U.S. patent application number 11/765335 was filed with the patent office on 2008-12-25 for magnetic recording disk drive with head positioning servo control system for disk surfaces with identical servo patterns.
This patent application is currently assigned to HITACHI GLOBAL STORAGE TECHNOLOGIES NETHERLANDS B.V.. Invention is credited to Thomas R. Albrecht, Zvonimir Z. Bandic.
Application Number | 20080316638 11/765335 |
Document ID | / |
Family ID | 39766870 |
Filed Date | 2008-12-25 |
United States Patent
Application |
20080316638 |
Kind Code |
A1 |
Albrecht; Thomas R. ; et
al. |
December 25, 2008 |
MAGNETIC RECORDING DISK DRIVE WITH HEAD POSITIONING SERVO CONTROL
SYSTEM FOR DISK SURFACES WITH IDENTICAL SERVO PATTERNS
Abstract
A magnetic recording disk drive has disks with identical
pre-patterned servo patterns on their front and back surfaces and a
servo control system for positioning the read/write heads using the
servo signals from the identical servo patterns. The servo sectors
on the two disk surfaces form identical patterns of angularly
spaced arcuate-shaped lines that extend generally radially across
the data tracks. The arcuate-shaped lines on one surface, the front
surface, generally replicate the path of the recording head as it
is moved across the data tracks by a rotary actuator, so that there
is a constant sampling rate of the servo sectors on the front
surface regardless of radial position of the head. However, the
arcuate-shaped lines on the other surface, the back surface, do not
replicate the path of the recording head so the servo sampling rate
is not constant but varies with radial position of the head. The
disk drive servo control system implements a method to enable track
seeking from one data track to another data track, regardless of
whether the initial disk surface and the destination disk surface
are front or back surfaces.
Inventors: |
Albrecht; Thomas R.; (San
Jose, CA) ; Bandic; Zvonimir Z.; (San Jose,
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: |
39766870 |
Appl. No.: |
11/765335 |
Filed: |
June 19, 2007 |
Current U.S.
Class: |
360/77.08 ;
G9B/5.192 |
Current CPC
Class: |
G11B 5/5547
20130101 |
Class at
Publication: |
360/77.08 |
International
Class: |
G11B 5/596 20060101
G11B005/596 |
Claims
1. A method for operating a magnetic recording disk drive, the disk
drive having: at least one rotatable magnetic recording disk having
a front generally planar surface, a back generally planar surface
opposite the front disk surface, and a central axis of rotation
perpendicular to said surfaces, each disk comprising a plurality of
generally concentric circular data tracks of magnetic material on
each of said surfaces, the data tracks being centered about said
axis; a plurality of servo sectors on the front surface and forming
a pattern of generally arcuate lines extending in a generally
radial direction across said data tracks between radially inner and
outer circles centered at said axis, each servo sector comprising a
plurality of discrete blocks of magnetic material, the blocks being
arranged in angularly spaced fields along the data tracks; and a
plurality of servo sectors on the back surface forming a pattern of
generally arcuate lines identical to the pattern of generally
arcuate lines on said front surface; the tracks on the disks being
aligned into radially-spaced cylinders; at least one first head,
each first head associated with a front surface of a disk for
reading and writing to data tracks and for detecting servo sectors
on the front surface of a disk; at least one second head, each
second head associated with a back surface of a disk for reading
and writing to data tracks and for detecting servo sectors on the
back surface of a disk; an actuator connected to the heads for
positioning the heads to different data tracks and maintaining the
heads on the data tracks, the actuator causing the first head to
follow a generally arcuate path across a front surface that
generally replicates the arcuate lines of servo sectors on said
front surface and the second head to follow a generally arcuate
path across a back surface that also generally replicates the
arcuate lines of servo sectors on said front surface, whereby the
generally arcuate path of the second head across said back surface
does not replicate the arcuate lines of servo sectors on said back
surface; and a servo control system coupled to the heads and the
actuator, the servo control system including a processor for
generating an actuator control signal in response to detection of
the servo fields by the heads; the method comprising the
processor-implemented steps of: in response to a request to read or
write to a data track on a back surface of a disk, causing the
actuator to seek from a first cylinder to a second cylinder
containing the requested data track; during a first phase of said
seek, receiving signals from the servo sectors on a front surface
of a disk; and during a second phase of said seek, receiving
signals from the servo sectors on a back surface of a disk.
2. The method of claim 1 further comprising, prior to receiving
said request, receiving signals from the servo sectors on a back
surface of a disk to maintain the second head associated with said
back surface on the data track contained within said first
cylinder.
3. The method of claim 1 wherein the second phase commences when
the actuator has positioned the second head to within a
predetermined number of tracks of the requested data track on said
back surface.
4. The method of claim 1 further comprising, in response to a
request to read or write to a data track on a back surface of a
disk, determining if the seek length is greater than a
predetermined number of data tracks, and wherein receiving signals
from the servo sectors on a front surface of a disk comprises
receiving signals from the servo sectors on a front surface of a
disk only if the seek length is greater than said predetermined
number.
5. The method of claim 1 wherein the step of receiving signals from
the servo sectors on a front surface of a disk comprises receiving
signals from the servo fields in a first order, and wherein the
step of receiving signals from the servo sectors on a back surface
of a disk comprises receiving signals from the servo fields in a
reverse order to said first order.
