U.S. patent application number 12/477765 was filed with the patent office on 2010-12-09 for patterned media magnetic recording disk drive with write clock phase adjustment for write head track misregistration.
This patent application is currently assigned to HITACHI GLOBAL STORAGE TECHNOLOGIES NETHERLANDS B.V.. Invention is credited to Thomas R. Albrecht, Manfred Ernst Schabes.
Application Number | 20100309576 12/477765 |
Document ID | / |
Family ID | 43244131 |
Filed Date | 2010-12-09 |
United States Patent
Application |
20100309576 |
Kind Code |
A1 |
Albrecht; Thomas R. ; et
al. |
December 9, 2010 |
PATTERNED MEDIA MAGNETIC RECORDING DISK DRIVE WITH WRITE CLOCK
PHASE ADJUSTMENT FOR WRITE HEAD TRACK MISREGISTRATION
Abstract
A patterned-media magnetic recording disk drive has compensation
for write head track misregistration (TMR) from the track
centerline. As the disk rotates, the read head detects angularly
spaced servo sectors and generates a position error signal (PES)
which is used by the servo control system to maintain the read head
on track. As the disk rotates, the read head also detects angularly
spaced synchronization marks, which are used to control the write
clock so that magnetization reversal of the magnetic write field
from the write head is synchronized with the position of the data
islands. If there is TMR of the write head, there will be an
effective shift of A(p in the timing of when the center of the data
islands pass through the write field. The disk drive includes write
clock phase adjustment circuitry that correlates the PES with
.DELTA..phi. to compensate for TMR of the write head.
Inventors: |
Albrecht; Thomas R.; (San
Jose, CA) ; Schabes; Manfred Ernst; (Saratoga,
CA) |
Correspondence
Address: |
THOMAS R. BERTHOLD
18938 CONGRESS JUNCTION COURT
SARATOGA
CA
95070
US
|
Assignee: |
HITACHI GLOBAL STORAGE TECHNOLOGIES
NETHERLANDS B.V.
San Jose
CA
|
Family ID: |
43244131 |
Appl. No.: |
12/477765 |
Filed: |
June 3, 2009 |
Current U.S.
Class: |
360/75 ;
G9B/21.003 |
Current CPC
Class: |
G11B 5/59627 20130101;
B82Y 10/00 20130101; G11B 5/743 20130101 |
Class at
Publication: |
360/75 ;
G9B/21.003 |
International
Class: |
G11B 21/02 20060101
G11B021/02 |
Claims
1. A magnetic recording disk drive comprising: a rotatable magnetic
recording disk having a plurality of generally circular data
tracks, a plurality of angularly spaced synchronization marks
extending generally radially across the data tracks, and a
plurality of angularly spaced servo sectors extending generally
radially across the data tracks, each data track patterned into
discrete magnetizable data islands; a read head for sensing the
synchronization marks and the servo sectors; a write head for
writing data to the data tracks by generating a magnetic write
field to magnetize the data islands in the data tracks; a head
carrier supporting the read head and write head; a rotary actuator
connected to the head carrier for moving the read head and write
head generally radially across the data tracks; a controller for
selecting a data track where data is to be written by the write
head; a write clock coupled to the write head for controlling
reversals of the magnetic write field to the data islands in the
selected data track; servo electronics coupled to the read head for
generating a head position error signal (PES) in response to servo
sectors detected by the read head, the PES representing track
misregistration (TMR) of the write head from the centerline of the
selected data track; synchronization mark detection circuitry
coupled to the read head for controlling the write clock in
response to synchronization marks detected by the read head; and
phase adjustment circuitry for adjusting the phase of the write
clock in response to the PES to thereby correct for TMR of the
write head.
2. The disk drive of claim 1 wherein the controller is coupled to
the servo electronics and the phase adjustment circuitry and
further comprising memory accessible by the controller, the memory
containing a table of PES values and associated TMR phase
adjustment values, and wherein the controller receives the PES
values and recalls from said table the associated TMR phase
adjustment values.
3. The disk drive of claim 1 wherein the controller is coupled to
the servo electronics and the phase adjustment circuitry and
further comprising memory accessible by the controller, the memory
containing a program of instructions readable by the controller for
calculating a TMR phase adjustment value from a PES value.
4. The disk drive of claim 1 wherein the servo electronics
generates a measured PES.
5. The disk drive of claim 1 wherein the servo electronics
generates an estimated PES.
6. The disk drive of claim 1 wherein each data track has an
associated head circumferential offset (HCO) value representing the
circumferential offset of the write head from the read head, and
wherein the phase adjustment circuitry adjusts the phase of the
write clock with an HCO phase adjustment value for said selected
data track.
7. The disk drive of claim 6 wherein the phase adjustment circuitry
adjusts the phase of the write clock in response to the PES with a
TMR phase adjustment value, and wherein the phase adjustment
circuitry sums the TMR phase adjustment value with the HCO phase
adjustment value for said selected data track.
