U.S. patent application number 11/214549 was filed with the patent office on 2007-03-01 for method of compensating for microjog error due to repeatable run-out.
This patent application is currently assigned to Iomega Corporation. Invention is credited to Gregory M. Allen.
Application Number | 20070047133 11/214549 |
Document ID | / |
Family ID | 37803734 |
Filed Date | 2007-03-01 |
United States Patent
Application |
20070047133 |
Kind Code |
A1 |
Allen; Gregory M. |
March 1, 2007 |
METHOD OF COMPENSATING FOR MICROJOG ERROR DUE TO REPEATABLE
RUN-OUT
Abstract
A process for continually compensating for the microjog error
resulting from RRO. Typically, the 1F RRO is the most significant,
but the method could be applied to the microjog error caused by RRO
of other frequencies. The process continually determines an
instantaneous microjog error based on the RRO and adjusts the read
element target position throughout one revolution, such that the
write element remains centered on its intended position.
Inventors: |
Allen; Gregory M.; (Layton,
UT) |
Correspondence
Address: |
PROCOPIO, CORY, HARGREAVES & SAVITCH LLP
530 B STREET
SUITE 2100
SAN DIEGO
CA
92101
US
|
Assignee: |
Iomega Corporation
|
Family ID: |
37803734 |
Appl. No.: |
11/214549 |
Filed: |
August 29, 2005 |
Current U.S.
Class: |
360/77.04 ;
G9B/5.221 |
Current CPC
Class: |
G11B 5/59627
20130101 |
Class at
Publication: |
360/077.04 |
International
Class: |
G11B 5/596 20060101
G11B005/596 |
Claims
1. A disk drive comprising: a read/write head having a read element
and a write element, wherein the read element and the write element
are separated by a preset distance thereby resulting in a microjog
distance between the read and write elements as the read/write head
is positioned at tracks of the disk; a control circuit which
positions the read/write head along a desired track of the disk,
wherein the control circuit adjusts the position of the read/write
head based on a repeatable runout measurement of the disk, and the
control circuit further adjusts the position of the read/write head
based on variations in the microjog distance resulting from
adjustments based on the repeatable runout.
2. The disk drive according to claim 1, wherein the control circuit
determines the repeatable runout measurement.
3. The disk drive according to claim 1, wherein the position
adjustment of the read/write head centers the write element on a
write target position.
4. The disk drive according to claim 1, wherein the control circuit
positions the read element based on the repeatable runout
measurement.
5. The disk drive according to claim 1, wherein the error due to
the repeatable runout measurement is determined by the equation:
sin (w)*(calc_calibrated_microjog (head,
track+RRO)-calc_calibrated_microjog (head, track)); where w refers
to the angle through which the disk rotates, synchronized with the
phase of the repeatable runout.
6. The disk drive according to claim 1, further comprising a
removable cartridge, wherein the cartridge contains a storage
medium.
7. The disk drive according to claim 6, wherein the storage medium
is a magnetic media.
8. The disk drive according to claim 6, wherein the control circuit
disables write functionality when the repeatable runout measurement
exceeds a predetermined threshold.
9. A method of compensating for microjog error due to repeatable
runout comprising: determining a nominal microjog distance based on
a location of a read/write head relative to a disk; obtaining a
repeatable runout measurement of the disk; calculating an
instantaneous microjog error due to the repeatable runout; and
adjusting the nominal microjog distance in accordance with the
instantaneous microjog error.
10. The method according to claim 9, further comprising applying
the instantaneous microjog error to position a write element.
11. The method according to claim 10, further comprising centering
the write element on a write target position.
12. The method according to claim 9, further comprising calculating
the instantaneous microjog error using the formula: sin
(w)*(calc_calibrated_microjog (head,
track+RRO)-calc_calibrated_microjog (head, track)) where w refers
to the angle through which the disk rotates, synchronized with the
phase of the repeatable runout.
13. The method according to claim 9, further comprising obtaining a
new repeatable runout measurement upon insertion of a removable
cartridge.
14. The method according to claim 13, wherein the removable
cartridge contains storage media.
15. The method according to claim 9, further comprising disabling
write functionality when the repeatable runout measurement exceeds
a predetermined threshold.
