U.S. patent application number 12/237550 was filed with the patent office on 2009-02-26 for compensation for variable servo track width.
This patent application is currently assigned to IOMEGA CORPORATION. Invention is credited to Gregory M. Allen, Scott E. Chase, Lawrence Moon, Daniel D. Rochat.
Application Number | 20090052081 12/237550 |
Document ID | / |
Family ID | 38876337 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090052081 |
Kind Code |
A1 |
Chase; Scott E. ; et
al. |
February 26, 2009 |
Compensation for Variable Servo Track Width
Abstract
A track map of a disk drive is generated to compensate for
various conditions. A track map is a stored measurement of every
"half" servo track width on a disk. Every servo half track width is
measured in the factory on a disk and then the data is stored on
the disk for use during drive operations. When an individual track
is accessed, the appropriate servo half track width data is pulled
from memory and is used to adjust a scale factor for that local
half track width. The track map may be used to compensate for
various conditions including varying servo track width, microjog
distance, and data track spacing.
Inventors: |
Chase; Scott E.; (South
Ogden, UT) ; Rochat; Daniel D.; (Ogden, UT) ;
Moon; Lawrence; (Layton, UT) ; Allen; Gregory M.;
(Layton, UT) |
Correspondence
Address: |
Procopio, Cory, Hargreaves & Savitch, LLP;IOMEGA/EMC
530 B Street, 21st Floor
San Diego
CA
92101
US
|
Assignee: |
IOMEGA CORPORATION
San Diego
CA
|
Family ID: |
38876337 |
Appl. No.: |
12/237550 |
Filed: |
September 25, 2008 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
|
11478245 |
Jun 29, 2006 |
7460328 |
|
|
12237550 |
|
|
|
|
Current U.S.
Class: |
360/77.08 ;
G9B/5.216 |
Current CPC
Class: |
G11B 5/59627
20130101 |
Class at
Publication: |
360/77.08 ;
G9B/5.216 |
International
Class: |
G11B 5/596 20060101
G11B005/596 |
Claims
1. A method for compensating for variable servo track width in an
information storage disk comprising: creating a track map of the
storage disk, wherein the track map includes a stored determination
of an average width of every half servo track on the information
storage disk; and adjusting a scale factor for a servo track width
based on the track map.
2. The method of claim 1, further comprising correcting a position
error signal for the servo track width based on the scale
factor.
3. The method of claim 1, wherein the scale factor is BSF1 ("Burst
Scale Factor 1").
4. The method of claim 3, further comprising calculating a
multiplicative scale factor BSF2 ("Burst Scale Factor 2") such that
the servo track width is represented by 4096 bits.
5. The method of claim 4, wherein BSF2 is greater than one for a
narrow track and less than one for a wide track.
6. The method of claim 1, further comprising computing an average
of the widths of adjacent half track to apply to a window of
positions including the boundary between the adjacent half
tracks.
7. The method of claim 6, wherein the window includes positions
within a positive and negative 5% range of the boundary between the
adjacent half tracks.
8. An information storage system comprising: information storage
media having a plurality of servo tracks; and a track map defining
a location for each of the plurality of servo tracks, wherein the
track map is stored on the information storage media, and wherein
the track map includes a stored determination of an average width
of every half servo track of the information storage media.
9. The information storage system of claim 8, further comprising: a
drive containing an actuator and read/write heads; and a removable
cartridge containing the information storage media.
10. The information storage system of claim 8, wherein the track
map is used to compensate for variable track width.
11. The information storage system of claim 10, wherein the track
map is used to create a scale factor to adjust for the variable
track width.
12. The information storage system of claim 8, wherein the track
map is used to compensate for variations in microjog distance.
13. The information storage system of claim 12, wherein the track
map is used to adjust the microjog distance based on track width to
position a write element.
14. The information storage system of claim 8, wherein the track
map is used to compensate for variations in servo track
spacing.
15. The information storage system of claim 13, wherein the track
map is used to place data tracks at a desired spacing.
16. A method of creating a track map for a data storage system
comprising: measuring the half servo track widths on an information
storage media, wherein the information storage media includes a
plurality of servo tracks, and wherein a half servo track width is
determined for every half servo track; and storing the half servo
track widths in a track map data file on the information storage
media.