6. The method of claim 5 wherein the discrete blocks in each servo
sector are arranged in angularly spaced fields including an
automatic gain control (AGC) field, a sector identification (SID)
field, a track identification field (TID), and a servo position
error signal (PES) field and wherein the step of receiving signals
from the servo fields on a front surface of a disk in a first order
comprises receiving signals from the servo fields in the order of
AGC, SID, TID and PES.
7. A magnetic recording disk drive comprising: a plurality of
magnetic recording disks, each having a front generally planar
surface and a back generally planar surface opposite the front disk
surface, the disks being rotatable about a common central axis
perpendicular to said disk surfaces, each disk comprising a
plurality of generally concentric circular data tracks of magnetic
material on each of said surfaces, the data tracks being centered
about said axis; a plurality of servo sectors on the front surface
and forming a pattern of generally arcuate lines angularly spaced
about said axis and extending in a generally radial direction
across said data tracks between radially inner and outer circles
centered at said axis and, each servo sector comprising a plurality
of discrete blocks of magnetic material, the blocks being arranged
in angularly spaced fields along the data tracks; and a plurality
of servo sectors on the back surface forming a pattern of generally
arcuate lines identical to the pattern of generally arcuate lines
on said front surface; a plurality of heads, each head associated
with a disk surface for reading and writing to data tracks and for
detecting servo sectors on its associated disk surface; an actuator
connected to the heads for moving the heads radially to different
data tracks and maintaining the heads on the data tracks, the
actuator causing the heads to follow a generally arcuate path
across said disk surfaces that is substantially identical to the
arcuate line of servo sectors on the front surfaces of the disks
and substantially dissimilar to the arcuate line of servo sectors
on the back surfaces of the disks; whereby, during rotation of the
disks, the arcuate lines of servo sectors on front surfaces pass
the heads associated with the front surfaces at a constant rate as
the heads associated with the front surfaces are moved radially by
the actuator, but the arcuate lines of servo sectors on back
surfaces do not pass the heads associated with the back surfaces at
a constant rate as the heads associated with the back surfaces are
moved radially by the actuator; a processor for receiving servo
signals from the heads in response to detection of the servo fields
by the heads and for generating an actuator control signal to
position the heads to different data tracks and to maintain the
heads on the data tracks; memory coupled to the processor; and a
program of instructions in the memory and readable by the processor
for undertaking acts comprising: receiving servo signals from the
head associated with a disk surface A to maintain said head on an
initial data track on disk surface A; receiving a request to read
or write to a target data track on a disk surface B; identifying
each of surfaces A and B as either a front surface or a back
surface; and if surface A is a front surface and surface B is a
back surface, then causing the actuator to position the head
associated with surface B to the target data track on surface B by
receiving signals from the servo sectors on surface A during a
first positioning phase and receiving signals from the servo
sectors on surface B during a second positioning phase.
8. The disk drive of claim 7 wherein the program of instructions
further comprises instructions for undertaking acts of: if surface
A is a back surface and surface B is a back surface, then causing
the actuator to position the head associated with surface B to the
target data track on surface B by receiving signals from the servo
sectors on a front disk surface C during a first positioning phase
and receiving signals from the servo sectors on surface B during a
second positioning phase.
9. The disk drive of claim 7 wherein the act of receiving servo
signals during a second positioning phase comprises commencing said
second positioning phase when the head associated with surface B is
within a predetermined number of data tracks from the target data
track.
10. The disk drive of claim 7 wherein the act of receiving signals
from the servo sectors on surface A during a first positioning
phase comprises receiving signals from the servo fields in a first
order, and the act of receiving signals from the servo sectors on
surface B during a second positioning phase comprises receiving
signals from the servo fields in a reverse order to said first
order.
11. The disk drive of claim 10 wherein the discrete blocks in each
servo sector are arranged in angularly spaced fields including an
automatic gain control (AGC) field, a sector identification (SID)
field, a track identification field (TID), and a servo position
error signal (PES) field and wherein the step of receiving signals
from the servo fields on surface A in a first order comprises
receiving signals from the servo fields in the order of AGC, SID,
TID and PES.