8. The disk drive of claim 1 wherein the write clock has adjustable
phases and the phase adjustment circuitry comprises a phase
register, and wherein the phase of the write clock is adjusted by
the value in said phase register.
9. The disk drive of claim 1 wherein the write clock includes a
phase rotator and wherein the phase register provides an output to
control the phase rotator.
10. The disk drive of claim 1 wherein the synchronization marks are
located within the servo sectors.
11. The disk drive of claim 1 wherein the data tracks are grouped
into a plurality of radially spaced bands, and wherein the write
clock is capable of operating at different frequencies, each band
being associated with a unique write clock frequency.
12. A method for synchronizing the writing of data on discrete
magnetizable data islands of a patterned-media disk drive, the disk
drive having (a) a rotatable magnetic recording disk having a
plurality of generally circular data tracks, a plurality of
angularly spaced synchronization marks extending generally radially
across the data tracks, and a plurality of angularly spaced servo
sectors extending generally radially across the data tracks, each
data track patterned into discrete magnetizable data islands; (b) a
write head for writing data in the data tracks by generating a
magnetic write field to magnetize the data islands; (c) a write
clock coupled to the write head for controlling reversals of the
magnetic write field to the data islands; (d) a read head for
sensing the synchronization marks and the servo sectors; (e) servo
electronics coupled to the read head for generating a head position
error signal (PES) in response to servo sectors sensed by the read
head, the PES representing track misregistration (TMR) of the write
head from the centerline of a data track where data is to be
written; (f) a processor for receiving synchronization mark signals
from the read head and generating a phase adjustment signal to the
write clock; and (g) memory coupled to the processor and containing
a program of instructions readable by the processor; the
processor-implemented method comprising: identifying the selected
data track on which data is to be written; receiving the PES when
the write head is on the selected data track; generating, from the
PES, a write clock TMR phase adjustment value; and transmitting to
the write clock the TMR phase adjustment value.
13. The method of claim 12 wherein the memory contains a table of
PES values and associated TMR phase adjustment values, and wherein
the controller generates the write clock TMR phase adjustment value
by recalling from said table the TMR phase adjustment value
associated with the PES.
14. The method of claim 12 wherein the processor-implemented method
of generating, from the PES, a write clock TMR phase adjustment
value comprises calculating, from an equation representing TMR
phase adjustment value as a function of PES value, the write clock
TMR phase adjustment value.
15. The method of claim 12 wherein the processor-implemented method
of receiving the PES when the write head is on the selected data
track comprises receiving the measured PES from the servo
electronics.
16. The method of claim 12 wherein the processor-implemented method
of receiving the PES when the write head is on the selected data
track comprises receiving the estimated PES from the servo
electronics.
17. The method of claim 12 wherein each data track has an
associated head circumferential offset (HCO) value representing the
circumferential offset of the write head from the read head, the
processor-implemented method further comprising: generating, from
the HCO value for said selected data track, a write clock HCO phase
adjustment value; and transmitting to the write clock the HCO phase
adjustment value.
18. The method of claim 17 further comprising summing the TMR phase
adjustment value with the HCO phase adjustment value, and wherein
transmitting the TMR phase adjustment value and the HCO phase
adjustment value comprises transmitting said summed value.
19. The method of claim 12 wherein the synchronization marks are
located within the servo sectors.
20. The method of claim 12 wherein the data tracks are grouped into
a plurality of radially spaced bands, and wherein the write clock
is capable of operating at different frequencies, each band being
associated with a unique write clock frequency.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to magnetic recording disk
drives for use with patterned media, wherein each data bit is
stored in a magnetically isolated block or island on the disk, and
more particularly to such a disk drive with an improved clock for
writing the data.
[0003] 2. Description of the Related Art
[0004] Magnetic recording hard disk drives with patterned magnetic
recording media have been proposed to increase the data density. In
patterned media, the magnetic material on the disk is patterned
into small isolated data blocks or islands arranged in concentric
data tracks. Each island contains a single magnetic "bit" and is
separated from neighboring islands by a nonmagnetic region. This is
in contrast to conventional continuous media wherein a single "bit"
is composed of multiple weakly-coupled neighboring magnetic grains
that form a single magnetic domain and the bits are physically
adjacent to one another. Patterned-media disks may be longitudinal
magnetic recording disks, wherein the magnetization directions are
parallel to or in the plane of the recording layer, or
perpendicular magnetic recording disks, wherein the magnetization
directions are perpendicular to or out-of-the-plane of the
recording layer. To produce the required magnetic isolation of the
patterned blocks, the magnetic moment of the regions between the
blocks must be destroyed or substantially reduced so as to render
these regions essentially nonmagnetic. Alternatively, the media may
be fabricated so that that there is no magnetic material in the
regions between the blocks.
