidate compensation
curves based on the offset.
31. The method of claim 29, wherein obtaining a plurality of samples of the waveform comprises:
obtaining a window of the samples of the waveform from the recording medium,
wherein the window comprises n frames each comprising m of the samples of the waveform,
and wherein the m samples of the waveform within at least one of the n frames comprises
the one of the synchronization symbols.
32. The method of claim 31, wherein determining an estimated location of a peak
amplitude of the waveform comprises:
determining the estimated location of a peak amplitude of the waveform with respect
to the window of the samples.
33. The method of claim 31, wherein determining a sampling phase comprises:
determining a sampling phase defined by a number of the samples between the one
of the synchronization symbols and the start of the one of the n frames comprising
the one of the synchronization symbols.
34. The method of claim 29, further comprising:
writing further servo data to the recording medium at a location based on the
determined location of the center of the servo track.
35. The method of claim 29, wherein determining an estimated location of the
peak amplitude of the waveform comprises:
computing a demodulation value of the samples within each frame;
integrating the demodulation values;
selecting the integrated demodulation value having a value that is one-half of
the value of the largest integrated demodulation value;
selecting the sample corresponding to the selected integrated demodulation value; and
determining the estimated location of a peak amplitude of the waveform based
on the selected sample.
36. A computer program embodied in a computer-readable recording medium embodying
instructions executable by a computer for determining the location of the center
of a servo track, wherein the servo track comprises servo data comprising one or
more synchronization symbols, and wherein a waveform is produced in a head as the
head moves across the servo track, the computer program comprising:
determining an estimated location of a peak amplitude of the waveform with respect
to a plurality of samples of the waveform, wherein the plurality of samples of
the waveform comprises one of the synchronization symbols;
determining a sampling phase defined by a number of the samples between one of
the synchronization symbols and a predetermined one of the samples of the waveform;
determining a compensation amount based on a compensation curve and the sampling phase;
determining a refined estimated location of the peak amplitude of the waveform
based on the estimated location of the peak amplitude of the waveform and the compensation
amount; and
determining the location of the center of the servo track based on the refined
estimated location of the peak amplitude of the waveform.
37. The computer program of claim 36, wherein the method further comprises:
determining an offset defined by a number of the samples of the waveform between
the estimated location of the peak amplitude of the waveform and a second predetermined
one of the samples; and
selecting as the compensation curve one of a plurality of candidate compensation
curves based on the offset.
38. The computer program of claim 36, further comprising:
obtaining a window of the samples of the waveform from the recording medium,
wherein the window comprises n frames each comprising m of the samples of the waveform,
and wherein the m samples of the waveform within at least one of the n frames comprises
the one of the synchronization symbols.
39. The computer program of claim 38, wherein determining an estimated location
of a peak amplitude of the waveform comprises:
determining the estimated location of a peak amplitude of the waveform with respect
to the window of the samples.
40. The computer program of claim 38, wherein determining a sampling phase comprises:
determining a sampling phase defined by a number of the samples between the one
of the synchronization symbols and the start of the one of the n frames comprising
the one of the synchronization symbols.
41. The computer program of claim 36, wherein the method further comprises:
writing further servo data to the recording medium at a location based on the
determined location of the center of the servo track.
42. The computer program of claim 36, wherein determining an estimated location
of the peak amplitude of the waveform comprises:
computing a demodulation value of the samples within each frame;
integrating the demodulation values;
selecting the integrated demodulation value having a value that is one-half of
the value of the largest integrated demodulation value;
selecting the sample corresponding to the selected integrated demodulation value; and
determining the estimated location of a peak amplitude of the waveform based
on the selected sample.
Description
BACKGROUND
The present invention relates generally to servo writing recording media disks.
More particularly, the present invention relates to self-servo writing recording
media disks having spiral servo tracks recorded thereupon.
Computer hard disk drives employ electromagnetic read/write heads to write
and read data on the magnetic layers of one or more rotating disks. The data is
stored on concentric data tracks on the disk surface. To guarantee the quality
of the information saved on and read back from the disk, it is important to position
the head at the center of the data track during both writing and reading. A closed-loop
servo system is often used to accurately position the head on the data track using
servo information embedded in a dedicated portion of each data track.