12. A method for operating a magnetic recording disk drive, the
disk drive having: at least one rotatable magnetic recording disk
having a front generally planar surface, a back generally planar
surface opposite the front disk surface, and a central axis of
rotation perpendicular to said surfaces, each disk comprising a
plurality of generally concentric circular data tracks of magnetic
material on each of said surfaces, the data tracks being centered
about said axis; a plurality of servo sectors on the front surface
and forming a pattern of generally arcuate lines extending in a
generally radial direction across said data tracks between radially
inner and outer circles centered at said axis, each servo sector
comprising a plurality of discrete blocks of magnetic material, the
blocks being arranged in angularly spaced fields along the data
tracks; and a plurality of servo sectors on the back surface
forming a pattern identical to the pattern on said front surface;
the tracks on the disks being aligned into radially-spaced
cylinders; at least one first head, each first head associated with
a front surface of a disk for reading and writing to data tracks
and for detecting servo sectors on the front surface of a disk; at
least one second head, each second head associated with a back
surface of a disk for reading and writing to data tracks and for
detecting servo sectors on the back surface of a disk; an actuator
connected to the heads for positioning the heads to different data
tracks and maintaining the heads on the data tracks, the actuator
causing the first and second heads to follow a generally arcuate
path across said disk surfaces that is substantially identical to
the arcuate line of servo sectors on a front surface of a disk; and
a servo control system coupled to the heads and the actuator, the
servo control system including a processor for generating an
actuator control signal in response to detection of the servo
fields by the heads; the method comprising the
processor-implemented steps of: in response to a request to read or
write to a data track on a back surface of a disk, before causing
the actuator to seek from a first cylinder to a second cylinder
containing the requested data track, determining if the seek length
to the requested data track is greater than a predetermined number
of data tracks; and if the seek length is greater than said
predetermined number, then during a first phase of said seek
receiving signals from the servo sectors on a front surface of a
disk and during a second phase of said seek receiving signals from
the servo sectors on a back surface of a disk.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] This invention relates generally to a magnetic recording
disk drive that has disks with pre-patterned servo patterns formed
with a master template, and more particularly to a magnetic
recording disk drive with a servo control system for positioning
the read/write heads on the disk surfaces by using the servo
patterns.
[0003] 2. Description of the Related Art
[0004] Conventional magnetic recording hard disk drives use either
horizontal recording wherein the magnetized regions that define the
magnetically recorded data bits are oriented in the plane of the
recording layer on the hard disks, or perpendicular recording
wherein the magnetized regions are oriented perpendicular to the
plane of the recording layer. Each recording surface on the disks
is a continuous layer of magnetic material that becomes formed into
concentric data tracks containing the magnetically recorded data
bits when the recording head writes on the magnetic material. Each
disk surface also includes a fixed, pre-recorded pattern of servo
sectors that cannot be written over by the recording heads and that
are used to position the heads to the desired data tracks and
maintain the heads on the data tracks.
[0005] The conventional method of generating the pattern of servo
sectors is by "servo-writing" the pattern on a track-by-track
basis, either with a special write head and servo-writer or with
the production recording head in the disk drive. Because this is a
time-consuming and therefore expensive process, other methods for
generating the servo patterns have been proposed.
[0006] In contact magnetic duplication or transfer (CMT), sometimes
referred to as magnetic printing or magnetic lithography (ML), a
"master" template is used that contains regions or islands of soft
(low-coercivity) magnetic material in a pattern corresponding to
the servo pattern that is to be transferred to the disk. The CMT
master template is typically a rigid substrate or a rigid substrate
with a plastic film formed on it, as described in U.S. Pat. Nos.
6,347,016 B1 and 6,433,944 B1; and by Ishida, T. et al., "Magnetic
Printing Technology-Application to HDD", IEEE Transactions on
Magnetics, Vol 39, No. 2, March 2003, pp 628-632. U.S. Pat. No.
6,791,774 B1, assigned to the same assignee as this application,
describes a CMT template and process for forming servo patterns in
perpendicular magnetic recording disks. Magnetic lithography (ML)
using a flexible master template is described in U.S. Pat. No.
6,798,590 B2, assigned to the same assignee as this application,
and by Bandic et al., "Magnetic lithography for servowriting
applications using flexible magnetic masks nanofabricated on
plastic substrates", Microsystems Technology, DOI
10.1007/s00542-006-0287-8.
[0007] The CMT process for forming servo patterns is applicable not
only to conventional "continuous" magnetic media wherein the
concentric data tracks are formed in the continuous layer of
magnetic material by the recording heads, but also to "discrete
track" media. In this type of media, as described for example in
U.S. Pat. No. 4,912,585, each data track consists of continuous
magnetic material, but the individual data tracks are separated by
nonmagnetic guard bands. The CMT process may be used to form not
only the servo patterns but also the discrete tracks.
[0008] Patterned magnetic media has been proposed to replace
conventional continuous magnetic media to increase the data storage
density in disk drives. In patterned media the magnetic material on
the disk surface is patterned into small isolated data islands such
that there is a single magnetic domain in each island or "bit". To
produce the required magnetic isolation of the patterned data
islands, the magnetic moment of the regions between the islands
must be destroyed or substantially reduced so as to render these
regions essentially nonmagnetic. Alternatively, the patterned media
may be fabricated so that that there is no magnetic material in the
regions between the islands. Patterned media can be produced by
replication from a master template via nanoimprinting. The
nanoimprinting process forms not only the isolated data islands in
the data tracks, but also the servo patterns. In nanoimprinting a
master mold or template replicates a topographic pattern onto a
polymeric resist coating on the disk substrate, followed by sputter
deposition of magnetic material over the pattern. Nanoimprinting of
patterned media is described by Bandic et al., "Patterned magnetic
media: impact of nanoscale patterning on hard disk drives", Solid
State Technology S7+ Suppl. S, SEP 2006; and by Terris et al.,
"TOPICAL REVIEW: Nanofabricated and self-assembled magnetic
structures as data storage media", J. Phys. D: Appl. Phys. 38
(2005) R199-R222.
