Dynamic magnetic information storage or retrieval – General recording or reproducing – Recording-or erasing-prevention
Reexamination Certificate
2001-09-28
2004-03-30
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
General recording or reproducing
Recording-or erasing-prevention
C360S031000, C360S075000, C360S077040
Reexamination Certificate
active
06714372
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to magnetic disk drives (disk drives), and more particularly to a method of manufacturing a disk drive by using a servo track writer (STW) for measuring the width of the read element to set the write unsafe (WUS) limit.
2. Description of the Related Art
This application is directed to varying an operating parameter known as the write-unsafe limit, or “WUS limit”, based on the width of a read element and, in some embodiment, on the width of a write element. As explained below, the WUS limit has historically been fixed for large groups of disk drives without regard to the actual widths of the read and write elements in a given disk drive.
1) An Exemplary Disk Drive and its Read/Write Elements
Referring to
FIG. 1
, a conventional disk drive
10
has a head disk assembly (HDA)
20
including at least one disk
23
, a spindle motor
22
for rapidly rotating the disk
23
, and a head stack assembly (HSA)
40
that includes an actuator assembly
50
and a head gimbal assembly (HGA) (not numbered) with a transducer head
80
for reading and writing data. The HSA
40
is part of a servo control system that positions the transducer head
80
over a particular track on the disk to read or write information from that track. The HSA
40
earns its name from the fact that it generally includes a plurality of HGAs that collectively provide a vertical arrangement of heads called a “head stack.”
The transducer heads
80
of several years ago were “merged” devices where reading and writing were accomplished with a single inductive element. The transducer head
80
commonly used today, however, is a composite (MR and inductive) transducer head
80
that has separate read and write elements.
FIG. 2
is a highly simplified representation of a composite transducer head
80
having it's a write element
81
of width W and it's a read element
82
of width R. The transducer head
80
shown is a “write wide, read narrow” device in that the read element's width R is typically about 50-65% of the write element's width W.
Composite transducer heads
80
are very small devices that are manufactured in large batches using photolithographic wafer process techniques. As a result, operating characteristics such as the widths of the read and write elements
81
,
82
tend to vary over a normal distribution curve for a given number of heads, wafers or an manufacturers. As explained further below, the wide variability of read width R and write width W is problematic when combined with a fixed WUS limit.
FIG. 3
is an exploded perspective view of a fully-assembled HDA
20
having servo-writing access ports
25
,
26
(discussed below) and the controller circuit board
30
that is usually installed after servo-writing. The controller circuit board
30
suitably positions the actuator assembly
50
and then reads or writes user data in accordance with commands from a host system (not shown).
Returning to
FIG. 1
, the industry presently prefers a “rotary” or “swing-type” actuator assembly
50
that conventionally comprises an actuator body
51
which rotates on a pivot assembly between limited positions, a coil
52
that extends from one side of the actuator body to interact with a pair of permanent magnets to form a voice coil motor (VCM), and an actuator arm
54
that extends from the opposite side of the actuator body to support the HGA.
2. An Exemplary Servo Pattern
A disk drive is ultimately used to store user data in one or more “data tracks” that are most commonly arranged as a plurality of concentric data tracks on the surface of its disk or disks. Special servo information is factory-recorded on at least one disk surface so that the disk drive's servo control system may control the actuator assembly
50
, via the VCM, to accurately position the transducer head to read or write user data to or from the data tracks. In colloquial terms, the servo information provides the servo control system with the position of the head relative to the written track. In operation, the disk drive's servo control system intermittently processes (read only) the pre-recorded servo information just before the disk drive processes (reads or writes) user data in the data tracks.
3. The Write Unsafe Limit
FIGS. 4A
,
4
B and
4
C are data path diagrams that explain why a WUS limit has been used to date and why it is generally set to a small, “narrow” or “tight” value when a single WUS limit is used for a family of drives.
FIG. 4A
shows a hypothetical data path
501
of a nominally wide write element
81
that is 70% as wide as the track pitch. As shown, the write element
81
settles in along a damped oscillatory path
501
after the servo control system has moved the write element
81
to the desired track in a track seek mode and then entered a track following mode. The WUS limit relates to when writing will be terminated as a function of the oscillatory deviations of the write element's path
501
relative to track center (T/C). The WUS limit, to put it another way, corresponds to the maximum off-track distance of the write element
81
before writing is disabled. The tighter the WUS limit, the more frequently that writing will be disabled. A higher frequency of disabling writing will reduce the performance of the drive.
The WUS limit is usually specified in terms of a percentage track pitch from track center T/C (e.g. ±16%). In
FIG. 4A
, the write element's excursions from track center T/C are signified by vertical arrows, varying from +5%, to −10%, to +18%, to −14%, to +5%, to −3%. The disk drive's servo control system stops writing the moment that the write element moves beyond the WUS limit due to resonant vibrations, a shock event, or the like. In
FIG. 4A
, assuming the WUS limit is set to 16%, and writing is disabled just prior to the 18% excursion. What may not be so apparent from
FIG. 4A
is that the WUS limit is chosen to minimize or eliminate the detrimental effect of reading erroneous data with a narrow read element. The WUS limit, in more detail, reduces so-called “sliver” errors, i.e. errors that arise from reading a sliver of old data that remains when new data is written to the same track.
FIG. 4B
shows a “new” data path
502
. As shown, most of the old data path
501
has been overwritten beneath the new data path
502
. Exposed adjacent to the new data path
502
, however, are some slivers of old data
501
-
1
,
501
-
2
,
501
-
3
and
501
-
4
.
FIG. 4C
shows a relatively narrow read element
82
attempting to read the data in the new data path
502
. This particular read element
82
is represented as being 32% as wide as the data track pitch. As shown, if the data had been written from position “A” onward, i.e., with an extremely liberal WUS limit of 33%, the read element
82
may read the old data track slivers
501
-
1
,
501
-
2
and
501
-
3
while trying to read the data on the new data path
502
. This is completely unacceptable, of course, because it constitutes a data integrity error. There is no resulting ECC error to alert the disk drive's firmware to the problem. The problem simply goes undetected and the disk drive provides the host with garbled data masquerading as good data.
A WUS limit is useful for preventing sliver errors. The problem, however, is that a single WUS limit is usually applied to an entire family of disk drives even though the width of the read element varies from drive to drive. Under this one size fits all approach, the WUS limit is set to 50% of: (1) the narrowest width of the read elements used in the drive family in order to guarantee that there are no sliver errors; (2) a compromise between (i) an overly-narrow WUS limit that causes too many disk drives to fail during Initial Burn-In (IBI) for repeatedly trying to satisfy the WUS limit and (ii) an overly-wide WUS limit that permits disk drives to pass through IBI with one or more narrow read elements that make the drive susceptible t
Codilian Raffi
Johns William D.
Hudspeth David
Mobarhan, Esq. Ramin
Myers Dawes Andras & Sherman
Negron Daniell L
Shara, Esq. Milad G.
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