For such a servo system to operate properly, the servo information must be written
to the data tracks with great position accuracy. Often the servo information is
written on each surface as a set of spokes or wedges extending radially from the
inner diameter of each disk. The portion of a servo wedge on each data track contains
a sync field, an index mark, a gray-coded track number, and two or more fine-positioned
offset bursts configured in an echelon across the data track. Head position relative
to the center of a data track can be determined and corrected, if necessary, by
reading and noting the respective amplitudes and timings of these offset bursts.
Traditionally, a machine called a "servo writer" is used to write
the servo information to the disk. A servo writer includes a massive granite base
to minimize the effects of vibration, precision fixtures to hold the disk drive,
a precision laser interferometer-based actuator mechanism to place the head radially
with respect to the axis of rotation of the disks in the drive, and an external
clock head to position the servo wedges in time. These features cause present servo
writers to be extremely large and expensive. And as the density of the tracks on
the disk increases, the time required by the servo writer to write the servo data
to the disk also increases, which can create a bottleneck in the disk drive manufacturing
process at the servo writer. Because the time that a disk drive is being servo
written is expensive, it is desirable to reduce the length of this costly process.
To reduce the time required by the servo writer, techniques called "self-servo
writing" have been developed. A disk produced by one such technique, called "spiral
servo," is shown in FIG. 1. Instead of slowly writing the servo information to
each concentric data track on each surface of each disk in a hard drive, the servo
writer writes only a limited and much smaller number of spiral "servo tracks" (ST)
having a known data pattern from the outer diameter (OD) to the inner diameter
(ID) on only one surface of one of the disks by moving the head from OD to ID at
a constant rate while the disk is spinning at a constant rate.
Then, without the aid of the servo writer, the hard disk drive uses timing
information in the spiral servo tracks to determine the radial and circumferential
position of the head, and writes conventional servo data to concentric data tracks
on the disks using the hard drive's internal read/write head and servo system.
Since the servo writer is used only to write the spiral servo tracks, the amount
of servo-writing time on the servo writer is greatly reduced. The pattern on the
disk surface after self servo writing is given in FIG. 2. The diagonal tracks are
the residual spiral servo tracks ST, and the vertical strips represent the conventional
servo spokes (SS). Once the self servo writing is finished, the spiral tracks are
no longer needed, and can be overwritten by data tracks.
SUMMARY
In general, in one aspect, the invention features a hard disk drive comprising
a hard disk having one or more spiral servo tracks written thereon, wherein each
of the servo tracks comprises servo data comprising one or more synchronization
symbols; a head adapted to produce a waveform as the head moves across the servo
track; a sampler adapted to obtain a plurality of samples of the waveform, wherein
the plurality of samples of the waveform comprises one of the synchronization symbols;
and a processor adapted to determine an estimated location of a peak amplitude
of the waveform with respect to the plurality of samples of the waveform, determine
a sampling phase defined by a number of the samples between one of the synchronization
symbols and a predetermined one of the samples of the waveform, determine a compensation
amount based on a compensation curve and the sampling phase, determine a refined
estimated location of the peak amplitude of the waveform based on the estimated
location of the peak amplitude of the waveform and the compensation amount, and
determine the location of the center of the servo track based on the refined estimated
location of the peak amplitude of the waveform.
Particular implementations can include one or more of the following features.
The processor is further adapted to determine an offset defined by a number of
the samples of the waveform between the estimated location of the peak amplitude
of the waveform and a second predetermined one of the samples; and select as the
compensation curve one of a plurality of candidate compensation curves based on
the offset. The sampler is further adapted to obtain a window of the samples of
the waveform from the hard disk, wherein the window comprises n frames each comprising
m of the samples of the waveform, and wherein the m samples of the waveform within
at least one of the n frames comprises the one of the synchronization symbols.
The processor is further adapted to determine the estimated location of a peak
amplitude of the waveform with respect to the window of the samples. The processor
is further adapted to determine a sampling phase defined by a number of the samples
between the one of the synchronization symbols and the start of the one of the
n frames comprising the one of the synchronization symbols. Implementations comprise
a head controller adapted to cause the head to write further servo data to the
hard disk at a location based on the determined location of the center of the servo
track. The processor is further adapted to compute a demodulation value of the
samples within each frame; integrate the demodulation values; select the integrated
demodulation value having a value that is one-half of the value of the largest
integrated demodulation value; select the sample corresponding to the selected
integrated demodulation value; and determine the estimated location of a peak amplitude
of the waveform based on the selected sample.