16. A method of positioning a read/write head in a disk drive, the
method comprising: positioning a read/write head along a desired
track of the disk, the read/write head having a read element and a
write element, wherein the read element and the write element are
separated by a preset distance thereby resulting in a microjog
distance between the read and write elements as the read/write head
is position at tracks of the disk; varying the position of the
read/write head based on a repeatable runout measurement of the
disk; and adjusting the position of the read/write head based on
variations in the microjog distance resulting from varying the
position of the read/write head based on the repeatable runout.
17. A disk drive comprising: a read/write head having a read
element and a write element, wherein the read element and the write
element are separated by a preset distance; a control circuit which
positions the read element centered on a read target position while
writing data with the write element to a write track, then when
reading the data written to the write tack, the controller
positions the read element to a nominal write track position and
further adjusts the position of the read element about the nominal
write track position based on microjog variations resulting from
adjustments based on instantaneous repeatable runout
measurements.
18. The disk drive according to claim 17, further comprising a
removable cartridge, wherein the cartridge contains a storage
medium.
19. The disk drive according to claim 17, further comprising a
fixed storage medium.
Description
TECHNICAL FIELD OF THE INVENTION
[0001] This invention relates in general to information storage
devices and, more particularly, to information storage devices
having read/write heads with spaced read and write elements.
BACKGROUND OF THE INVENTION
[0002] Most computers include a disk drive which is used for data
storage. The disk drive includes a rotatable disk having a magnetic
coating on at least one side thereof. A read/write head is disposed
adjacent the surface, and an actuator can move the read/write head
approximately radially with respect to the surface, so that the
head can write data to the surface and/or read data from the
surface. The surface on the disk is conceptually divided into a
plurality of concentric data tracks, which can each store data.
[0003] Early disk drives included a read/write head having a single
read/write element, which was used both for writing data and
reading data. However, there has been a progressively increasing
demand for disk drives with significantly higher storage densities,
and one result is that new types of heads have come into common
use, examples of which include the magneto-resistive (MR) head, and
the giant maaneto-resistive (GMR) head. These MR and GMR heads
typically have one element for writing data and a separate element
for reading data, and these read and write elements are physically
spaced from each other.
[0004] As is known in the art, a head can be positioned with
respect to a disk by using feedback control based on servo
information read from the disk with a read element of the head. In
a head with spaced read and write elements, the read element is
used to position the head relative to the disk not only for
reading, but also for writing. One aspect of this is that, as the
head is moved relative to the disk, the orientation of the read and
write elements varies with respect to the tracks on the disk, such
that the write element is typically aligned with a track that is
different from the track with which the read element is aligned.
Consequently, in order to correctly position the write element over
a selected track for the purpose of writing data to that track, the
read element must be positioned at a location which is radially
offset from the selected track. This radial offset is referred to
as a "microjog", and has a magnitude which varies as the head moves
radially with respect to the disk. Techniques have been developed
for calculating microjog values, and have been generally adequate
for their intended purposes, but they have not been satisfactory in
all respects.
[0005] As one aspect of this, there are existing disk drives in
which the disk is rotatably supported in a removable cartridge, and
in which the head is movably supported in a drive unit that can
removably receive the cartridge. A given drive unit must be able to
work with any of several similar and interchangeable cartridges,
and any given cartridge must be capable of working in any of a
number of compatible drive units. The removability of the cartridge
introduces a number of real-world considerations into the system,
and these considerations affect the accurate calculation of a
microjog value.
[0006] For example, the cartridges have manufacturing tolerances
which vary from cartridge. Thus, from cartridge to cartridge, there
will be some variation relative to the cartridge housing of the
exact position of the axis of rotation of the disk. As another
example, two different cartridges may have slightly different
mechanical seatings when they are inserted into the same drive
unit. In fact, a given cartridge may experience different
mechanical seatings on two successive insertions into the same
drive unit. Real-world variations of this type cause small
variations in the orientation of the read/write head with respect
to the tracks on the disk, and thus affect accurate calculation of
a microjog value.
[0007] One of the major components of head position error is called
repeatable runout (RRO). RRO that occurs at the disk rotating
frequency may be called 1F runout. There are several possible
causes for 1F runout, such as an unbalanced spindle, or a non-ideal
spindle bearing.
[0008] In order to realize higher data storage densities in systems
of the type which utilize removable cartridges, it is desirable to
be able to use read/write heads that facilitate high storage
densities, especially read/write heads that have spaced read and
write elements, such as MR and GMR heads. What is needed is a
system that compensates for any changes in the microjog that may
occur.
