Dynamic magnetic information storage or retrieval – General processing of a digital signal – Data clocking
Reexamination Certificate
2001-03-02
2004-03-30
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
General processing of a digital signal
Data clocking
C360S075000, C360S076000
Reexamination Certificate
active
06714369
ABSTRACT:
The present invention relates to a method and an apparatus for writing clock data to a storage medium.
Information for systems such as data processing systems is typically stored on storage media. Particular use is made of storage disks such as magnetic disks, opto-magnetic disks, and the like. One type of magnetic disk arrangement is a so-called “head disk assembly” which is intended normally to be permanently fixed in a data processing system; the head disk assembly includes the magnetic disk medium itself and the associated read and write head or heads (the “product” head or heads) which write data to the disk and read data from the disk. Another type of magnetic disk is of a type known as “removable media” which normally consists of a magnetic disk medium in a protective plastics case which can be used to transfer data between data processing systems by physical transfer of the disk itself from one machine to another.
In a known method of manufacturing storage media such as hard disk drives, a head disk assembly consisting of the product head(s), the disk or disks, the motor and arm electronics, is mounted in a mastering station known as a servo-writer. The servo-writer writes a pattern of magnetic information (the “servo track pattern”) onto the disk. The servo track pattern becomes the master reference which is used by the disk drive during normal operation in order to locate the tracks and sectors on the disk for data storage and retrieval. Clearly, the servo track pattern has to be accurately written to the disk at very well defined positions.
In order to be able to write the servo pattern onto the correct positions on the disk during manufacture of the disk, in one commonly used process, a (usually temporary) “clock” track is written onto the disk to serve as a timing reference during writing of the servo pattern. Conventionally, a separate clock head is used to write the clock track onto the disk and to read the clock track from the disk so that the servo tracks, which are written with the disk drive's product head, can be phase aligned with respect to the clock track and each other. However, the use of a dedicated clock track writing head is an expensive addition to the manufacturing process and further requires that the servo tracks be written in a clean room because the clock track writing head has to be inserted into the open (unsealed) head disk assembly. Moreover, in practice the clock heads have to be replaced on a daily basis because of damage which occurs during use.
There have been several proposals for methods and apparatus for writing the servo tracks to a disk without the need for a dedicated clock head. Typically, and put briefly, an internal write head of a head disk assembly is used to write an initial clock track around the disk. That initial clock track is then read by an internal read head of the head disk assembly and the next clock track is written by the internal write head, with reading of the previous clock track being interleaved with writing of the present clock track. The servo patterns or tracks are written at appropriate locations on the disk, interleaved with the clock tracks, the positions of the servo patterns or tracks being determined in accordance with timing information obtained from the clock tracks. Such methods, or similar methods, are disclosed in for example U.S. Pat. No. 5,485,322, U.S. Pat. No. 5,448,429, and U.S. Pat. No. 5,668,679.
In our WO-A-98/31015, the entire disclosure of which is incorporated herein by reference, there is disclosed another method and apparatus for writing clock data to a storage medium, such as a disk. The storage medium has tracks on which data can be stored. Clock data is written to a present track on the storage medium. The written clock data is read from that track. Clock data for a subsequent track on the storage medium is generated from the clock data read from said present track. The phase of the generated clock data for the subsequent track is compared with a reference timing signal and the phase of the generated clock data for the subsequent track is adjusted in accordance with said comparison. The phase adjusted clock data for the subsequent track is then written to said subsequent track. In this method and apparatus, it is again not necessary to provide a dedicated clock read/write head. As in other proposals of this type, the servo information is written to the disk using the clock track data to position the servo information very accurately. To save time, it is preferred that the servo information in a track be written alternately with the clock data in that track, i.e. for any track, a portion of clock data is written, that clock data being phase aligned with the clock data in the previous track, and then a burst of servo data is written, again appropriately phase aligned with the previous track, followed by the next portion of clock data, and so on.
In such self-clocking methods, as mentioned above, it is necessary to read clock data from a previously written track whilst writing, in an interleaved manner, clock data to the next (present) track. Modern disk drives currently use a magnetoresistive read element in tandem with a conventional thin film write element in which the read and write elements are offset by a fixed distance from each other. An example of such an arrangement is shown in
FIG. 1
which shows schematically a magnetoresistive read element
1
and an associated write element
2
. The centres of the read and write elements
1
,
2
are separated by an offset x and the read and write elements
1
,
2
are shown located over a track
3
on a disk. Typically, x may be in the range 1 to 3 &mgr;m and the width of the write element
2
is 1.8 &mgr;m. Where the read and write elements
1
,
2
are aligned with the direction of the track
3
as shown in
FIG. 1
, there is a write-to-read delay which depends on the rotational speed &ngr; and also on the distance of the track
3
from the centre of the disk. For example, if the rotational speed &ngr; of the disk is 5400 rpm and the offset x between the read and write elements
1
,
2
is 1 &mgr;m, then for a track at a 20 mm radius, the write-to-read delay is approximately 90 ns, and for a track at a radius of 46 mm, the write-to-read delay is approximately 40 ns. This delay, which as shown varies according to radial position across the disk, has to be compensated for during the clock track copying process.
Referring to
FIG. 2
, there is shown a disk
4
rotating about a centre O. The arm
5
which carries the read and write elements
1
,
2
is mounted for pivotal movement about a pivot point P which is displaced from the centre of rotation O of the disk
4
. This arrangement allows the arm
5
to be rigidly mounted and allows the disk to be rotated with a constant angular velocity. As can be appreciated from a study of
FIG. 2
, the pivotal mounting of the arm
5
about a pivot point P which is displaced from the centre of rotation O of the disk
4
means that the arm
5
will not be tangential to the disk
4
for most of the pivotal travel &agr; of the arm
5
. Instead, the arm
5
and therefore the head containing the read and write elements
1
,
2
will be skewed relative to the tracks on the disk
4
. Skew of the head containing the read and write elements
1
,
2
relative to the tracks on the disk
4
causes a degradation of the signal read back from a clock track on the disk
4
and can make it very difficult or impossible to read back a previously written track with the read element
2
and then immediately write the next track with the write element
1
. One theoretical solution to this would be to jog the read/write head between successive read and write steps but, at the frequency with which the clock tracks are written in order to obtain high throughput during manufacturing, this is technically not feasible nor practical. It is therefore desirable to minimise the amount of skew of the head during the self-servowriting process.
According to a first aspect of the present invention, there is provided a method
Figueroa Natalia
Pillsbury & Winthrop LLP
Xyratex Technology Limited
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