[0009] In hard disk drives, the servo pattern on the back surface
of the disks is not identical to, but is rather the mirror image
of, the servo pattern on the front surface of the disks. Thus, to
form servo patterns by either CMT or nanoimprinting, it is
necessary to fabricate two master templates, one for the front
surfaces of the disks and one for the back surfaces of the disks.
This doubles the time and cost to fabricate the master templates.
In the case of nanoimprinting, the master template can be very
expensive and require several days to fabricate because it is
typically generated by relatively costly and slow e-beam
lithography equipment.
[0010] Pending application Ser. No. 11/740,289, filed Apr. 26, 2007
and assigned to the same assignee as this application, describes a
disk drive with disks that have identical pre-patterned servo
patterns on the front and back disk surfaces. The servo sectors on
the two disk surfaces of each disk form identical patterns of
angularly spaced arcuate-shaped lines that extend generally
radially across the data tracks. The arcuate-shaped lines on one
surface, e.g., the front surface, generally replicate the path of
the recording head as it is moved across the data tracks by the
rotary actuator, so that there is a constant sampling rate of the
servo sectors on the front surface regardless of radial position of
the head. However, the arcuate-shaped lines on the other surface,
i.e., the back surface, do not replicate the path of the recording
head so the servo sampling rate is not constant but varies with
radial position of the head. Thus when the servo control system is
operating from servo sectors on the back surface, the servo control
processor calculates a timing adjustment from an estimate of the
radial position of the head. This timing adjustment is then used to
adjust the time to open a time window to allow detection of the
servo sectors on the back surface.
[0011] What is needed is a magnetic recording disk drive that has
disks with identical servo patterns on the front and back disk
surfaces, and a disk drive with a servo control system that can
operate with the identical servo patterns but that does not require
the calculation of a timing adjustment.
SUMMARY OF THE INVENTION
[0012] The invention is a magnetic recording disk drive with disks
that have identical pre-patterned servo patterns on the front and
back surfaces and a servo control system for positioning the
read/write heads using the servo signals from the identical servo
patterns. The servo patterns on each disk surface are pre-patterned
with a single master template, resulting in the identical pattern
on each disk surface. The servo sectors on the two disk surfaces
form identical patterns of angularly spaced arcuate-shaped lines
that extend generally radially across the data tracks. The
arcuate-shaped lines on one surface, the front surface, generally
replicate the path of the recording head as it is moved across the
data tracks by the rotary actuator, so that there is a constant
sampling rate of the servo sectors on the front surface regardless
of radial position of the head.
[0013] However, the arcuate-shaped lines on the other surface,
i.e., the back surface, do not replicate the path of the recording
head so the servo sampling rate is not constant but varies with
radial position of the head. The disk drive servo control system
implements a method to enable track seeking from one data track to
another data track, regardless of whether the initial disk surface
and the destination disk surface are front or back surfaces. The
servo signal from a front surface is used during a first seek phase
and the servo signal from the destination surface is used during
the last phase of the seek, even if the destination surface is a
back surface. For example, if the target sector where data is to be
read or written is on a back surface, then during seeking the servo
signal from a front surface, which may or may not be the initial
disk surface, is used for the first phase of the seek, and the
servo signal from the destination surface, which is a back surface,
is used during the last phase of the seek. During the first phase
of the seek, the constant frequency of the servo sectors from the
front surface applies. The servo control system switches over to
receive the servo signals from the destination surface during the
second phase of the seek, i.e., when the head is within a
predetermined number of tracks where variation in servo timing is
small enough to be tolerated.
[0014] 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 DRAWINGS
[0015] FIG. 1 is a schematic of a prior art disk drive with a
rotary actuator and a rigid magnetic recording disk having
pre-patterned servo sectors formed on a first or "front"
surface.
[0016] FIG. 2 is an expanded view of a portion of a typical servo
sector and portions of three data tracks of the prior art disk
shown in FIG. 1.
[0017] FIG. 3 is a block diagram of a disk drive servo control
system for use with this invention.
[0018] FIGS. 4A and 4B show the comparison of the servo patterns on
the front (FIG. 4A) and back (FIG. 4B) surfaces of a prior art
disk.
[0019] FIGS. 5A and 5B show the identical servo patterns of the
front surface (FIG. 5A) and back surface (FIG. 5B) of one
embodiment of a disk according to this invention.
[0020] FIG. 6 is a view of a portion of a servo sector according to
this invention with substantially symmetric servo fields.
[0021] FIG. 7 is a view of a portion of a servo sector according to
this invention with substantially symmetric servo fields for use
with a phase-based servo system (also called a timing-based servo
system).
[0022] FIG. 8 is an illustration of typical disk drive
geometry.
[0023] FIG. 9 is a graph of timing adjustment as a function of
radius, relative to zero timing adjustment at the inner radius
(r.sub.ID), for a disk surface with straight-line servo
sectors.
[0024] FIG. 10 is a flow chart illustrating the method of this
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0025] FIG. 1 illustrates a disk drive with a rotary actuator 2 and
a rigid magnetic recording disk 10 having pre-patterned servo
sectors 18 formed on a first or "front" surface 11. The disk 10
rotates in the direction 102 about a central axis 100. The front
surface 11 has an annular data band 12 which is defined by an
inside diameter (ID) 14 and an outside diameter (OD) 16. The
portions of the data band between the servo sectors 18 are used for
the storage of user data and contain circular data tracks, with
each data track being typically divided into physical data sectors.