[0009] Further, if a removable cartridge is dropped, the disk may
slip within the clamp, resulting in large RRO. As the head moves
back and forth in order for the read element to follow the RRO, the
write element, which is spaced some distance away from the read
element, does not remain centered over the intended write position.
If the track density is high enough, the microjog error caused by
the RRO will increase, eventually resulting in degraded performance
in reading the data. In addition, if the RRO changes once the disk
has been written, a subsequent write may cause encroachment. What
is needed is a system that can compensate for microjog error caused
by the RRO.
SUMMARY OF THE INVENTION
[0010] A process for continually compensating for the microjog
error resulting from RRO. Typically, the 1F RRO is the most
significant, but the method could be applied to the microjog error
caused by RRO of other frequencies. The process continually
determines an instantaneous microjog error based on the RRO and
adjusts the read element target position throughout one revolution,
such that the write element remains centered on its intended
position.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] A better understanding of the present invention will be
realized from the detailed description which follows, taken in
conjunction with the accompanying drawings, in which:
[0012] FIG. 1 is a diagrammatic view of an apparatus which is an
information storage system that embodies aspects of the present
invention;
[0013] FIG. 2 is a fragmentary diagrammatic view which shows a
portion of the system of FIG. 1 in a substantially enlarged
scale;
[0014] FIG. 3 is a fragmentary diagrammatic view similar to FIG. 2,
but showing a different operational position;
[0015] FIG. 4 is a fragmentary diagrammatic view similar to FIGS. 2
and 3, but showing still another operational position;
[0016] FIG. 5 is a diagrammatic view showing a geometric
relationship between selected elements of the system of FIG. 1;
[0017] FIG. 6 is a diagrammatic view showing different geometric
relationships involving other elements of the system of FIG. 1;
and
[0018] FIG. 7 is a flowchart illustrating a method of positioning
the read/write head based on an instantaneous RRO-induced microjog
error.
DETAILED DESCRIPTION
[0019] FIG. 1 is a diagrammatic view of an apparatus which is an
information storage system 10, and which embodies aspects of the
present invention. The system 10 includes a receiving unit or drive
12 which has a recess 14, and includes a cartridge 16 which can be
removably inserted into the recess 14.
[0020] The cartridge 16 has a housing, and has within the housing a
motor 21 with a rotatable shaft 22. A disk 23 is clamped on the
shaft 22 for rotation therewith. The side of the disk 23 which is
visible in FIG. 1 is coated with a magnetic material of a known
type, and serves as an information storage medium. This disk
surface is conceptually divided into a plurality of concentric data
tracks. In the disclosed embodiment, there are about 50,000 data
tracks, not all of which are available for use in storing user
data.
[0021] The disk surface is also conceptually configured to have a
plurality of circumferentially spaced sectors, two of which are
shown diagrammatically at 26 and 27. These sectors are sometimes
referred to as servo wedges. The portions of the data tracks which
fall within these sectors or servo wedges are not used to store
data. Data is stored in the portions of the data tracks which are
located between the servo wedges. The servo wedges are used to
store servo information of a type which is known in the art. The
servo information in the servo wedges conceptually defines a
plurality of concentric servo tracks, which have a smaller width or
pitch than the data tracks. In the disclosed embodiment, each servo
track has a pitch or width that is approximately two-thirds of the
pitch or width of a data track. Consequently, the disclosed disk 23
has about 73,000 servo tracks. The servo tracks effectively define
the positions of the data tracks, in a manner known in the art.
[0022] Approximately 60 of the data tracks, which are the radially
innermost tracks, are set aside as a first reserved area 36.
Approximately 60 more data tracks, which are the radially outermost
tracks, are set aside as a second reserved area 37. The reserved
areas 36 and 37 are not available to store user data, but instead
are used for a special purpose which is discussed later. User data
is stored in the many data tracks that are disposed between the
reserved areas 36 and 37 (except in the regions of the servo
wedges).