[0005] In one type of patterned media, the data islands are
elevated, spaced-apart pillars that extend above the disk substrate
surface to define troughs or trenches on the substrate surface
between the pillars. This type of patterned media is of interest
because substrates with the pre-etched pattern of pillars and
trenches can be produced with relatively low-cost, high volume
processes such as nanoimprint lithography using a master template
created by e-beam lithography and self-assembly, along with an
appropriate etching method to transfer the pattern to the
substrate. The magnetic recording layer material is then deposited
over the entire surface of the pre-etched substrate to cover both
the ends of the pillars and the trenches. The trenches are recessed
far enough from the read/write head to not adversely affect reading
or writing. This type of patterned media is described by Moritz et
al., "Patterned Media Made From Pre-Etched Wafers: A Promising
Route Toward Ultrahigh-Density Magnetic Recording", IEEE
Transactions on Magnetics, Vol. 38, No. 4, July 2002, pp.
1731-1736.
[0006] In patterned media, because the data islands are
single-domain, the transitions between bits occur only between the
islands. Since the magnetic transitions are restricted to
predetermined locations governed by the locations of individual
data islands, it is necessary to synchronize the switching of the
write current and thus the reversal of the write field from the
write head with the passing of individual data islands past the
write head. The optimal phase of the write clock that controls the
reversal of the write field depends on the position of the write
head with respect to the centerline of the data track. This is
because the outer boundary or contour of the write "bubble", i.e.,
the locus of magnetic field strength sufficient to magnetize the
data islands, is curved at its trailing edge. Thus if the write
head is not well-centered on the data track, but shifted by an
amount of track misregistration (TMR) away from the track
centerline, the data islands will not be located within the write
bubble when the write field is reversed. This may lead to write
errors on the selected track.
[0007] What is needed is a magnetic recording disk drive with
patterned media that has a write-clock with phase adjustment to
compensate for TMR of the write head.
SUMMARY OF THE INVENTION
[0008] The invention relates to a patterned-media magnetic
recording disk drive with compensation for write head track
misregistration (TMR) from the track centerline. The disk drive has
a read head and a write head located on an air bearing slider
associated with each disk surface. There is an effective radial
offset between the read head and write head, with the radial offset
being a known function of track number, so that when the read head
is aligned with one data track the write head is aligned with a
different data track. As the disk rotates, the read head detects
angularly spaced servo sectors and generates a position error
signal (PES) which is used by the servo control system to maintain
the read head on track. As the disk rotates, the read head also
detects angularly spaced synchronization marks, which are used to
control the write clock so that magnetization reversal of the
magnetic write field is synchronized with the position of the data
islands beneath the write head. The write head produces a magnetic
write field within a write "bubble", i.e., the region where the
magnetic field has a strength sufficient to switch the
magnetization of a data island. If the write head is shifted
relative to the track centerline by the TMR, then the center of a
data island whose magnetization is to be reversed passes through
the boundary of the write bubble of the shifted write head sooner
than it would pass through the boundary of the write bubble of the
unshifted write head. This results in an effective shift of A(p in
the timing of when the center of the island passes through the
boundary of the write bubble. The disk drive includes circuitry to
adjust the phase of the write clock by the amount of A(p to thus
compensate for TMR of the write head. The phase adjustment
circuitry uses a lookup table of PES values (which correspond to
TMR values) and associated phase adjustment (.DELTA..phi.) values.
As an alternative to the use of a lookup table, the value of
.DELTA..phi. may be calculated from an equation generated by a
curve-fitting algorithm for values of PES and measured .DELTA..phi.
values.
[0009] For a fuller understanding of the nature and advantages of
the present invention, reference should be made to the following
detailed description taken together with the accompanying
figures.
BRIEF DESCRIPTION OF THE DRAWING
[0010] FIG. 1 is a top view of a patterned-media disk drive like
that to which the present invention relates.
[0011] FIG. 2 is a block diagram of the electronics associated with
the disk drive of the present invention and also shows a sectional
view of the patterned-media magnetic recording disk.
[0012] FIG. 3 is an illustration of a portion of a patterned-media
disk with a typical patterned servo sector spanning several data
tracks.
[0013] FIG. 4 is a top view of a portion of the slider on a surface
of a patterned-media disk and shows the head circumferential offset
(HCO) between the read head and write head.
[0014] FIG. 5 is a lookup table showing effective radial offset
(R.sub.e) values and HCO values as a function of track number.
[0015] FIG. 6 is a schematic showing a portion of patterned-media
disk with data islands and the write bubble contour from the write
head when the write head is aligned with a track centerline and
when the write head has a track misregistration (TMR) from the
track centerline.
[0016] FIG. 7 is a lookup table showing position error signal (PES)
values and associated write clock phase adjustment values for TMR
(.DELTA..phi.).
[0017] FIG. 8 is a block diagram of the write clock controlled by
input from the synchronization (sync) mark detector with phase
rotator controlled by input from the controller electronics.