The rotary actuator 2 rotates about pivot 4 and supports a
read/write head 6 at its end. As the actuator 2 rotates, the head 6
follows a generally arcuate path between ID 14 and OD 16.
[0026] The servo sectors 18 are not formed by conventional
servowriting but by a patterning process using a master template.
In contact magnetic transfer (CMT), also called magnetic
lithography (ML), a magnetic mask serves as the master template. In
nanoimprinting a master template replicates a topographic pattern
onto a polymeric resist coating on the disk substrate, followed by
sputter deposition of magnetic material over the pattern. The servo
sectors 18 form a pattern of angularly spaced arcuate lines that
extend generally radially from ID 14 to OD 16. The arcuate shape of
the servo sectors matches the arcuate path of head 6. During
operation of the disk drive, the head 6 reads or writes data on a
selected one of a number of concentric circular data tracks located
between the ID 14 and OD 16 of the annular data band 12. To
accurately read or write data from a selected track, the head 6 is
required to be maintained over the centerline of the track.
Accordingly, each time one of the servo sectors 18 passes beneath
the head 6, the head 6 detects discrete magnetized servo blocks in
the position error signal (PES) field in the servo sector. A PES is
generated and used by the disk drive's head positioning control
system to move the head 6 towards the track centerline. Thus,
during a complete rotation of the disk 10, the head 6 is
continually maintained over the track centerline by servo
information from the servo blocks in successive angularly spaced
servo sectors 18.
[0027] An expanded top view of a typical servo sector 18 and
portions of three data tracks is shown in FIG. 2. The three data
tracks 20, 22, 24 are shown in outline. If the disk 10 is the type
with a continuous layer of magnetic recording material in the data
portions of surface 11, then the tracks 20, 22, 24 are continuous
tracks whose radial width is defined generally by the head 6 when
it records on the continuous recording layer. If the disk 10 is the
type with discrete tracks, then the tracks 20, 22, 24 would contain
continuous recording material along the tracks but the tracks would
be separated from each other by nonmagnetic guard bands. If the
disk 10 is the type with patterned media, then the tracks 20, 22,
24 would each contain discrete islands of magnetizable
material.
[0028] All of the shaded portions of FIG. 2 represent discrete
servo blocks magnetized in the same direction. They may all be
magnetized in the same direction horizontally, i.e., in the plane
parallel to the plane of the paper in FIG. 2, if the disk drive is
designed for longitudinal or horizontal magnetic recording, or
perpendicularly, i.e., into or out of the plane of the paper, if
the disk drive is for perpendicular magnetic recording. It is also
possible that every other shaded region in FIG. 2 might have
opposite polarity, with the unshaded regions being nonmagnetic,
which improves the signal quality of the servo pattern, as
described in application Ser. No. 11/149,028, published as
US20060279871 A1 and assigned to the same assignee as this
application. If the servo sectors 18 are formed by CMT then the
non-shaded portions of FIG. 2 represent regions that are magnetized
in the opposite direction from the magnetization of the servo
blocks because they retain this opposite magnetization from a DC
magnetization process prior to the CMT process. If the servo
sectors 18 are formed by nanoimprinting then the non-shaded
portions of FIG. 2 represent nonmagnetic regions, i.e. either
regions of nonmagnetic material or regions of magnetic material
generally incapable of being magnetized by the write head.
[0029] The servo blocks that make up servo sector 18 are arranged
in fields 30, 40, 50 and 60, as shown in FIG. 2. Servo field 30 is
an automatic gain control (AGC) field of blocks 31-35 that are used
to measure the amplitude of the signal and adjust the provide gain
for the subsequently read servo blocks. Servo field 40 is sector
identification (SID) field, also called a servo timing mark or STM
field, to provide a timing mark to establish start/stop timing
windows for subsequent servo blocks. Servo field 50 is a track
identification (TID), also called the cylinder or CYL field because
the tracks from all of the disk surfaces in a disk drive with a
multiple stacked disks form a "cylinder" of tracks. The TID field
50 contains the track number, typically Gray-coded, and determines
the integer part of the radial position. Servo field 60 is the
position error signal (PES) field, which in this example contain A,
B, C, D subfields of servo blocks as part of the well-known
"quad-burst" PES pattern, and are used to determine the fractional
part of the radial position. In some cases, a separate servo sector
counter field (not shown) may be located between the TID and PES
fields for encoding the servo sector number.
[0030] FIG. 3 is a block diagram of the disk drive servo control
system and illustrates the read/write electronics 210, servo
electronics 220, interface electronics 230, and controller
electronics 240. Read/write electronics 210 receives signals from
head 6, passes servo information from the servo sectors to servo
electronics 220, and passes data signals to controller electronics
240. Servo electronics 220 uses the servo information to produce a
signal at 221 which drives actuator 2 to position the head 6.
Interface electronics 230 communicates with a host system (not
shown) over interface 231, passing data and command information,
including requests from the host system for reading from or writing
to the data sectors of disk 10. Interface electronics 230
communicates with controller electronics 240 over interface
233.
