Dynamic magnetic information storage or retrieval – General processing of a digital signal – Data in specific format
Reexamination Certificate
1998-04-30
2001-03-06
Sniezek, Andrew L. (Department: 2651)
Dynamic magnetic information storage or retrieval
General processing of a digital signal
Data in specific format
C360S075000, C360S077120
Reexamination Certificate
active
06198584
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a disk drive with a unique servo pattern of special calibration bursts that are “disposably” written in a data region, used for independently calibrating a read head and, if desired, later disposed of by being written over with data.
2. Description of the Prior Art and Related Information
A conventional disk drive contains a disk with a plurality of concentric data tracks and a “head” which generally comprises a “slider” that carries a read transducer and a write transducer. The drive has “servo” information recorded on this disk or on another disk to determine the position of the head. The most popular form of servo is called “embedded servo” wherein the servo information is written on the disk in a plurality of servo sectors or “wedges” that are interspersed between data regions. Data is conventionally written in the data regions in a plurality of discrete data sectors. Each data regions is preceded by a servo wedge. Each servo wedge generally comprises a servo header (HDR) containing a track identification (TKID) field and a wedge number (W#) field, followed by at least two angularly successive servo burst regions that define a plurality of burst pair centerlines. Each servo burst is conventionally formed from a series of magnetic transitions defined by an alternating pattern of magnetic domains. The servo control system samples the servo bursts with the read transducer to align the transducer with or relative to a burst pair centerline and, therefore, with or relative to a particular data track.
The servo control system moves the transducer toward a desired track during a “seek” mode using the TKID field as a control input. Once the transducer head is generally over the desired track, the servo control system uses the servo bursts to keep the transducer head over that track in a “track follow” mode. The transducer reads the servo bursts to produce a position error signal (PES). The PES has a particular value when the transducer is at a particular radial position relative to a burst pair centerline defined by the bursts and, therefore, relative to the data track center. The desired track following position may or may not be at the burst pair centerline or data track center.
The width of the write transducer is desirably narrower than the data track pitch. The servo information is recorded, therefore, to define a data track pitch that is slightly wider than the write transducer to provide room for tracking error. The servo information is usually recorded to define a data track pitch that is about 25% wider than the nominal width of the write transducer and conversely, therefore, the nominal write transducer is about 85% of the track pitch. The percentage is not exactly 85% for every transducer, however, since the width of the write transducer will vary from nominal due to typical manufacturing distributions.
FIGS. 1A and 3A
show disks
12
having a plurality of servo wedges
211
comprising servo sectors
511
disposed in concentric tracks across the disk, and corresponding data regions
212
comprising data sectors
512
disposed in the concentric tracks between the servo wedges
211
. For clarity, the disks
12
are simplified to show only four wedge pairs
211
,
212
whereas a typical disk is divided into 70-90 wedge pairs. The servo wedges
211
, moreover, are greatly exaggerated in width relative to the width of the data regions
212
. Finally, the disks
12
of
FIGS. 1A and 3A
have only one annular “zone” of data sectors
512
from the inner diameter (ID) of the disk
12
to the outer diameter (OD), whereas an actual disk
12
usually has multiple concentric zones in order to increase the data capacity of the drive by packing more data sectors
512
in the larger circumference tracks near the OD.
FIGS. 1A and 1B
, in particular, schematically represent an older disk drive having a disk
12
wherein each servo wedge
211
only has two angularly successive servo burst regions
221
,
222
that contain A and B bursts, respectively. The servo bursts A,B moreover, are 100% bursts that stretch radially from data track center to data track center such that there is only one burst pair centerline (e.g.
412
) per data track (an annular collection of data sectors
512
). As shown in
FIG. 1B
, for example, the A and B bursts A(
12
), B(
12
) define a burst pair centerline
412
that coincides with the data track centerline (not separately numbered).
The disk drive of
FIGS. 1A and 1B
has a head slider
200
in which the same inductive transducer
201
both reads and writes data. When the R/W transducer
201
is on track center in such arrangement, it detects equal signal amplitudes from the two angularly successive, radially offset A and B bursts such that A−B=0. If the RAW transducer
201
is offset one way or the other from the burst pair centerline
412
, an inequality exists between the signal amplitudes, such that A−B≠0. The degree of inequality is normally derived from an algebraic position error signal (PES), such as A−B or A−B/A+B, that is proportional to the position error (PE) or mechanical offset of the RAW transducer
201
relative to the burst pair centerline
412
.
The servo pattern of
FIGS. 1A and 1B
, however, contains nonlinear regions because the R/W transducer
201
becomes saturated when it is in certain positions. In particular, with only two 100% bursts and one burst pair centerline per data track, if the transducer is displaced too far from the burst pair centerline, the R/W transducer
201
may be completely over one of the bursts and no longer pass over any part of the other burst. The maximum linear signal (PES) occurs when displacement is one half of the transducer's physical width. As shown in
FIG. 2
, for example, the 80% R/W transducer
201
of
FIG. 1B
can only be displaced by a maximum of 40% of a data track pitch from the burst pair centerline
412
and still pass over at least a portion of both bursts A(
12
), B(
12
) to develop a linearly varying A-B position error signal. The nonlinear regions where the head position is ambiguous are called “blind spots” or “gaps”
413
,
414
.
FIGS. 3A and 3B
schematically represent the industry's solution to the gaps
413
,
414
of FIG.
2
. Here, the disk drive includes two additional, angularly successive servo burst regions
231
,
232
to provide C and D bursts that fill the gaps between the A and B bursts. The C and D bursts are placed in “quadrature” with the A and B bursts in that the edges of the C and D bursts are aligned with the centers of the A and B bursts. With four 100% bursts A, B, C, D positioned in quadrature, there are two burst pair centerlines (e.g.
411
,
412
) per data track pitch, i.e. one burst pair centerline every 50% of a data track pitch. The 80% R/W transducer
201
, therefore, will always pass over both parts of an A/B pair or a C/D pair because it is always within 25% of a data track pitch from an A/B or C/D burst pair centerline.
FIG. 4
shows how the linear portions of the C-D position error signal which surround the C/D burst pair centerlines are radially aligned with the gaps in the A-B position error signal, and vice versa.
The industry recently began using magnetoresistive heads (MR heads) which contain two separate transducers—an inductive transducer for writing and a magnetoresistive transducer for reading.
An MR head is advantageous in providing an improved signal-to-noise-ratio (SNR) to recover data in disk drives of high areal density. However an MR head also presents a number of disadvantages. In particular, the separate read and write transducers are necessarily spaced apart from one another along the length of the slider. As a result, their relative radial separation varies from ID to OD as the MR head is moved in an arc by a swing-type actuator.
The drive industry presently compensates for the variable radial separation between the transducers by “micro-jogging” the read transducer relative to a given burst pai
Codilian Raffi
Nazarian Ara W.
Tanner Brian
Shara Milad G
Sniezek Andrew L.
Western Digital Corporation
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