17. The method of claim 16, further comprising compensating for
variable track width using the track map.
18. The method of claim 16, further comprising compensating for
microjog distance using the track map.
19. The method of claim 16, further comprising compensating for
servo track spacing using the track map.
20. The method of claim 16, wherein the information storage media
is housed in a removable data cartridge.
Description
CROSS-REFERENCE TO RELATED APPLICATION
[0001] This application is a continuation of application Ser. No.
11/478,245 filed Jun. 29, 2006, which is incorporated by reference
herein.
TECHNICAL FIELD
[0002] This invention relates to computer storage products, and
more particularly to compensation for variable servo track width in
removable data cartridges.
BACKGROUND
[0003] Over the past twenty years, computer technology has evolved
very rapidly. One aspect of this evolution has been a progressively
growing demand for increased storage capacity in memory devices. In
order to provide a high storage density at a reasonable cost, one
of the most enduring techniques has been to provide a rotatable
hard disk with a layer of magnetic material thereon, and a
read/write head which is supported for movement adjacent the disk
and can transfer information to and from the disk.
[0004] Early disk drives included a read/write head having a single
read/write element, which was used both for writing data and
reading data. However, there has been a progressively increasing
demand for disk drives with significantly higher storage densities,
and one result is that new types of heads have come into common
use, examples of which include the magneto-resistive (MR) head, and
the giant magneto-resistive (GMR) head. These MR and GMR heads
typically have one element for writing data and a separate element
for reading data, and these read and write elements are physically
spaced from each other.
[0005] As is known in the art, a head can be positioned with
respect to a disk by using feedback control based on servo
information read from the disk with a read element of the head. In
a head with spaced read and write elements, the read element is
used to position the head relative to the disk not only for
reading, but also for writing. One aspect of this is that, as the
head is moved relative to the disk, the orientation of the read and
write elements varies with respect to the tracks on the disk, such
that the write element is typically aligned with a track that is
different from the track with which the read element is aligned.
Consequently, in order to correctly position the write element over
a selected track for the purpose of writing data to that track, the
read element must be positioned at a location which is radially
offset from the selected track. This radial offset is referred to
as a "microjog", and has a magnitude which varies as the head moves
radially with respect to the disk. Techniques have been developed
for calculating microjog values, and have been generally adequate
for their intended purposes, but they have not been satisfactory in
all respects.
[0006] As one aspect of this, there are existing disk drives in
which the disk is rotatably supported in a removable cartridge, and
in which the head is movably supported in a drive unit that can
removably receive the cartridge. A given drive unit must be able to
work with any of several similar and interchangeable cartridges,
and any given cartridge must be capable of working in any of a
number of compatible drive units. The removability of the cartridge
introduces a number of real-world considerations into the system,
and these considerations affect the accurate calculation of a
microjog value.
[0007] For example, the cartridges have manufacturing tolerances
which vary from cartridge. Thus, from cartridge to cartridge, there
will be some variation relative to the cartridge housing of the
exact position of the axis of rotation of the disk. As another
example, two different cartridges may have slightly different
mechanical seatings when they are inserted into the same drive
unit. In fact, a given cartridge may experience different
mechanical seatings on two successive insertions into the same
drive unit. Real-world variations of this type cause small
variations in the orientation of the read/write head with respect
to the tracks on the disk, and thus affect accurate calculation of
a microjog value.
[0008] In order to realize higher data storage densities in systems
of the type which utilize removable cartridges, it is desirable to
be able to use read/write heads that facilitate high storage
densities, especially read/write heads that have spaced read and
write elements, such as MR and GMR heads. However, due to
real-world considerations of the type discussed above, accurate
calculation of a microjog value has presented problems in the
context of a removable cartridge. Accordingly, existing systems
that use removable cartridges have continued to use read/write
heads with a single read/write element, with the consequence that
the storage capacities are significantly less than the storage
capacities desired by consumers.
[0009] Current magnetic recording devices use an inductive-write
and GMR-read dual element head (FIG. 1). Due to head manufacturing
limitations, the read and write elements are not necessarily
aligned along a centerline.