[0031] Controller electronics 240 includes a microprocessor 241 and
associated memory 242 with stored computer programs for executing
various algorithms, including a control program 245 that executes
the control algorithm. The control algorithm uses a set of
parameters stored in memory 242 and based on the static and dynamic
characteristics of the actuator 2. The control algorithm is
essentially a matrix multiplication algorithm, and the parameters
are coefficients used in the multiplication.
[0032] Controller electronics 240 receives a list of requested data
sectors from interface electronics 230 and converts them into
cylinder (i.e., track), head, and data sector numbers which
uniquely identify the physical location of the desired data sectors
on disk 10. The head and cylinder numbers are passed to servo
electronics 220, which positions head 6 over the appropriate data
sector on the appropriate cylinder. If the cylinder number provided
to servo electronics 220 is not the same as the cylinder number
over which head 6 is presently positioned, servo electronics 220
first executes a "seek" operation to move the head 6 from its
present cylinder to the desired cylinder.
[0033] The servo electronics 220 first begins executing sector
computations to locate and identify the desired data sector. As
servo sectors pass under head 6, each servo sector is detected. In
brief, the SID is used to locate servo sectors, and a count of SIDs
from a servo sector containing an index mark uniquely identifies
each servo sector. SID decoder 400 receives a control input 430
from the controller electronics 240 that opens a time window for
detection of the next SID. SID decoder 400 then receives a clocked
data stream 211 as input from the read/write electronics 210. Once
a SID has been detected, a SID found signal 420 is generated. The
SID found signal 420 is used to adjust timing circuit 401, which
controls the operating sequence for the remainder of the servo
sector. After detection of a SID, the track identification (TID)
decoder 402 receives timing information 422 from timing circuit
401, reads the signals generated by TID field 50 (FIG. 2), and then
passes the decoded TID information 424 to controller electronics
240. During a seek operation the controller electronics 240 uses
the TID information to estimate the position and velocity of the
head from a stored program of instructions represented as control
program 245.
[0034] Once servo electronics 220 has positioned head 6 over the
appropriate cylinder, the servo fields are read by the head 6 and
read/write electronics 210 inputs signals 211 to the servo
electronics 220. Subsequently, PES decoder 403 captures the signals
from PES field 60 (FIG. 2), then passes the PES 426 to controller
electronics 240. Controller electronics 240 uses the PES as input
to a control algorithm to calculate the signal 428 to actuator
position controller 404 to maintain the head 6 on the centerline of
the desired track.
[0035] Referring again to FIG. 1, it can be seen that the servo
sectors 18 are shaped as an arc whose center of whose center of
rotation is the pivot 4 of actuator 2. This arcuate shape for the
servo sectors assures that the time interval between successive
sector sectors passing the head remains fixed, regardless of which
track the head is on. This simplifies the design and operation of
the head-positioning servo system because a constant servo sampling
rate is achieved regardless of head motion. This arcuate shape is
also the shape of the servo sectors when the servo pattern is
created using conventional track-by-track servowriting methods
(external servowriting and self-servowriting).
[0036] However, this requirement for the shape of the servo sectors
means that the second or "back" surface of each disk must be the
mirror image of the first or front surface. This also assures that
the order of the servo fields (FIG. 2) detected by the head is the
same for each disk surface so that no modification of the servo
control system is required. FIGS. 4A and 4B show the comparison of
the front surface 11 (FIG. 4A) and back surface 11a (FIG. 4B) of a
prior art disk 10 as the disk rotates in the direction 102. A
comparison of the direction of curvature of the arcuate servo
sectors 18 (FIG. 4A) with the direction of curvature of the arcuate
servo sectors 18a (FIG. 4B) shows that the two servo patterns are
not identical, but mirror images of one another. Thus the master
template used to pattern servo sectors 18a on back surface 11a must
be the mirror image of the master template used to pattern servo
sectors 18 on front surface 11. However, this requires that two
different master templates must be fabricated and used to form the
servo patterns for a single disk.
[0037] In the previously-cited pending application Ser. No.
11/740,289, a single master template is used for both disk
surfaces, resulting in the front and back surfaces having identical
servo patterns. FIGS. 5A and 5B show the identical servo patterns
of the front surface 511 (FIG. 5A) and back surface 511a (FIG. 4B)
of a disk 510 as described in pending application Ser. No.
11/740,289. A comparison of the direction of curvature of the
arcuate servo sectors 518 (FIG. 5A) with the direction of curvature
of the arcuate servo sectors 518a (FIG. 5B) shows that the two
servo patterns are identical. However, it is apparent that the
servo sectors 518a on back surface 511a do not have a shape that
follows the arc of actuator 2, so that a constant servo sample rate
will not be achieved on surface 511a.
[0038] While the above description and figures have shown only a
single disk, disk drives typically have multiple disks stacked on
the rotatable spindle. Thus the disk drive of the present invention
may have one or more disks, each with front and back disk surfaces,
511, 511a, respectively.
[0039] Also, because the servo patterns on surfaces 511 and 511a
are identical, the order of the servo fields detected by the head
6a on back surface 511a will be the reverse of the order of the
servo fields detected by the head 6 on front surface 511. Thus, the
arrangement of the fields within the servo sectors are also
modified, as described in the pending application Ser. No.