DETAILED DESCRIPTION OF THE INVENTION
[0018] FIG. 1 is a top view of a patterned-media disk drive 100
like that to which the present invention relates. The drive 100 has
a housing or base 101 that supports an actuator 130 and a spindle
motor (not shown) for rotating the patterned-media magnetic
recording disk 10 about its center 13. The actuator 130 may be a
voice coil motor (VCM) rotary actuator that has a rigid arm 134 and
rotates about pivot 132 as shown by arrow 124. A head-suspension
assembly includes a suspension 121 that has one end attached to the
end of actuator arm 134 and a head carrier, such as an air-bearing
slider 122, attached to the other end of suspension 121. The
suspension 121 permits the head carrier 122 to be maintained very
close to the surface of disk 10. The slider 122 supports the
read/write or recording head 109. The recording head 109 is
typically a combination of an inductive write head with a
magnetoresistive read head (also called a read/write head) and is
located on the trailing end or end face of the slider 122. Only one
disk surface with associated slider and recording head is shown in
FIG. 1, but there are typically multiple disks stacked on a hub
that is rotated by a spindle motor, with a separate slider and
recording head associated with each surface of each disk.
[0019] The patterned magnetic recording disk 10 includes a disk
substrate with discrete data blocks or islands 30 of magnetizable
material on the substrate. The data islands 30 function as discrete
magnetic bits for the storage of data. Each discrete data island 30
is a magnetized block separated from other blocks by nonmagnetic
regions or spaces. The term "nonmagnetic" means that the spaces
between the data islands are formed of a nonferromagnetic material,
such as a dielectric, or a material that has no substantial
remanent moment in the absence of an applied magnetic field, or a
magnetic material in a groove or trench recessed far enough below
the blocks to not adversely affect reading or writing. The
nonmagnetic spaces between the data islands may also be the absence
of magnetic material, such as grooves or troughs in the magnetic
recording layer or disk substrate.
[0020] The data islands 30 are arranged in radially-spaced circular
tracks, with the tracks being grouped into annular bands or zones
151, 152, 153. Within each track, the data islands 30 are typically
arranged in fixed-byte-length data sectors (e.g., 512 bytes plus
additional bytes for error correction coding (ECC) and data sector
header). The number of data sectors is different in each zone. The
grouping of the data tracks into annular zones permits banded
recording, wherein the angular spacing of the data islands, and
thus the data rate, is different in each zone. In FIG. 1, three
zones 151, 152, 153 are shown, with only portions of representative
concentric data tracks, 161, 162, 163, being shown for each
respective zone. While only three zones are depicted in FIG. 1,
modem disk drives typically have about 20 zones. In each zone there
are also generally radially-directed synchronization (sync) marks,
like typical marks 173 in zone 153. Each sync mark 173 may be
plurality of circumferentially-spaced marks, with the spacing being
different in each zone, that are detected by the read head to
enable the write head to be synchronized with the specific spacing
of the data islands in that zone. The sync marks may be located in
the sector headers for the data sectors. The physical location
where data is to be written or read is identified by a head number,
track number (also called "cylinder" number when there are multiple
disks) and data sector number.
[0021] As the disk 10 rotates about its center 13 in the direction
of arrow 20, the movement of actuator 130 allows the read/write
head 109 on the trailing end of head carrier 122 to access
different data tracks and zones on disk 10. Because the actuator
130 is a rotary actuator that pivots about pivot 132, the path of
the read/write head 109 across the disk 10 is not a perfect radius
but instead an arcuate line 135.
[0022] Each data track also includes a plurality of
circumferentially or angularly-spaced servo sectors 120 that
contain positioning information detectable by the read head for
moving the head 109 to the desired data tracks and maintaining the
head 109 on the data tracks. The servo sectors in each track are
aligned circumferentially with the servo sectors in the other
tracks so that they extend across the tracks in a generally radial
direction, as represented by radially-directed servo sectors 120.
The servo sectors 120 have an arcuate shape that generally
replicates the arcuate path 135 of the head 109. The servo sectors
120 are nondata regions on the disk that are magnetized once,
typically during manufacturing or formatting of the disk, and are
not intended to be erased during normal operation of the disk
drive. While the sync marks (like sync marks 173) may be located in
the sector headers for the data sectors, as an alternative they may
be located in the servo sectors 120.
[0023] FIG. 2 is a block diagram of the electronics associated with
disk drive 100 and also shows a sectional view of the magnetic
recording disk 10 with a magnetic recording layer of patterned
media in the form of discrete magnetizable data islands 1-9. FIG. 2
also shows a portion of slider 122 with read/write head 109 that
includes the read element or head 109a and the write element or
head 109b. The read head 109a and write head 109b are formed on the
trailing end 122a of slider 122. The arrows depicted in the islands
1-9 represent the magnetic moments or magnetization directions in
the islands, and are depicted for perpendicular or out-of-plane
magnetic recording. The recording or writing of data occurs by an
inductive coil write head 1 09b that has a write pole that
generates a magnetic field to magnetize the islands in one of the
two magnetization directions, depending on the direction of current
through the coil of the write head. Because there is no magnetic
material between the islands 1-9, the write field magnetization
reversals must be precisely timed to magnetize the appropriate
islands. While FIG. 2 illustrates perpendicular patterned media,
wherein the islands 1-9 are depicted with their moments oriented
out of the plane of the recording layer, the invention is fully
applicable to horizontal or longitudinal patterned media, wherein
the islands 1-9 would have their moments oriented in the plane of
the magnetic recording layer.