[0010] In addition, most hard drives place the head on a rotary
actuator. When the actuator rotates around its pivot to position
the head over a particular track, the GMR reader and inductive
writer can be several servo tracks apart due to the finite distance
between them (FIG. 2).
[0011] Hard drives minimize the complications caused by this head
and actuator geometry by positioning the reader at a servo track
center and writing data wherever the writer happens to be. Adjacent
tracks are written in the same manner. If the servo tracks are
evenly spaced there should always be a constant distance between
the centerlines of the written data (FIG. 3). The hard drive
disk(s) can not be removed, so the read and write elements will
always be the same. Knowledge of the reader to writer spacing and
offset is required to accurately place the reader over the written
data upon read back.
[0012] A removable hard disk drive such as the REV drive from
Iomega Corporation uses the same dual element head, but allows the
user to remove the disk from the drive. This means the data written
on a given disk can come from multiple REV drives. Each drive has
its own set of heads, and the separation and offset between the
reader and writer can vary from head to head. Using the hard drive
technique described above, which places the reader at the track
center and writing data wherever the writer is, will not work
because of this drive to drive variation. To ensure all REV drives
can read and write any cartridge, the REV drive requires the writer
to be placed over the data track center. This implies the head
geometry for each drive has to be pre-determined. During data
writes, the GMR reader is positioned wherever necessary so the data
is always written down the data track center. All REV drives expect
to find the data in the center of the defined data track.
[0013] FIG. 4 illustrates a typical servo and data track layout.
Servo tracks are defined by writing specific patterns to the disk
which are never overwritten. The reader width is designed to be
narrower (.about.60%) than the writer. This allows the servo track
pitch to be higher than the data track pitch. Ideally the servo
tracks would be evenly spaced. Due to spindle motor runout,
windage, vibration, temperature variations, etc., the servo tracks
vary in width. Inaccuracies in the servo track widths can cause
inaccuracies in positioning the GMR reader and inductive writer.
Positioning inaccuracies ultimately cause two problems. The first
is known as "data encroachment". Data encroachment occurs when the
writer is positioned away from the data track center and overwrites
part of the adjacent track data. This is catastrophic if the
adjacent track data is unrecoverable. The second issue occurs
during data read back. Since the data is expected to be on the data
track center, the servo system will position the reader there
first. If problems occur reading the data, the servo system will
re-position the head at varying off track locations in an attempt
to find it. This process takes time which affects the overall data
throughput.
[0014] As described above, accurately positioning the reader and
writer is important to successfully storing data without data
encroachment, and efficiently reading that data back. Varying servo
track widths cause position errors in two different ways. The first
involves inaccuracies in generating a linear position error signal
from the servo information written on the disk within a given servo
track. The second comes from the fact that the GMR reader can be
several servo tracks away from the inductive writer. This is the
cumulative effect of many servo track widths in error. Since the
writer needs to be positioned over a data track center, the reader
to writer distance at a particular disk radius must be
pre-determined. Using the servo tracks as the measuring tool, any
inaccuracy in servo track width directly translates to writer
positioning errors. What is needed is an efficient manner to
compensate for these errors.
SUMMARY
[0015] A track map of a disk drive is generated to compensate for
various conditions. A track map is a stored measurement of the
average width of every "half" servo track on a disk. The average
servo half track width is measured for all servo half tracks on a
disk in the factory and then the data is stored on the disk for use
during drive operations. When an individual track is accessed, the
appropriate servo half track width data is pulled from memory and
is used to adjust a scale factor for that local half track width.
The track map may be used to compensate for various conditions
including varying servo track width, microjog distance, and servo
track spacing.
DESCRIPTION OF DRAWINGS
[0016] These and other features and advantages of the invention
will become more apparent upon reading the following detailed
description and upon reference to the accompanying drawings.
[0017] FIG. 1 dual element heads used in a typical prior art hard
drive.
[0018] FIG. 2 shows a prior art GMR reader and inductive writer
separation and offset.
[0019] FIG. 3 shows evenly spaced servo tracks on a prior art hard
drive.
[0020] FIG. 4 illustrates a typical prior art track layout.
[0021] FIG. 5 shows a typical removable hard disk servo burst
pattern.
[0022] FIG. 6 illustrates an ideal Position Error Signal.
[0023] FIG. 7 shows the Position Error Signal saturation.