In general, in one aspect, the invention features an apparatus for determining
the location of the center of a servo track embodied in a recording medium, wherein
the servo track comprises servo data comprising one or more synchronization symbols,
the apparatus comprising a sampler adapted to obtain a plurality of samples of
a waveform produced as a head moves across the servo track, wherein the plurality
of samples of the waveform comprises one of the synchronization symbols; and a
processor adapted to determine an estimated location of a peak amplitude of the
waveform with respect to the plurality of samples of the waveform, determine a
sampling phase defined by a number of the samples between one of the synchronization
symbols and a predetermined one of the samples of the waveform, determine a compensation
amount based on a compensation curve and the sampling phase, determine a refined
estimated location of the peak amplitude of the waveform based on the estimated
location of the peak amplitude of the waveform and the compensation amount, and
determine the location of the center of the servo track based on the refined estimated
location of the peak amplitude of the waveform.
Particular implementations can include one or more of the following features.
The processor is further adapted to determine an offset defined by a number of
the samples of the waveform between the estimated location of the peak amplitude
of the waveform and a second predetermined one of the samples; and select as the
compensation curve one of a plurality of candidate compensation curves based on
the offset. The sampler is further adapted to obtain a window of the samples of
the waveform from the recording medium, wherein the window comprises n frames each
comprising m of the samples of the waveform, and wherein the m samples of the waveform
within at least one of the n frames comprises the one of the synchronization symbols.
The processor is further adapted to determine the estimated location of a peak
amplitude of the waveform with respect to the window of the samples. The processor
is further adapted to determine a sampling phase defined by a number of the samples
between the one of the synchronization symbols and the start of the one of the
n frames comprising the one of the synchronization symbols. Implementations comprise
a head controller adapted to cause the head to write further servo data to the
recording medium at a location based on the determined location of the center of
the servo track. The processor is further adapted to compute a demodulation value
of the samples within each frame; integrate the demodulation values; select the
integrated demodulation value having a value that is one-half of the value of the
largest integrated demodulation value; select the sample corresponding to the selected
integrated demodulation value; and determine the estimated location of a peak amplitude
of the waveform based on the selected sample.
In general, in one aspect, the invention features a method and computer program
for determining the location of the center of a servo track embodied in a recording
medium, wherein the servo track comprises servo data comprising one or more synchronization
symbols, and wherein a waveform is produced in a head as the head moves across
the servo track. It comprises obtaining a plurality of samples of the waveform,
wherein the plurality of samples of the waveform comprises one of the synchronization
symbols; determining an estimated location of a peak amplitude of the waveform
with respect to the plurality of samples of the waveform; determining a sampling
phase defined by a number of the samples between one of the synchronization symbols
and a predetermined one of the samples of the waveform; determining a compensation
amount based on a compensation curve and the sampling phase; determining a refined
estimated location of the peak amplitude of the waveform based on the estimated
location of the peak amplitude of the waveform and the compensation amount; and
determining the location of the center of the servo track based on the refined
estimated location of the peak amplitude of the waveform.
Particular implementations can include one or more of the following features.
Implementations comprise determining an offset defined by a number of the samples
of the waveform between the estimated location of the peak amplitude of the waveform
and a second predetermined one of the samples; and selecting as the compensation
curve one of a plurality of candidate compensation curves based on the offset.
Obtaining a plurality of samples of the waveform comprises obtaining a window of
the samples of the waveform from the recording medium, wherein the window comprises
n frames each comprising m of the samples of the waveform, and wherein the m samples
of the waveform within at least one of the n frames comprises the one of the synchronization
symbols. Determining an estimated location of a peak amplitude of the waveform
comprises determining the estimated location of a peak amplitude of the waveform
with respect to the window of the samples. Determining a sampling phase comprises
determining a sampling phase defined by a number of the samples between the one
of the synchronization symbols and the start of the one of the n frames comprising
the one of the synchronization symbols. Implementations comprise writing further
servo data to the recording medium at a location based on the determined location
of the center of the servo track. Determining an estimated location of the peak
amplitude of the waveform comprises computing a demodulation value of the samples
within each frame; integrating the demodulation values; selecting the integrated
demodulation value having a value that is one-half of the value of the largest
integrated demodulation value; selecting the sample corresponding to the selected
integrated demodulation value; and determining the estimated location of a peak
amplitude of the waveform based on the selected sample.