[0024] The disk drive electronics shown in FIG. 2 include
read/write (R/W) electronics 113, servo electronics 112, controller
electronics 115 and interface electronics 1 14. The R/W electronics
113 receives signals from read head 109a and passes servo
information from the servo sectors 120 to servo electronics 112 and
data signals from the data sectors to controller electronics
115.
[0025] Servo electronics 112 typically includes a servo control
processor that uses the servo information from the servo sectors
120 to run a control algorithm that produces a control signal. The
control signal is converted to a current that drives rotary
actuator 130 to position the head 109. The operation of servo
electronics 112 is explained in more detail with respect to FIG. 3,
which is a schematic showing a portion of the patterned-media disk
10 (FIG. 1) with a typical patterned servo sector 120 spanning
several data tracks. Three full data tracks and one half-track are
depicted: 308, 309, 310 and half-track 311, each having a
respective track centerline 328, 329, 330 and 331. The tracks are
spaced about by the track pitch (Tp). The read head 109a is shown
as positioned in data track 308 and will detect the islands in
servo sector 120 as the disk rotates in the direction of arrow
20.
[0026] The servo sector 120 is a conventional servo pattern of the
type commonly used in sector servo systems and shows a greatly
simplified pattern for clarity. The servo pattern includes several
fields containing nondata islands, which are shown as automatic
gain control (AGC) field 301, servo-timing-mark (STM) field 302,
track ID (TID) field 304 and position-error-signal (PES) field 306.
PES field 306 is depicted as the well-known quadrature pattern of
PES blocks or islands A-D. The PES islands A-D are used to
determine the fractional part of the radial position of the head.
When the head is at the track centers the read-back signal
amplitudes from the A islands and the B islands are equal. When the
head is at the half-track positions the amplitudes from the C
islands and the D islands are equal. As the head moves off-track
the amplitudes from all the islands will increase or decrease. The
amplitudes of the PES islands are decoded or demodulated in servo
electronics 112 to generate a PES that is used in the control
algorithm run by the servo control processor to generate control
signals to the actuator 130 (FIG. 1) to reposition the head. The
PES from servo electronics 112 is also available to controller
electronics 115 via line 160 (FIG. 2).
[0027] In FIG. 3 all of the islands in servo sector 120 are
discrete blocks or islands of magnetic material and are magnetized
in the same direction, either perpendicular to the recording layer
(either into or out of the paper in FIG. 3) for
perpendicular-recording media, or in the plane of the recording
layer (either to the right or left in the along-the-track direction
in FIG. 3) for horizontal-recording media. The islands are
typically DC-magnetized by a large magnet during manufacturing.
Each discrete island is a magnetized island separated from other
islands by nonmagnetic regions or spaces represented as 200. The
term "nonmagnetic" means that the spaces 200 between the islands
are formed of a nonferromagnetic material, such as a dielectric, or
a material that has no substantial remanent moment in the absence
of an applied magnetic field, or a magnetic material in a groove or
trench recessed far enough below the islands to not adversely
affect reading or writing. The nonmagnetic spaces 200 may also be
the absence of magnetic material, such as grooves or troughs in the
magnetic recording layer or disk substrate.
[0028] Referring again to FIG. 2, interface electronics 114
communicates with a host system (not shown) over interface 116,
passing data and command information. Interface electronics 114
also communicates with controller electronics 115 over interface
118. Interface electronics 114 receives a request from the host
system, such as a personal computer (PC), for reading from or
writing to the data sectors over interface 116. Controller
electronics 115 includes a microprocessor and associated memory
115a. Controller electronics 115 receives a list of requested data
sectors from interface electronics 114 and converts them into a set
of numbers that uniquely identify the disk surface (head number
associated with that disk surface), track (cylinder) and data
sector. The numbers are passed to servo electronics 112 to enable
positioning head 109 to the appropriate data sector.
[0029] FIG. 2 also shows schematically the transfer of data between
a host system, such as a PC, and the disk drive 100. The signals
from recorded data islands in the data sectors are detected by read
head 109a, and amplified and decoded by read/write electronics 113.
Data is sent to controller electronics 115 and through interface
electronics 114 to the host via interface 116. The data to be
written to the disk 10 is sent from the host to interface
electronics 114 and controller electronics 115 and then as a data
queue to pattern generator 117 and then to write driver 11 9. The
write driver 119 generates current whose direction is switched at
high-frequency to the coil of write head 109b. This results in
reversal of the direction of the magnetic write fields that
magnetize the data islands 1-9. The write clock 140, which is
capable of operating at different frequencies corresponding to the
different data zones, outputs a clock signal on line 144 to control
the timing of pattern generator 117 and write driver 119. A sync
mark detector 141 receives the readback signal from R/W electronics
113 on input line 142 and outputs a signal on line 143 to control
the timing of write clock 140. The sync mark detector 141 detects
the sync marks (like sync marks 173 in FIG. 1) that are sensed by
the read head 109a and sent to R/W electronics 113. The sync mark
spacing in each zone is different so sync mark detector 141 enables
the write clock 140 to be synchronized with the spacing of the data
islands in each of the different zones. FIG. 2 also shows a
physical spacing D between the read head 109a and the write head
109b.