[0023] The drive 12 includes an actuator 51 of a known type, such
as a voice coil motor (VCM). The actuator 51 can effect limited
pivotal movement of a pivot 52. An actuator arm 53 has one end
fixedly secured to the pivot 52, and extends radially outwardly
from the pivot 52. The housing of the cartridge 16 has an opening
in one side thereof. When the cartridge 16 is removably disposed
within the drive 12, the arm 53 extends through the opening in the
housing, and into the interior of the cartridge 16. At the outer
end of the arm 53 is a suspension 56 of a known type, which
supports a read/write head 57. In the disclosed embodiment, the
head 57 is a component of a known type, which is commonly referred
to as a giant magneto-resistive (GMR) head. However, it could
alternatively be some other type of head, such as a
magneto-resistive (MR) head.
[0024] During normal operation, the head 57 is disposed adjacent
the magnetic surface on the disk 23, and pivotal movement of the
arm 53 causes the head 57 to move approximately radially with
respect to the disk 23, within a range which includes the reserved
areas 36 and 37. When the disk 23 is rotating at a normal
operational speed, the rotation of the disk induces the formation
between the disk surface and the head 57 of an air cushion, which
is commonly known as an air bearing. Consequently, the head 57
floats on the air bearing while reading and writing information to
and from the disk, without direct physical contact with the disk.
However, the invention is not limited to systems in which the head
is spaced from the disk by an air bearing, and can be used in
systems where the head physically contacts the disk.
[0025] The drive 12 includes a control circuit 71, which is
operationally coupled to the motor 21 in the cartridge 16, as shown
diagrammatically at 72. The control circuit 71 selectively supplies
power to the motor 21 and, when the motor 21 is receiving power,
the motor 21 effects rotation of the disk 23. The control circuit
71 also provides control signals at 73 to the actuator 51, in order
to control the pivotal position of the arm 53. At 74, the control
circuit 71 receives an output signal from the head 57, which is
commonly known as a channel signal. When the disk 23 is rotating,
segments of servo information and data will alternately move past
the head 57, and the channel signal at 74 will thus include
alternating segments or bursts of servo information and data.
[0026] The control circuit 71 includes a channel circuit of a known
type, which processes the channel signal received at 74. The
channel circuit includes an automatic gain control (AGC) circuit,
which is shown at 77. The AGC circuit 77 effect variation, in a
known manner, of a gain factor that influences the amplitude of the
channel signal 74. In particular, the AGC circuit uses a higher
gain factor when the amplitude of the channel signal 74 is low, and
uses a lower gain factor when the amplitude of the channel signal
74 is high. Consequently, the amplitude of the channel signal has
less variation at the output of the AGC circuit 77 than at the
input thereof.
[0027] The control circuit 71 also includes a processor 81 of a
known type, as well as a read only memory (ROM) 82 and a random
access memory (RAM) 83. The ROM 82 stores a program which is
executed by the processor 81, and also stores data that does not
change. The processor 81 uses the RAM 83 to store data or other
information that changes dynamically during program execution.
[0028] The control circuit 71 of the drive 12 is coupled through a
host interface 86 to a not-illustrated host computer. The host
computer can send user data to the drive 12, which the drive 12
then stores on the disk 23 of the cartridge 16. The host computer
can also request that the drive 12 read specified user data back
from the disk 23, and the drive 12 then reads the specified user
data and sends it to the host computer. In the disclosed
embodiment, the host interface 86 conforms to an industry standard
protocol which is commonly known as the Universal Serial Bus (USB)
protocol, but could alternatively conform to any other suitable
protocol, including but not limited to the IEEE 1394 protocol.
[0029] FIG. 2 is a fragmentary diagrammatic view which shows, in a
substantially enlarged scale, a portion of the structure of FIG. 1,
including the head 57, the suspension 56, and portions of the arm
53 and disk 23. It should be understood that the depiction of all
of these components is highly diagrammatic. Reference numerals
101-108 identify eight adjacent data tracks on the disk 23, which
are close to but not within the reserved area 37 (FIG. 1). The
tracks 101-108 are circular and concentric but, due to the degree
of enlargement involved in FIG. 2, the curvature is sufficiently
gradual that these tracks appear to be straight lines. The
read/write head 57 has a read element 112 and a write element 113,
which are shown diagrammatically, and which are spaced from each
other. The write element 113 has a length which is somewhat longer
than the length of the read element 112.