11/740,289. In one embodiment, there is no change to the servo
fields, but the servo sectors on back surface 511a are read in the
opposite order, i.e., PES, CYL, SID, AGC, and stored in memory.
After the AGC is read and interpreted for servo signal amplitude,
the SID is analyzed and timing is established. Even though the SID
mark is time-reversed compared to expected, still the same
correlation procedure used for front surface 511 may be used, with
the exception that there is an accommodation for the inverted bit
sequence. After timing is established, the CYL values are decoded
(again taking into account the inverted sequence of bits), and the
PES is decoded. In this embodiment there is a time delay on back
surface 511a corresponding generally to the length of one servo
sector because the servo sector is read and stored in memory before
any calculations are performed by controller electronics 240.
[0040] In another embodiment servo sectors with substantially
symmetric servo fields are used, as shown in FIG. 6. In the
embodiment of FIG. 6, the order in which the servo fields is read
is the same for the front and back surfaces. For example, for front
surface 511 as shown in FIG. 5A, the servo fields would move in the
direction indicated by arrow 102, while for back surface 511a as
shown in FIG. 5B, the servo fields would move in the direction
indicated by arrow 102a. The servo fields are substantially
symmetric about the center of the servo sector. The PES field is
located in the center of the servo sector, with the CYL code
distributed as CYL1 and CYL2 on opposite ends of the PES field. The
complete CYL field normally contains m bits that contain actual
track information and n error correction code (ECC) bits, where
typically m and n have similar values. For the symmetric format of
FIG. 6, the CYL field is split into two subfields (CL1 and CYL2).
CYL1 and CYL2 may each provide partial information about the
cylinder number, whereas both are needed to obtain accurate track
number involving ECC as well. For example, CYL1 and CYL2 can each
have m+n/2 bits. This allows enough information for long seeks.
Identical AGC fields AGC1 and AGC2) are located at each end of the
servo sector, and identical SID fields (SID1 and SID2) are located
between respective AGC and CYL fields. In the servo sector format
shown in FIG. 6, the additional disk surface "overhead" is for the
second SID field as well as the extra m bits of the second CYL
field. In a typical servo system, for example 140 servo sectors
angularly spaced around each disk surface, this would result in
approximately 16 bits of overhead for having two CYL fields
(assuming no ECC) and about 12-16 bits for the second SID field. A
typical servo sector may have approximately 40 bits of AGC, 12 bits
of SID, 32 bits of track code, 12 bits of sector code and 48 bits
of PES code, for total of approximately 144 bits. The additional
28-32 bits is thus approximately a 20% increase in servo overhead
over the conventional servo pattern (FIG. 2).
[0041] Another embodiment of servo sectors with substantially
symmetric servo fields uses a phase-based servo system (also called
a timing-based servo system), whose pattern is shown in FIG. 7. The
PES field 700 includes two symmetric sets 702, 704 of generally
slanted position marks extending generally radially across multiple
tracks. Identical start-of-field (SOF) marks 701, 703 are located
on respective ends of PES field 700 and extend radially across the
tracks. The time from detection of a SOF mark to detection of a
slanted position mark indicates the radial position of the head.
This type of PES field is different from the conventional
quad-burst PES field 60 in FIG. 2, and thus a different type of PES
decoding system is used. The phase-based servo system and decoding
method is well-known, as described for example in U.S. Pat. Nos.
5,689,384; 5,923,272 and 5,930,065. In the embodiment of FIG. 7,
the CYL fields can be encoded in additional patterns located before
or after the sets 702, 704. Alternatively, the CYL fields can be
encoded within the sets 702, 704 of slanted position marks by
shifting pairs of position marks early or late relative to other
position marks in the pattern, in a manner which does not affect
the overall phase relationship between the position marks. An
example of such encoding with timing-based patterns is described in
the previously-cited U.S. Pat. Nos. 5,923,272 and 5,930,065. Since
this method of encoding a CYL field embeds only a single bit or a
few bits within each servo sector, a complete reading of a complete
CYL address requires several successive servo sectors.
[0042] The present invention relates to a disk drive servo control
system for positioning the heads on the disks that have disk
surfaces with identical servo patterns, like the arcuate-shaped
servo sectors as shown in FIGS. 5A-5B. In the method of this
invention, the servo signal from a "front" surface 511 is used for
a first phase of a seek, and the servo signal from the surface with
the target data sector is used during the last phase of the seek.
For example, if the target sector is on a "back" surface 511a then,
depending on the length of the seek, the controller electronics 240
uses the servo signal from a surface with the "correct" servo
pattern, such as surface 511, for the first portion of the seek,
and the servo signal from the surface 511a that has the target data
sector during the last portion of the seek. During the first phase
of the seek, the constant frequency of the servo sectors 518 from
front surface 511 applies. The controller electronics 240 switches
over to receive the servo signals from the target surface 511a
during the second phase of the seek. During the seek's second
phase, the timing variation from the servo sectors 518a on back
surface 511a is small enough to ignore.