[0030] FIG. 4 is a top view of a portion of slider 122 on a surface
of patterned-media disk 10 and shows the relationship between the
read head 109a, write head 109b and typical patterned data tracks
163 with typical data islands 30. The tracks are spaced apart
radially by a distance called the track pitch (Tp). All the data
island patterns in the tracks 163 are shown as being precisely
aligned circumferentially with one another, with an intended
one-half block circumferential shift of the patterns in alternate
tracks. The advantage of having the data island pattern in each
track shifted in the along-the-track direction by one-half the
block spacing from the blocks in adjacent tracks is that any
readback signal interference from an adjacent track will be out of
phase with the readback signal from the track being read, which
results in a substantially reduced error rate for the data being
read. Also, in FIG. 4 the data islands are depicted as circular,
but the data islands may have other shapes, including rectangular
with different aspect ratios (radial height to circumferential
width).
[0031] FIG. 4 shows the physical spacing D in the generally
circumferential or along-the-track direction between the read head
109a and the write head 109b. The physical spacing D does not vary
much from head to head, but there still is some variation due to
variations in the thicknesses of the films deposited in the head
fabrication process. Additionally, due to tolerances in fabrication
there is typically also a radial or generally cross-track physical
spacing X between the read head 109a and write head 109b. The
cross-track spacing X is not the same for each head but typically
has a statistical variation among the heads in the same fabrication
process. Because the slider 122 is mounted to the rotary actuator
that rotates about pivot 132 its path is an arcuate path 135 that
is not aligned with the disk radius 149. As shown in FIG. 4, the
end face 122a of slider 122 makes an angle .alpha. (called the skew
angle) with the disk radius 149, with skew angle .alpha. being a
known function of disk radius and thus track number. As a result of
the circumferential spacing D, cross-track spacing X and skew angle
.alpha., there is an effective radial offset R.sub.e between read
head 109a and write head 109b, with radial offset R.sub.e being a
known function of track number. In a disk drive using patterned
media with an areal density around 1 Terabit per square inch, the
track pitch (the radial spacing between adjacent tracks) may be in
the range of about 25 to 50 nm and the maximum skew angle .alpha.
may be about 15 degrees. Manufacturing tolerances result in X being
between .+-.1 micron and D is typically about 8 microns. As a
result, the maximum R.sub.e may be about 3 microns, or as much as
120 times Tp. In FIG. 4, to fully illustrate the relationship among
the values of .alpha., D, X and R.sub.e, these values are not shown
as being precisely to scale. Conventional approaches exist for
measuring the effective read head/write head radial offset R.sub.e
as a function of radius in a disk drive and storing this
information in the disk drive's drive electronics for
track-following control during reading and writing of data. For
example, the disk drive may include a lookup table stored in memory
115a accessible by controller electronics 115. FIG. 5 is an example
of a portion of a lookup table that shows track numbers (TR) and
associated values of skew angle .alpha. and radial offset R.sub.e
for every 10.sup.th track between tracks n and n+30 for one
specific head. A table like FIG. 5 will typically be required for
each head in the disk drive because the value X is not the same for
each head. It is typically not necessary for the lookup table to
store values of R.sub.e for every track. Values for every N tracks,
for example where N is 100 or 1000, can be stored in the lookup
table and an interpolation method used to determine the values for
a selected track. As an alternative to the use of a lookup table,
the processor in controller electronics 115 may calculate R.sub.e
for a selected track number from an equation stored in memory 115a,
with the equation generated by a curve-fitting algorithm for values
of track number and measured R.sub.e values. The values of R.sub.e
are typically a non-integer number of Tp.
[0032] This effective radial offset R.sub.e between the read head
109a and write head 109b means that when the write head 109b is
positioned to write on a selected track 163b, the read head 109a
will be positioned over a different track 163a. In FIG. 4, for ease
of illustration, the read head 109a is depicted as being on the
centerline of track 163a when the write head 109b is positioned on
the centerline of track 163b, so that R.sub.e in this example would
be an integer number of Tp. However, R.sub.e is typically a
non-integer number of Tp. For example, if R.sub.e is equal to 90.12
Tp, this means that the servo electronics 112 will maintain the
read head 109a at a position 0.12 Tp from the track centerline of a
track that is 90 tracks displaced from track 163b, the track where
the write head 109b is writing.