[0024] FIG. 8 illustrates the generation of a linear Position Error
Signal.
[0025] FIG. 9 shows the affect of varying magnetic read width on
the Position Error Signal.
[0026] FIG. 10 illustrates the head width calibration.
[0027] FIG. 11 illustrates the head width scaling using BSF1.
[0028] FIG. 12 shows the nonlinear position error signal on a
narrow servo track.
[0029] FIG. 13 shows the nonlinear position error signal on a wide
servo track.
[0030] FIG. 14 illustrates the Position Error Signal gain
correction using BSF2.
[0031] FIG. 15 shows the servo half track width measurement.
[0032] FIG. 16 shows the average BSF2 at half track boundaries.
[0033] FIG. 17 illustrates the microjog distance between the reader
and writer.
[0034] FIG. 18 shows varying servo track widths causing microjog
errors.
[0035] FIG. 19 illustrates the present invention correcting
microjog errors with the track map.
[0036] FIG. 20 illustrates an adjusted data track layout using the
track map.
DETAILED DESCRIPTION
[0037] To accurately position the GMR read element anywhere on a
servo track, a linear error signal needs to be developed. This is
accomplished using the servo patterns written on the disk. FIG. 5
is an example of the REV servo pattern. It consists of four bursts
of information labeled A, B, C, and D. The read element has an
effective read width known as the magnetic read width (MRW). This
characteristic describes the physical distance over which the head
senses magnetic fields. The read element senses the magnetic field
of each burst as it passes underneath the head. The digital read
channel chip, that receives the signal from the head, calculates
the amplitude for each burst. The amplitude is represented in a ten
bit word. The center of a servo track is defined to be where the
amplitudes of A and B are equal. As the head moves away from the
track center, the measured amplitudes of A, B, C, and D vary.
[0038] FIG. 6 illustrates an ideal position error signal generated
as the head moves across several servo tracks. In reality the error
signal begins to saturate just beyond the 1/4 track as shown in
FIG. 7. This is why the quadrature pattern is used. Notice as the
A-B signal begins to saturate, the C-D signal is in its linear
region.
[0039] By piecing the appropriate regions together, a continuously
linear error signal can be mathematically developed across any
servo track (refer to FIG. 8).
[0040] There are several important pieces of information in FIG. 8.
The first is the odd quarter (1/4 and 3/4) track positions are
unique in that the absolute value of A-B equals the absolute value
of C-D (|A-B|=|C-D|). This fact is used to calibrate the MRW of the
GMR head and will be described in detail later. The second is that
A-B is linear between -1/4 track to +1/4 track and C-D is linear
between +1/4 track and +3/4 track. This feature must be true for
the position error signal to be linear across any given servo track
as the segments are mathematically pieced together. The third is
that, ideally, the position error signal for all servo tracks is
the same. In reality, the MRW of the GMR heads vary head to head as
well as the servo track widths. Both issues can be major sources of
positioning error if not dealt with carefully.
[0041] Assuming a nominal width servo track, FIG. 9 shows the
affect the varying MRW has on the ideal A-B (or C-D) position error
signal.
[0042] In an effort to make all heads look the same, the drive code
performs a "head width" calibration at power up. To measure the MRW
of the GMR element, the servo system moves the head to the 1/4
track and 1/4 track position (refer to FIG. 10).
[0043] Notice at these two points |A-B|=|C-D|. These points are
unique in that this relationship holds true regardless of the MRW.
The overall goal is to have the generated position error signal
represent one servo track with 4096 bits. For example, using a
nominal MRW head, the |1/4 track| is measured to be 252 bits. The
head width calibration algorithm then scales this to the desired
4096 bits/servo track. The scale factor is known as Burst Scale
Factor 1 (BSF1) and can be calculated as follows:
252 bits measured/(1/4 servo track)=1008 (bits/servo track)
BSF1=4096 (bits/servo track)/1008 (bits/servo track)
BSF1=4.06
[0044] From this point forward, every position measurement made
with that head is first scaled by BSF1. The answer is then
referenced to 4096 to determine the fractional track position. FIG.
11 graphically shows the head width measurement on a nominal width
servo track and the affect of BSF1.