[0043] It is possible to estimate at what point the switch between
the starting surface and the target surface can be made. The
arcuate shape of the servo sectors on a back surface 511a cause a
timing error in the servo sector detection window from the
"correct" arcuate shape on a front surface 511. This will be
explained with the use of FIG. 8, which shows a typical disk drive
geometry. The actuator is located at distance p between pivot 4 and
disk central axis 100, and has an actuator length a, which is the
distance from pivot 4 to the actuator tip RW where the head is
located. The head on the tip of the actuator makes an arc or path
Z1 across the disk from a point b1 at disk inner radius r.sub.ID to
point b2 at disk outer radius r.sub.OD. In this case conventional
servo sectors 518 like those on front surface 511 will have a
curvature with radius a. It is straightforward to conclude from
FIG. 8 that the "skew" angle as a function of radius is given
by:
.alpha. ( r ) = arccos p 2 + r 2 - a 2 2 pr - arccos p 2 + r ID 2 -
a 2 2 pr ID ##EQU00001##
[0044] Because the head is fixed at the tip of the actuator, the
magnetic transitions written by head are not collinear with the
radius (or not orthogonal to the track direction). This is called
head "skew". From the triangle (4-100-RW) the head skew is given
by:
skew = 90 - arccos a 2 + r 2 - p 2 2 ar ##EQU00002##
The time delay of the arcuate sector, when compared to a linear
sector (the one that would result from drawing a straight line from
b1 to b2) is given by:
.tau. ( r ) = 60 rpm .times. .alpha. ( r ) 360 .degree.
##EQU00003##
where the angle alpha is expressed in degrees. This timing
adjustment is plotted in FIG. 9 as a function of radius, and
relative to the timing adjustment at r.sub.ID, which is assumed to
be 0. FIG. 9 is generated based on the dimensions listed in FIG. 8
at a disk rpm of 15,000.
[0045] From FIG. 9 the maximum rate of time delay change is about
10 .mu.s/mm. A typical servo system can tolerate SID timing
variations of up to about 100 ns=0.1 .mu.s (exact tolerable timing
variation depends on the implemented size of the SID timing
window). Therefore, during the final phase on the target surface it
may be possible to tolerate head "wandering" of up to 0.1 .mu.s/(10
.mu.s/mm) or about 0.01 mm (10 .mu.m). This distance may contain
about 50-100 tracks in a typical disk drive. This 10 .mu.m limit
also indicates the level of preferred concentricity between front
and back surface of the disks, i.e., the difference in disk runout
between the front and back surface of a disk which will necessarily
be present due to the disk fabrication process. Thus if the seek
length is less than this distance (a predetermined number of data
tracks) there is no need to switch from one surface during a first
phase of the seek to a second surface during the second phase of
the seek.
[0046] FIG. 10 is a flow chart illustrating the method of the
present invention in more detail. The flow chart is for the purpose
of illustrating an algorithm that can be implemented as computer
program instructions stored in memory 242 as part of the control
program 245 (FIG. 3).
[0047] Prior to the initiation of a track seek from one cylinder to
another cylinder, the servo control system is in a track
"following" mode, wherein the head associated with a disk surface A
is maintained on an initial data track contained within the initial
or first cylinder. First, a determination is made as to whether the
seek length is greater than a predetermined value, e.g., 50 tracks
in the example given above. If it is less then this value, then a
switch is made to the target or destination surface and the entire
seek is performed on the target surface. If the seek length is less
than this predetermined value and the initial surface (surface A)
is also the destination surface then the entire seek is performed
on the initial surface.
[0048] However, if the seek length is greater than this
predetermined value, then a determination is made whether the
initial surface on which track following is taking place (surface
A) is a front or back surface (block 800).
[0049] If surface A is a front surface then a determination is made
as to whether the target data sector (and thus the target data
track) is also on surface A (block 805). If it is then both phases
of the complete seek are performed using the servo signals from
surface A (block 810). If the target data track is not on surface
A, then it is on the destination surface B. Then, at block 815, a
determination is made whether destination surface B is a front or
back surface. If surface B is a front surface then the servo
control system switches from track following on surface A using the
servo signals from surface A, to receive servo signals from surface
B, with both phases of the complete seek being performed using the
servo signals from surface B (block 820). However, if destination
surface B is a back surface then the servo control system continues
to use the servo signals from surface A during the first phase
(block 825), but switches over to receive servo signals from
surface B during the second phase (block 830) when the head is
within the predetermined number of tracks from the target
track.
[0050] Referring back to block 800, if surface A is a back surface
then a determination is made as to whether the target data track is
also on surface A (block 835). If it is then the servo control
system switches from track following on surface A using the servo
signals from surface A, to receive servo signals from surface C,
where surface C is any front disk surface, during the first phase
(block 840). The servo control system then switches back to receive
servo signals from surface A during the second phase (block 845).
If the target data track is not on surface A, then it is on the
destination surface B. Then, at block 850, a determination is made
whether destination surface B is a front or back surface. If
surface B is a front surface then the servo control system switches
from track following on surface A using the servo signals from
surface A, to receive servo signals from surface B, with both
phases of the complete seek being performed using the servo signals
from surface B (block 855). However, if destination surface B is a
back surface then the servo control system switches over to receive
servo signals from surface C, where surface C is any front disk
surface, during the first phase (block 860). The servo control
system then switches back to receive servo signals from surface B
during the second phase (block 865).
[0051] 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.
* * * * *