[0033] As shown in FIG. 4, the value of R.sub.e together with the
skew angle .alpha. results in a head circumferential offset (HCO)
between the read head 109a and the write head 109b. As can be
appreciated from FIG. 4, because the skew angle .alpha. varies with
disk radius, the value of HCO also varies with disk radius and can
be calculated from the known values of D, .alpha. and the measured
values of X. Thus, when the read head 109a detects a sync mark 173
as the disk rotates in the direction of arrow 20, the write head
109b will not be precisely aligned with that sync mark, but will be
circumferentially offset by an amount HCO. Thus an adjustment to
the phase of write clock 140 must be made to compensate for HCO to
assure that the write current reversals are synchronized to the
location of the data islands 30 in the selected track 163b.
[0034] FIG. 6 is a schematic showing a portion of patterned-media
disk 10 with data tracks, including data track 163b with data
islands 30a, 30b and 30c. The data islands 30a -30c are shown as
being generally rectangularly shaped with dimensions of 20 nm by 20
nm. The center of the write head 109b is shown aligned with the
centerline of track 163b. The write head 109b generates a
three-dimensional magnetic field as the disk and thus the data
islands 30a -30c move in the direction of arrow 20 past the write
head 109b. The curved line 205 represents an intersection of this
three-dimensional magnetic field with the plane of the recording
layer that contains the data islands 30. This intersection (curved
line 205) is the outer contour or boundary of the magnetic field
write "bubble", i.e., the location where the magnetic field has a
strength sufficient to switch the magnetization of the individual
data islands. Because the islands do not have precisely identical
magnetic properties as a result of tolerances in the fabrication
process, they will have a switching field distribution, meaning
that some islands will require a slightly higher magnetic field
than others to switch their magnetization. This introduces some
uncertainty or jitter into when each island reverses its
magnetization. However, for the purpose of this invention the
magnetization reversal of the islands is based on the switching
behavior of an "average" island. Also, the magnetic field within
the write bubble is not constant over the whole island. The writing
(magnetization reversal) of data island 30b occurs when the
geometric center of the island is within the write bubble, and the
permanent fixing of the magnetic state of the island occurs when
the geometric center of the island passes through the boundary 205
of the write bubble into the region where the field (averaged over
the whole island) is below the switching field for the island. The
optimum phase of the write clock for minimizing write errors
depends on the position of the write head 109b with respect to the
center of track 163b. This is because, as shown in FIG. 6, the
write bubble is curved at its trailing edge or boundary 205. If the
write head 109b position is shifted relative to the track
centerline by the TMR, such that the center of the write head
follows 163b' instead of 163b, then the center of the island 30b
passes through the boundary 205' of the write bubble of the shifted
write head sooner than it would pass through the boundary 205 of
the write bubble of the unshifted write head. This results in an
effective shift of .DELTA..phi. in the timing of when the center of
the island 30b passes through the boundary of the write bubble.
[0035] In this invention an adjustment to the phase of write clock
140 (FIG. 2) is made to compensate for .DELTA..phi. to assure that
the magnetization reversals of the write field are synchronized to
the location of the data islands 30a -30c in the selected data
track 163b. The TMR of the write head away from the track
centerline 163b corresponds to the PES from the servo electronics
112. During writing, if the read head 109a is maintained precisely
on the data track 163a (FIG. 4), then the value of PES will be
zero, the write head 109b will be precisely on the centerline of
selected data track 163b and no write clock phase adjustment for
TMR is necessary. However, because of the finite response time of
the servo system to maintain the read head 109a precisely on the
centerline of track 163a (FIG. 4) and because of disturbances due
to electrical noise and mechanical vibration, the write head 109b
will typically be offset by some value of TMR from the centerline
of data track 163b. Thus by adjusting the phase of the write clock
140 to compensate for TMR the likelihood of writing the correct
data on the selected track is increased.
[0036] To compensate for TMR of the write head the disk drive may
include a lookup table stored in memory 115a accessible by
controller electronics 115. FIG. 7 is an example of a portion of a
lookup table that shows PES values (which correspond to TMR values)
in units of nm and associated values of the phase adjustment due to
TMR (.DELTA..phi.) in nm. A table like FIG. 7 will typically be
required for each head in the disk drive. As an alternative to the
use of a lookup table, the processor in controller electronics 115
may calculate .DELTA..phi. from a set of computer program
instructions stored in memory 115a, with the instructions
representing an equation generated by a curve-fitting algorithm for
values of PES and measured .DELTA..phi. values.
[0037] One method for generating the lookup table of FIG. 7 is
described as follows. First, the optimum write phase for writing
with the write head well-centered on a track is determined. This is
accomplished by writing pseudo-random data with the write clock
having a known phase relationship with the write synchronization
pattern (sync marks 173 in FIG. 1) read by the read head. A large
enough number of data sectors are written, for example all the data
sectors in one complete circular track, so that good statistics can
be gathered on error rate. Then the error rate of this written data
is checked by reading back the written track and comparing it to
the known written pseudo-random data. Alternatively, the disk
drive's error correction coding (ECC) can be used to analyze the
number of errors. Then this writing of pseudo-random data and
checking of the error rate is performed multiple times, each time
after making a small shift in the write clock phase. This is
continued until the write clock has been stepped through a full
cycle of possible phases. The error rate will follow an expected
behavior, with a minimum error rate occurring at one particular
write clock phase and increasing to a maximum error rate when the
write clock phase is shifted by approximately 180 degrees from the
optimum. By analyzing the error rate of all iterations, the optimum
write phase can be determined which corresponds to the case where
the write head is well-centered on the track.