[0030] As the disk 23 rotates, segments of servo information and
segments of data on the disk alternately move past the read element
112. The read element 112 produces the channel signal, which is
supplied at 74 to the control circuit 71 (FIG. 1), and which
includes alternating bursts of data and servo information. By
analyzing the successive bursts of servo information received from
the read element 112, the control circuit 71 can make an accurate
determination of the current radial position of the read element
112. In particular, the control circuit 71 can use the servo
information to accurately determine the radial position of the read
element 112 with respect to the not-illustrated servo tracks. Since
the positions of the data tracks are defined by the servo tracks,
knowledge of the radial position of the read element 112 with
respect to the servo tracks also constitutes knowledge of the
radial position of the read element 112 with respect to the data
tracks. Thus, in FIG. 2, the control circuit 71 knows from servo
information read by the read element 112 that the read element 112
is currently disposed at a location straddling data tracks 104 and
105, with slightly more of the read element over track 105 than
over track 104. Using this servo information read by the read
element 112, the control circuit can affect feedback control to
maintain the read element in a given radial position, or to
radially reposition the read element 112.
[0031] Positioning the head 57 with respect to the disk 23 for the
purpose of reading data is relatively straightforward, because the
read element 112 is used both to read the data of interest and also
to read the servo information which is used to position the read
element 112. On the other hand, the write element 113 is used to
write data to the disk 23, but does not read any information from
the disk 23. Consequently, in order to write data to the disk 23,
the write element 113 must be positioned indirectly, through the
approach of positioning the read element 112 using the servo
information which it is reading from the disk, and knowing where
the write element 113 is in relation to the read element 112. A
degree of complexity is introduced by the fact that the write
element 113 is typically not aligned with the same data track as
the read element 112. In fact, the radial position of the write
element 113 in relation to the read element 112 is not a constant,
but varies as the head 57 is moved radially of the disk.
[0032] In FIG. 2, for example, due to the angle of the actuator arm
53 with respect to the disk 23, the write element 113 is offset in
a radial direction by approximately 2.33 data tracks from the read
element 112. As explained above, there are 1.5 of the
not-illustrated servo tracks for each illustrated data track, and
so the offset can also be expressed as 2.33 data
tracks.times.1.5=3.5 servo tracks. This radial offset, which is
also referred to as a "microjog", is indicated diagrammatically at
116 by an arrow. Consequently, if the control circuit 71 wants to
use the write element 113 to write data to the data track 107, the
control circuit must use the servo information received through the
read element 112 to accurately position the read element 112 so
that it straddles data tracks 104 and 105 in the manner shown in
FIG. 2, thereby centering the write element 113 over the data track
107 so that the write element can be used to write data to the data
track 107.
[0033] FIG. 3 is a fragmentary diagrammatic view similar to FIG. 2,
but showing a different operational position. In particular, the
actuator arm 53 has been rotated counterclockwise from the position
shown in FIG. 2, so that in FIG. 3 the head 57 is near but not
within the reserved area 36. FIG. 3 shows eight data tracks
121-128. It will be noted that the write element 113 is centered
over the data track 122, and the read element 112 straddles the
data tracks 124 and 125, with slightly more of the read element
over the track 124 than the track 125. Thus, when the control
circuit 71 wants to use the write element 113 to write data to the
track 122, it uses servo information read by the read element 112
to accurately position the read element 112 so that the read
element straddles the tracks 124-125 in the manner shown in FIG.
3.
[0034] In this situation, the read element 112 is offset by
approximately 2.33 data tracks (3.5 servo tracks) from the write
element 113, which is the microjog indicated by the arrow 131 in
FIG. 3. However, it will be noted that the radial direction of the
arrow 131 in FIG. 3 is opposite to the radial direction of the
arrow 116 in FIG. 2. Stated differently, in order to position the
write element 113 over the track 107 in FIG. 2, the control circuit
71 must position the read element 112 so that it is disposed 2.33
data tracks (3.5 servo tracks) in a direction radially inwardly
from the track 107. In contrast, in order to position the write
element 113 over the track 122 in FIG. 3, the control circuit 71
must position the read element 112 so that it is disposed 2.33 data
tracks (3.5 servo tracks) in a direction radially outwardly from
the track 122.