The details of one or more implementations are set forth in the accompanying
drawings and the description below. Other features will be apparent from the description
and drawings, and from the claims.
DESCRIPTION OF DRAWINGS
FIG. 1 shows a disk produced by one such technique, called "spiral servo."
FIG. 2 shows the pattern on the disk surface after self servo writing.
FIG. 3 shows a hard disk drive according to a preferred embodiment.
FIG. 4 shows a waveform produced as a head passes over a servo track on the disk.
FIG. 5 is a flowchart of a process 500 for self-servo writing that can
be performed by the hard disk drive of FIG. 3 according to a preferred embodiment.
FIG. 6 shows a typical distribution of demodulation values for the waveform
of FIG. 4.
FIG. 7 shows an integration curve with the integration values normalized to
one for the demodulation values of FIG. 6.
FIG. 8 shows an error curve for the waveform of FIG. 4.
FIG. 9 shows a compensation curve for the waveform of FIG. 4.
FIG. 10 shows the residual peak positioning error after compensation.
FIG. 11 shows the range of a compensation curve as a function of the window offset.
FIG. 12 shows the DC offset of a compensation curve as a function of the window offset.
FIG. 13 shows the residual peak positioning error after compensation using window offset.
The leading digit(s) of each reference numeral used in this specification indicates
the number of the drawing in which the reference numeral first appears.
DETAILED DESCRIPTION
FIG. 3 shows a hard disk drive 300 according to a preferred embodiment
comprising a disk 302 having one or more spiral servo tracks ST written
thereupon, and a head 304 connected to a servo 306 by an arm 308.
Servo 306 comprises a sampler 310, a processor 312, and a
head controller 314. Processor 312 can be implemented as a general-purpose
processor or as a dedicated digital circuit. As head 304 passes over a servo
track ST, a waveform is generated in head 304, such as the waveform depicted
in FIG. 4. Because head 304 crosses each spiral servo track ST at an angle,
the waveform has a diamond-shaped amplitude. Assuming that head 304 has
the same width as the servo spiral track, the peak of the waveform occurs only
at the moment when head 304 is completely inside the spiral servo track
during an intersection. The position of head 304 with respect to disk 302
can be determined by accurately determining the location of the peak of the waveform.
FIG. 5 is a flowchart of a process 500 for self-servo writing that can
be performed by hard disk drive 300 of FIG. 3 according to a preferred embodiment.
Sampler 310 obtains a plurality of samples of the waveform (step 502).
The spiral servo tracks are written using a repeated pattern of channel bits
that is generated using Gray-coded symbols. The symbols are coded according to
the following simple rules:
- symbol '1' is encoded as channel bits '0011'; and
- symbol '0' is encoded as channel bits '1100'.
A typical pattern consists of a certain number of consecutive '1' symbols followed
by one '0' symbol. The following description considers a pattern consisting of
29 '1' symbols followed by one '0' symbol. In the present description, the '0'
symbol is referred to as a "synchronization symbol." Of course, other patterns
and encoding methods can be used with the present invention instead. In the present
example, the length of the pattern is 30 Gray-coded symbols which collectively
consist of 120 channel bits. One complete pattern is referred as one frame. A typical
length of the diamond-shaped waveform is six frames, which collectively consist
of 720 channel bits.
Using the samples obtained by sampler 310, processor 312 determines
an estimated location of the peak amplitude of the waveform (step 504).
In one embodiment this estimated location is determined according to the following
conventional technique.
Because of the uncertainty of the position of the diamond-shaped waveform,
the demodulation window opened by sampler 310 to sample the waveform is
longer than the waveform. In the present example, for a waveform six frames long,
a seven-frame window is opened. Therefore, even if the position of the waveform
is off by 15 symbols (60 channel bits), the demodulation window still captures
the complete diamond-shaped waveform.