[0045] As was mentioned earlier, the servo tracks are not all the
same width. This fact can also produce inaccuracies in head
positioning. The servo demodulation algorithm pieces the
appropriate position error segments of A-B and C-D to produce a
linear error signal across the disk (refer to FIG. 8). The
algorithm accomplishes this by measuring the amplitudes of all the
bursts (A, B, C, D) and determines the appropriate mathematical
operation to develop the head position.
[0046] FIGS. 12 and 13 graphically show what happens to the
position error signal if the head width calibration occurs on a
nominal width track and is then applied to servo tracks that are
narrow and wide respectively.
[0047] As seen in both figures, a discontinuity occurs at the 1/4
track position. The head width calibration produces a scaling
factor (BSF1) such that all nominal width servo tracks will be
represented by 4096 bits/track (1024 bits/1/4 tracks). In the wide
or narrow servo track case, the gain of 4096 bits/servo track is
not accurate. Therefore, when the demodulation algorithm blindly
pieces the linear sections together, a discontinuity occurs. If the
servo is commanded to position the GMR element in these regions,
the head position becomes inaccurate.
[0048] The quality of the position error signal, for any given
track, is dependent on reader width and servo track width.
Separating the two variables can be difficult. As discussed
earlier, the head width calibration makes use of the 1/4 track
points. When the servo tracks vary in width it makes it very
difficult to accurately measure the MRW of the head. Performing the
head width calibration on one random track could produce inaccurate
results if that track happened to be wide or narrow. On a REV disk
the average servo track width is nominal. Therefore, measuring the
head width on many servo tracks (taking measurements at the 1/4
track points) and averaging the results produces a very accurate
measure of the MRW for a nominal width track. Unfortunately, the
time to do this measurement is long and unacceptable to most
operating systems. In addition, the varying servo track widths
which produce nonlinear position error signals, is still a problem.
The solution to both issues is a "track map".
[0049] A track map is simply a stored measurement of the average
width of every "half" servo track on a disk. The data is stored on
the disk for use during drive operations. When an individual track
is accessed, the appropriate servo half track width data is pulled
from memory and is used to adjust BSF1 for that local half track
width. The half track width data is turned into a multiplicative
scalar (Burst Scale Factor 2 or BSF2) such that wide and narrow
tracks are correctly represented by 4096 bits. FIG. 14 graphically
shows how the servo half track data (BSF2) corrects the position
error signal for a given wide or narrow track.
[0050] The method used to turn half track width data into BSF2
deserves some explanation. Recall from the head width measurement
discussion above, BSF1 is a multiplicative scalar that is used to
adjust the raw servo burst amplitudes (A, B, C, D) such that a
nominal width track is represented by 4096 bits. FIGS. 12 and 13
show what happens to the position error signal when BSF1 is blindly
applied to a wide or narrow servo track. In other words, a wide or
narrow servo track cause the position error signal gain of 4096
bits/servo track to be incorrect. This can be corrected by knowing
the width of a given track and making a local adjustment to
BSF1.
[0051] The first step in generating BSF2 is measuring the servo
half track widths. This is done after an accurate head width
calibration takes place. Once the head width is known, (BSF1 is
calculated) the 1/4 track points can be used again. Recall from the
discussion above, regardless of head width, |A-B|=|C-D| at the 1/4
track points. The servo system is commanded to position the head at
this location. If the servo track is nominal in width, |A-B| and
|C-D| (after using BSF1) will both be equal to 1024 bits (4096
bits/track/4). If the servo track is narrow, the answer will be
less than 1024. If the servo track is wide the answer will be
greater than 1024. Therefore, using the measurement of |A-B| and
|C-D| at the 1/4 track, the half track width can be calculated as
follows: 1/2 servo track width=|A-B|+|C-D|. The half servo track
width is measured because the 1/4 track point defined by
|A-B|=|C-D| divides the distance between the A and B burst
boundary, which defines the track center, and the C and D burst
boundary, which defines the half servo track position. FIG. 15
graphically shows this relationship.