[0035] FIG. 4 is a fragmentary diagrammatic view similar to FIGS. 2
and 3, but showing yet another operational position. In FIG. 4, the
actuator arm 53 is disposed approximately halfway between the
positions shown in FIGS. 2 and 3. FIG. 4 shows eight data tracks
141-148. The read element 112 and the write element 113 are both
relatively accurately centered over the same data track 144. Thus,
in FIG. 4, the microjog value is zero, because the read element 112
does not need to be positioned with an offset from the track 144 in
order to center the write element 113 over the track 144. Stated
differently, if the read element 112 is radially centered over the
track 144, the write element will also be radially centered over
the track 144.
[0036] With reference to FIGS. 2-4, it will be noted that, as the
actuator arm 53 is pivoted and moves the head 57 radially across
the disk, the appropriate microjog value varies progressively from
a positive value through zero to a negative value. This is due in
part to the spacing between the read element 112 and the write
element 113, and is also due in part to the fact that there is
variation in the angle of the read and write elements with respect
to the tracks on the disk as the head is moved radially with
respect to the disk. Consequently, when the write element 113 is to
be used to write data to any given data track, an appropriate
microjog value must be determined for that data track in order to
know where to position the read element 112 while that write
operation is carried out.
[0037] As discussed above in association with FIG. 1, the cartridge
16 can be removed from the drive 12. In fact, the drive 12 is
designed with the intent that any one of a number of similar
cartridges can be interchangeably inserted into the drive 12, and
that the drive 12 will work reliably and accurately with any of the
cartridges. The removability of the cartridge 16 introduces
additional considerations into the determination of an appropriate
microjog value, because there will be factors that vary from
cartridge to cartridge, and factors that vary from insertion to
insertion. For example, there will be mechanical tolerances
involved in how different cartridges seat within the recess 14
within the drive 12. In fact, if a given cartridge is disposed in
the drive 12, and is then removed and reinserted, the mechanical
seating may change somewhat. If that cartridge is then removed and
replaced with a different cartridge, the replacement cartridge may
seat differently than the original cartridge. Consequently, the
exact position of the disk with respect to the head may vary from
one cartridge insertion to another, for either the same or
different cartridges.
[0038] Further, internal variations can exist from cartridge to
cartridge. For example, due to mechanical tolerances, the physical
location of the motor shaft 22 with respect to the housing of its
cartridge may be slightly different in one cartridge as compared to
another cartridge. These tolerance and/or seating variations can
cause variation in the distance between the motor spindle 22 and
the actuator pivot 52, which in turn can affect the appropriate
microjog value.
[0039] A further consideration is that the servo information on the
disk in one cartridge may have been written to the disk at the
factory by one servo-writer machine, while the servo information on
the disk in a different cartridge may have been written by a
different servo-writer machine. As a result, each track on one disk
may not be in precisely the same radial location as the equivalent
track on another disk.
[0040] Yet another consideration is that the foregoing discussion
has focused on how a particular drive must be able to accurately
and reliably work with any of a number of different cartridges, but
the converse is also true. In particular, a given cartridge must be
able to reliably and accurately work in a number of different
drives.
[0041] Still another consideration is that the spacing between and
orientation of the read and write elements 112 and 113 may vary
from head to head (and thus from drive to drive), for example due
to process variations involved in manufacturing the head. In order
to be able to use exactly the same firmware program for the
processor 81 in each drive 12, without customization for each
drive, the firmware must be capable of accommodating real-world
variations such as variations from one read/write head to
another.
[0042] Consequently, in the context of a removable cartridge, there
are a variety of factors, including those discussed above, which
can affect proper calculation of an accurate microjog value. One
feature of the present invention relates to techniques that allow
accurate determination of a microjog value, despite factors of this
type. These techniques for accurately calculating microjog are
explained in detail below. First, however, an overview is
provided.
[0043] In particular, with reference to FIG. 1, the control circuit
71 of the drive 12 responds to insertion of a cartridge 16 by
erasing data in at least part of the reserved area 36, and then
positioning the write element 113 approximately over a central
portion of the reserved area 36, using servo information read by
the read element 112. The control circuit 71 then uses the write
element 113 to write some predetermined data in the reserved area
36. The control circuit 71 then moves the head 57 radially while
searching for this data with the read element 112, until the
control circuit 71 determines a radial position in which the read
element 112 would be radially centered over this data. Based on
servo information read by the read element 112 while the data is
being written, and also servo information read by the read element
112 while the same data is later being read, the control circuit 71
knows the exact radial position of the read element 112 when the
data was being written, and the exact radial position of the read
element 112 when the data was being read. The control circuit 71
can then take the difference between these two positions, in order
to accurately determine an actual microjog value for one specific
data track within the reserved area 36.