Once the waveform is captured, a frame-wise demodulation is performed. The demodulation
window is divided into 7 frames. Within each frame, the waveform is demodulated
using a conventional approach by correlating the Baud-rate sampled waveform with
two demodulation vectors: [1, 0, -1, 0, . . . ] and [0, 1, 0, -1, . . . ].
Suppose the Baud-rate sampled waveform is
X=[x
0 . . . x
4N-1] (1)
and the two demodulation vectors are
V0=[1,0,-1,0, . . . 1,0,-1,0] (2)
and
V1=[0,1,0,-1, . . . 0,1,0,-1] (3)
where N is the number of Gray-coded symbols within each frame, and vectors
V
0 and V
1 vectors are of the same length as vector X.
For each frame i, process 500 calculates the two inner products
si=X·V0T (4)
and
ci=X·V1T (5)
and computes a demodulation value given by
ai=√{square root over (si2+ci2)}. (6)
Given that the center of the seven-frame demodulation window is perfectly aligned
with the center of the diamond-shaped waveform, a typical distribution of the demodulation
values is as shown in FIG. 6.
The demodulation values are integrated over the seven frames. FIG. 7 shows the
integration curve with the integration values normalized to one. Using this curve,
the point on the horizontal axis that corresponds to a 50-percent integration value
is taken as the estimated location of the peak amplitude of the diamond-shaped waveform.
The accuracy of the above-described conventional method for determining an estimated
location of the peak amplitude of the waveform is affected by the phase of the
waveform. Because the head can intercept the spiral servo track at any place, a
synchronization symbol can be located anywhere within a frame. This uncertainty
is referred to as the sampling phase of the waveform.
To illustrate the effect of the sampling phase on accuracy, consider a 16-times
over-sampled waveform having six frames of 30 Gray-coded symbols: 29 '1's and one
'0'. The sampling phase can be described by the location of the synchronization
symbol (here, the '0' symbol) within one of the frames. Because there is a synchronization
symbol in each frame, the underlying pattern is periodic over all of the frames.
Therefore, there is no need to extend the phase determination beyond one frame.
The length of one frame is 120 bit cells. With an over-sampling rate of 16, there
are 120×16=1920 different possible sampling phases.
Assuming no noise, and that the 6-frame waveform is perfectly centered in
the 7-frame window, the error in the estimated location of the peak amplitude of
the waveform versus different sampling phases can be determined. An error curve
for the present example is shown in FIG. 8. As can be seen, the error has a well-defined
relationship with the sampling phase. Therefore, if the sampling phase is known,
then based on this curve, a certain amount of compensation can be applied so that
the error in peak positioning may be greatly reduced. Because of the specific way
the spiral tracks are written on the disk, the sampling phase can be obtained with
resolution as good as one tenth of a clock cycle.
Note that on the curve of FIG. 8, there is a small-amplitude, high-frequency
ripple superimposed on a low-frequency component. The resolution of the knowledge
on sampling phase is good enough to compensate the low frequency component. But
the high frequency ripple is left uncompensated.
Based on the curve of FIG. 8, a compensation curve is easily generated. A compensation
curve for the present example is shown in FIG. 9. After such compensation, the
residual peak positioning error is shown in FIG. 10. As can be seen, residual peak
positioning error is reduced by approximately a factor 10 by this compensation technique.
Processor 312 determines the sampling phase of the samples obtained
by sampler 310, preferably as described above (step 506). Processor
312 then determines a compensation amount based on the sampling phase and
a compensation curve such as that of FIG. 9 (step 508). Processor 312
then determines a refined estimated location of the peak amplitude of the waveform
based on the estimated location of the peak amplitude of the waveform and the compensation
amount (step 510), and determines the location of the center of the spiral
servo track based on the refined estimated location of the peak amplitude of the
waveform (step 512). In some embodiments, steps 506 through 510
are repeated one or more times to further refine the estimated location of the
peak amplitude of the waveform.