[0038] Once the optimum phase for writing on the center of the
track has been established as described above, a similar test is
then performed with the write head intentionally offset from the
track centerline by a known amount of TMR. By iterating this
procedure at a number of off-track positions (for example, stepping
1 nm at a time to both sides of the track centerline) the optimum
phase (.DELTA..phi.) as a function of offset (TMR) from the track
centerline is determined. Since TMR is directly related to PES,
this procedure results in the table of FIG. 7, i.e., the table of
values of PES and associated values of phase adjustment due to TMR
(.DELTA..phi.). This procedure is then repeated for each head in
the disk drive, resulting in a table like FIG. 7 for each head.
[0039] FIG. 8 is a block diagram of the write clock 140 and will be
used to explain the method of adjusting the phase of the write
clock to compensate for both HCO and TMR. The write clock 140 may
be a voltage-controlled oscillator (VCO) 211 in a conventional
phase-locked loop (PLL) with a crystal reference 210 and loop
filter 212. The frequency and phase of write clock 140 is initially
set by the sync mark detection signal 143 from sync mark detector
141 which detects sync marks 173 (FIG. 1) as the disk 10 rotates in
the direction of arrow 20. The divider 214 allows the write clock
frequency to be adjusted, for example in multiples of a small
fraction of the frequency of the crystal reference 210. This
enables the PLL output 215 to be set to different output
frequencies depending on the frequency of the data zone containing
the selected track where data is to be written. The settings in
divider 214 should be selected so the PLL output is as close as
possible to the desired write clock frequency to minimize the
average size of phase updates at phase rotator 230. The write clock
signal is generated in equally-spaced primary phases, and by analog
interpolation it is possible to generate clocks with a phase
intermediate the primary phases. For example, the clock output at
line 144 may be capable of 64 equally-spaced clock phases. The
phase rotator 230, also called a "mixer", controls which clock
phase is selected for output on line 144.
[0040] Also shown in FIG. 8 is the phase adjustment for HCO and
TMR, shown as input 145 from controller electronics 115 to the
phase rotator 230. The controller electronics 115 calculates HCO
from the known relationship between HCO and the track number for
the selected data track where data is to be written. This is done
as described above by use of the lookup table (FIG. 5) or by
calculating HCO for a selected track number from an equation stored
in memory 115a. After the read head 109a has settled generally
along the centerline of track 163a, the servo sectors 120 (FIG. 1)
are detected as the disk rotates in the direction of arrow 20. The
servo electronics 112 (FIG. 2) generates a PES for each servo
sector 120. The PES is passed to controller electronics 115 which
then calculates .DELTA..phi. from the known relationship between
.DELTA..phi. and PES. This is done as described above by use of the
lookup table (FIG. 7) or by calculating .DELTA..phi. from an
equation stored in memory 115a. The phase adjustment value
corresponding to HCO and the value of .DELTA..phi. (the phase
adjustment due to TMR) are summed as a total phase adjustment. This
total phase adjustment value may stored in an optional phase
register 231 for input to phase rotator 230. The output of phase
register 231 signals phase rotator 230 to advance or retard the
write clock phase, and thus to adjust its frequency and phase so as
to be synchronized for writing to the data islands. Thus with the
correct phase adjustment, when the read head 109a detects sync
marks in its track 163a, and with the write head off the centerline
of track 163b by some amount of TMR as determined by the PES value,
the write clock 140 will cause write current reversals from write
driver 119 to be precisely synchronized with the data islands in
the selected data track 163b.
[0041] The PES value generated by servo electronics 112 and used by
controller 115 may be the measured PES value as determined from the
demodulated signal from PES blocks 306 (FIG. 3) in the servo
sectors 120. However, disk drive digital servo control systems
typically use a state estimator, which is a standard control system
element in which the dynamics of the system are expressed as a
system of state equations. The state of the system is represented
as a vector of real numbers and the estimate for the current state
is calculated from the estimated state from the previous samples.
Thus the PES value used by controller 115 may be the PES estimate
from previous measured PES samples.
[0042] The write synchronization system and method as described
above and illustrated with various block diagrams may be
implemented in conventional analog or digital hardware components
or in software. The servo control processor, the processor in the
controller electronics, or other microprocessor in the disk drive,
may perform the method, or portions of the method, using algorithms
implemented in computer programs stored in memory accessible to the
processor.
[0043] While the present invention has been particularly shown and
described with reference to the preferred embodiments, it will be
understood by those skilled in the art that various changes in form
and detail may be made without departing from the spirit and scope
of the invention. Accordingly, the disclosed invention is to be
considered merely as illustrative and limited in scope only as
specified in the appended claims.
* * * * *