[0044] The control circuit 71 then carries out a similar sequence
of operations for the other reserved area 37. This results in a
very accurate determination of an actual microjog value for one
specific data track within the reserved area 37. The information
obtained in this manner, which includes the two actual microjog
values, serves as compensation information that is specific to the
particular cartridge 16 that has been inserted into the drive 12,
and the particular current seating of that cartridge.
[0045] Thereafter, when the control circuit 71 needs to write data
to a selected data track on the disk 23, it carries out a two-step
procedure. First, it uses a predetermined translation technique,
which is independent of the particular cartridge and its present
seating, to determine a nominal or ideal microjog value for the
selected track. Second, the control circuit 71 uses the
compensation information to adjust the nominal microjog value, in
order to obtain an actual microjog value which accurately takes
into account the particular cartridge and its current seating,
thereby permitting the write element 113 of the head 57 to be
accurately positioned over the selected track. This microjog value
is the calibrated microjog value. The specific manner in which this
is all carried out will now be described in greater detail.
[0046] FIG. 5 is a diagrammatic view of selected components from
the system of FIG. 1, including the motor spindle 22, the actuator
pivot 52, and the head 57. Line 201 extends radially from the motor
pivot 22 to the head 57. Line 202 extends radially between the
motor spindle 22 and the actuator pivot 52, and has a length M.
Line 203, which corresponds conceptually to the actuator arm 53,
extends radially from the actuator pivot 52 to the write element
113 on the head 57, and has a length A. The angle between the lines
202 and 203 is identified by .theta.. The angle formed by the line
201 with the read and write elements is identified by .phi., and
can be referred to as head skew angle. It will be noted that the
angles .theta. and .phi. are not constant, but vary as the arm 53
is pivoted by the actuator 51 to move the head 57 toward or away
from the motor spindle 22 in FIG. 5.
[0047] FIG. 6 is a diagrammatic view showing the read and write
elements 112 and 113 of the head 57. As mentioned above, line 203
extends radially from the actuator pivot 52 to the write element
113, and in particular to the center of the write element 113. The
distance between the read element 112 and the write element 113, in
a direction parallel to the line 203, is a distance S which is
identified in FIG. 6 by a double-headed arrow.
[0048] In FIG. 6, the center of the read element 112 is depicted as
being laterally offset from the line 203 by a distance .delta.,
which is identified by reference numeral 212. The head 57 in the
disclosed embodiment is designed so that, in theory, the read
element 112 should have its center disposed on the line 203, such
that .delta.=0. However, the offset .delta. is depicted in FIG. 6
because, due to practical considerations such as manufacturing
process variations, the read element 112 may not actually be
centered accurately on the line 203. In FIG. 6, the total microjog
amount is indicated at MJ, and is made up of two portions, which
are respectively MJ1 and MJ2. Using standard trigonometric
principles, MJ1 and MJ2 can be expressed as: MJ1=Ssin(.phi.)
MJ2=.delta.cos(.phi.)
[0049] Consequently, the microjog amount mJ can be expressed as: MJ
= MJ .times. .times. 1 + MJ .times. .times. 2 = S sin .function. (
.PHI. ) + .delta. cos .function. ( .PHI. ) ##EQU1##
[0050] The microjog amount mJ can be normalized with absolute
dimensions to the track pitch TP of the servo tracks, thereby
yielding a microjog distance MJD in servo tracks, as follows: MJD =
S sin .function. ( .PHI. ) + .delta. cos .function. ( .PHI. ) cos
.function. ( .PHI. ) TP = S tan .function. ( .PHI. ) + .delta. TP (
1 ) ##EQU2##
[0051] With reference to FIG. 5, it can be shown with trigonometry
that: tan .function. ( .PHI. ) = A M - cos .function. ( .theta. )
sin .function. ( .theta. ) ( 2 ) ##EQU3##
[0052] Inserting Equation (2) into Equation (1) yields: MJD nom
.function. ( track ) = S ( A M - cos .function. ( .theta.