Finally, head controller 314 positions head 304 over the recording
medium at a location based on the determined location of the center of the spiral
servo track (step 514). the purpose of this invention is not to determine
the location of the center of the spiral tracks, but to determine the head's current
position, both radially and circumferentially. Processor 312 determines
the circumferential location of head 304 with respect to disk 302
based on the peak of the waveform, which corresponds to the center of the spiral
servo track. Processor 312 then determines the radial position of head 304
with respect to disk 302 based on the identity of the spiral servo track
and the circumferential location of head 304. Once the radial and circumferential
position of head 304 are determined, head controller 314 can precisely
move head 304 to the target positions to starting writing the conventional
servo wedges there. Head 304 then writes servo data, such as the conventional
servo data described above, to the recording medium (step 516).
Another factor that affects the accuracy of peak positioning is called "window
offset." Due to the nature of the peak-locating process, the position of the waveform
may not be known exactly. Therefore, the waveform may not be centered perfectly
in the demodulation window. The amount by which the window is misaligned with the
waveform, called the "window offset," affects the amount of compensation that should
be applied to minimize the peak positioning error. In fact, both the range and
the DC offset of compensation curves, such as that of FIG. 9, are functions of
the window offset. But once these two parameters, range and DC offset, are determined,
the compensation curve is well-defined. The range and the DC offset as functions
of the window offset are shown in FIGS. 11 and 12, respectively. These two functions
are also well-defined.
Some embodiments of the present invention compensate for window offset according
to the following process. A plurality of compensation curves are generated using
the known range and DC offsets for each window offset. These compensation curves
can be stored in a memory within servo 306.
Processor 312 determines the window offset, which is the distance
between the center of the seven-frame demodulation window and the center of the
six-frame diamond-shaped waveform. In the first cycle of waveform processing, the
center of the diamond-shaped waveform may not be known exactly. However, even this
inaccurate information about the center of the waveform is useful to begin an iterative
process of demodulation and compensation. With each cycle, the estimate of window
offset improves, as does the subsequent compensation. The magnitude of the window
offset can be defined as the number of the samples of the waveform between the
estimated location of the peak amplitude of the waveform obtained in step 504
of FIG. 5 and a predetermined one of the samples, such as the center sample in
the frame comprising the peak amplitude. Based on the window offset, processor
312 selects one of the stored compensation curves. The selected compensation
curve is then used as the compensation curve in step 508.
With this compensation scheme, which is uniform over all possible integration
window offsets, the residual error in peak positioning is slightly increased, compared
to the case of no window offset. Because this compensation scheme here works well
for all possible window offset values, it greatly reduces the error even when the
window offset is zero, as shown in FIG. 13.
Before compensation, the maximum position error is about 0.04 frame, or 5
clock cycles in the noiseless case. This translates into 0.7 percent of a track
width in the radial position error. After the compensation described above, the
peak position error is reduced to 0.006 frame, or 0.7 clock cycle. The corresponding
radial position error is then about 0.1 percent, which is very acceptable.
The invention can be implemented in digital electronic circuitry, or in computer
hardware, firmware, software, or in combinations of them. Apparatus of the invention
can be implemented in a computer program product tangibly embodied in a machine-readable
storage device for execution by a programmable processor; and method steps of the
invention can be performed by a programmable processor executing a program of instructions
to perform functions of the invention by operating on input data and generating
output. The invention can be implemented advantageously in one or more computer
programs that are executable on a programmable system including at least one programmable
processor coupled to receive data and instructions from, and to transmit data and
instructions to, a data storage system, at least one input device, and at least
one output device. Each computer program can be implemented in a high-level procedural
or object-oriented programming language, or in assembly or machine language if
desired; and in any case, the language can be a compiled or interpreted language.
Suitable processors include, by way of example, both general and special purpose
microprocessors. Generally, a processor will receive instructions and data from
a read-only memory and/or a random access memory. Generally, a computer will include
one or more mass storage devices for storing data files; such devices include magnetic
disks, such as internal hard disks and removable disks; magneto-optical disks;
and optical disks. Storage devices suitable for tangibly embodying computer program
instructions and data include all forms of non-volatile memory, including by way
of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory
devices; magnetic disks such as internal hard disks and removable disks; magneto-optical
disks; and CD-ROM disks. Any of the foregoing can be supplemented by, or incorporated
in, ASICs (application-specific integrated circuits).
A number of implementations of the invention have been described. Nevertheless,
it will be understood that various modifications may be made without departing
from the spirit and scope of the invention. Please list any additional modifications
or variations. Accordingly, other implementations are within the scope of the following claims.
*