[0052] To scale wide or narrow servo tracks to the nominal 4096
bits/track, BSF2 needs to be an inverse multiplicative scalar. For
example, on a narrow track, the 1/2 track measurement could be
equal to 1966. To scale the position error signal back to 4096
bits/track (2048 bits/1/2 track), BSF2=2048/1966. Again, BSF2 is
the multiplicative factor necessary to adjust the measured Position
Error Signal (PES) to an overall gain of 4096 bits/servo track
(BSF2 would equal 1 (2048/2048) for a nominal width half servo
track. BSF2 will be >1 for a narrow half track and BSF2 will be
<1 for a wide half track).
[0053] The final detail is deciding what half track measurement to
use in calculating BSF2 for cases when the head is positioned on
the half track boundaries. In these cases it was decided to average
the adjacent half track widths and calculate BSF2. This BSF2 value
is used any time the servo system is commanded to position the head
within .+-.5% of the desired half track boundary. FIG. 16
graphically shows an example of different BSF2 values for a series
of half tracks highlighting the boundary issue. The second diagram
in FIG. 16 shows the solution to the half track boundary case.
[0054] As described earlier, removability requires the REV drive to
write data down the data track center. Due to the reader/writer
separation and offset, positioning the writer over each data track
requires a unique reader position. At disk insertion, the drive
runs a calibration routine that determines the individual
reader/writer relationship. Using this information coupled with the
drive geometry, the reader position for any data track can be
calculated. This distance is known as the micro-jog distance (refer
to FIG. 17).
[0055] The micro-jog distance is the physical distance away from
the data track center the reader must be positioned such that the
writer is positioned directly over the data track. The micro-jog
distance is measured in servo tracks. Servo tracks are the only
measure of distance in the drive and ideally the servo tracks are
written at a precise spacing. In reality the servo track spacing is
not ideal, so the ability to accurately place the writer on the
data track center is degraded. As stated earlier, positioning
errors can cause data encroachment during write operations as well
as reduced data throughput during read operations.
[0056] As described above, a linear position error signal is
created over varying servo track widths. The result is that all
servo tracks, regardless of width, are represented by 4096 bits. A
servo command to position the reader 1/2 tracks away from center
would result in an offset of 2048 bits. The distance the head moves
will be 1/2 of the adjacent track, but the actual physical distance
moved depends on the true half track width. The micro-jog distance
is a physical distance determined by the reader/writer relationship
modified by the geometry of the drive (actuator/head assembly
moving in an arc). As an example, consider the case where the
micro-jog distance of 1/2 a "nominal" servo track is required to
place the writer over a data track center. If the adjacent half
track is not nominal in width, but wide or narrow, blindly moving
the reader to the adjacent half track position will not place the
write element over the data track center (refer to FIG. 18).
[0057] The solution to this problem is contained in the track map.
The track map is a measurement of all half track widths on the
disk. Thus, the micro-jog distance for any data track can be
adjusted to more accurately position the write element. FIG. 19
graphically shows the micro-jog distance correction for the local
half track width dimensions. Obviously, as the micro-jog distance
increases spanning multiple half tracks, the potential for
positioning errors increases. Therefore, the use of the track map
becomes more important in new products where the track pitch is
expected to increase.
[0058] The data track layout is based on the servo track spacing.
As can be seen in FIG. 4, if the servo track spacing is not
correct, the data track spacing is also incorrect. If several
adjacent servo tracks are narrow, the corresponding data tracks are
also squeezed together. This is one cause of data encroachment.
[0059] The solution to this problem is also the track map. With
specific knowledge of the servo track spacing, the drive code could
easily be designed to place the data tracks at whatever spacing is
desired (refer to FIG. 20). This feature has several advantages.
Presently, all servo half track widths on all cartridges are
measured. If adjacent servo half tracks are too narrow, the
corresponding data tracks are deleted from customer use. If too
many of these areas are found, the entire cartridge is failed. The
sensitivity to this issue is decreased by using the track map to
place the data tracks at the desired spacing. Another advantage
comes from improving the soft error rate (correctable data errors).
The drive soft error rate is used in the factory as a metric to
pass or fail drives. The soft error rate can be affected by many
drive parameters. Data encroachment is one of those variables, so
any improvement in this area can directly affect factory drive
yields.
[0060] Numerous variations and modifications of the invention will
become readily apparent to those skilled in the art. Accordingly,
the invention may be embodied in other specific forms without
departing from its spirit or essential characteristics.
* * * * *