.function. ( track ) ) sin .function. ( .theta. .function. ( track
) ) ) + .delta. TP ( 3 ) ##EQU4##
[0053] Given a particular value of the angle .theta., which
corresponds to a particular data track and an associated servo
track, Equation (3) can be used to determine the nominal or ideal
microjog distance in servo tracks (MJD.sub.nom), which is the
radial offset in servo tracks that the read element 112 must have
from the selected data track in order to center the write element
113 over the selected data track. Equation (3) basically represents
circumstances in an ideal system that is not subject to various
real-world factors of the type discussed above, such as those
relating to removability. The exception is the presence in Equation
(3) of .delta., which in an ideal system would be zero.
[0054] A calibrated microjog distance can be determined from the
nominal microjog obtained in Equation (3). The calibrated microjog
is determined based on the function calc_calibrated_microjog (head,
track) which calibrates the nominal microjog for the position and
geometry of the head and drive. A technique for obtaining this
value is disclosed in U.S. patent application Ser. No. 10/612,810,
filed Jul. 2, 2003, the contents of which are hereby incorporated
by reference.
[0055] FIG. 7 is a flowchart illustrating a process 300 of
positioning the read/write head based on an instantaneous
RRO-induced microjog error. The process begins in START block 305.
Proceeding to block 310, the process 300 calculates the microjog
distance, assuming no RRO, for the intended head and write target
position as described above. The microjog distance is an offset,
which is summed with the write target position to determine a read
target position. The read target position is the initial position
of the read element, such that on average, the write element is
over the write target position.
[0056] Proceeding to block 315, the process 300 obtains a
measurement of the peak RRO in tracks, and its phase with respect
to an index that occurs once per revolution. The RRO measurement
may be obtained by analyzing sector to sector timing measurements
taken during a revolution. Or the measurement could be calculated
from coefficients in the RRO cancellation algorithm and the loop
gain at the desired frequency. This RRO measurement may be obtained
once during spin-up calibrations of the disk drive, or it may be
obtained each time the head is positioned for a write. For
simplicity, it is assumed that the magnitude and phase of the RRO
remain constant for all tracks on the disk. In one embodiment, it
may be determined that the RRO measurement is too high to safely
perform data writes. In this circumstance, the write functionality
may be disabled, allowing a user to access data on the disk but not
write any data to the disk.
[0057] Proceeding to block 320, the process 300 calculates the
microjog distance assuming the head has moved the distance of the
known RRO in tracks. The difference between the microjog distance
with and without RRO represents the peak microjog error caused by
the RRO: calc_calibrated_microjog (head,
track+RRO)-calc_calibrated_microjog (head, track)
[0058] Note that if the RRO as measured in tracks is 0, then the
peak microjog error caused by the RRO is 0.
[0059] Proceeding to block 325, the process 300 determines the
instantaneous microjog error due to the RRO as the disk rotates as:
sin (w)*(calc_calibrated_microjog (head,
track+RRO)-calc_calibrated_microjog (head, track))
[0060] where w refers to the angle through which the disk rotates,
synchronized with the phase of the RRO previously measured.
[0061] Proceeding to block 330, the process 300 feeds the
instantaneous estimate of the microjog error into the track-follow
control loop, which is trying to keep the read element over the
read target position defined previously. Now, the read element is
no longer centered on the read target position, but it moves back
and forth with the estimate of the RRO-induced microjog error. The
result is that the write element remains centered on the write
target position. The process 300 then terminates in END block
335.
[0062] The process 300 described up to this point applies mainly to
a disk drive with a removable disk. It is important that any drive
be able to write and re-write any data track on any disk. So
keeping the write element centered on the write target position is
critical. But in a drive with fixed media, it may be desirable to
keep the read element centered on the read target position during
the write. As was described previously, the write element will
wander back and forth about some average position if RRO is
present. When the read element is eventually used to read back the
data, there may be some signal loss because the data is not always
centered throughout the revolution. In a modification to the method
described previously, the estimate of the RRO-induced microjog
error could be introduced into the track-follow loop at the time
the data is read. Thus, the read element would move in a position
very close to where the write element actually wrote the data,
improving the signal loss problem. However, if there is a chance
that the disk could ever slip and significantly change the RRO,
then it would be wise to keep the data centered during the
write.
[0063] Although one embodiment has been illustrated and described
in detail, it will be understood that various substitutions and
alterations are possible without departing from the spirit and
scope of the present invention, as defined by the following
claims.
* * * * *