Dynamic magnetic information storage or retrieval – General processing of a digital signal – Head amplifier circuit
Reexamination Certificate
1999-06-29
2001-10-30
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
General processing of a digital signal
Head amplifier circuit
C360S068000, C360S075000
Reexamination Certificate
active
06310740
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to disk drives. More particularly, the present invention relates to a disk drive comprising a n-current preamplifier for generating n-independently controlled write currents for a plurality of heads.
2. Description of the Prior Art
Magnetic disk drives for computer systems typically employ an array of disks and associated read/write heads together with head positioning and spindle mechanics. This arrangement of heads and fixed disk array is referred to as a head disk assembly or HDA, an overview of which is provided in FIG.
1
A. Several magnetic disks
2
connected in an array are rotated by a spindle motor. Each recording surface (top and bottom) of each magnetic disk is accessed through a dedicated head
4
; as the disks spin, a thin layer air-bearing forms between the heads
4
and the recording surface such that the heads
4
are said to “fly” just above the recording surface. The heads
4
are connected to the distal end of actuator arms
6
which are connected to a pivot
8
actuated by a rotary voice coil motor (VCM). As the VCM rotates the actuator arms
6
about the pivot
8
, the heads
4
are positioned radially over the recording surface so that information can be written to and read from the recording surface.
The recording surface of the magnetic disk is coated with a thin film medium (e.g., cobalt alloy) which is magnetized inductively by a write coil of the head
4
. The digital data being recorded modulates a current passing through the write coil in order to inductively write a series of magnetic transitions onto the disk surface (recording surface) of the disk, where a preamplifier chip incorporated within the HDA performs the modulation function in response to the digital data. As shown in
FIG. 1B
, the data is written in the radially spaced, concentric tracks
10
which are partitioned into blocks of data referred to as data sectors
12
. Because the circumferential recording area increases from the inner to outer diameter tracks, more data can be stored in the outer diameter tracks. Thus, in order to maintain a more constant linear bit density and thereby maximize the overall storage capacity, the recording surface is normally partitioned into a number of zones where each zone comprises a predetermined number of tracks. Data is then written to the recording surface at an increasing rate as the head traverses radially from the inner to outer diameter zones, thereby increasing the amount of data stored in the outer diameter tracks. This is illustrated in
FIG. 1B
which shows a disk partitioned into an inner diameter zone
14
comprising seven data sectors per track, and an outer diameter zone
16
comprising fourteen data sectors per track. In practice, the recording surface is actually partitioned into several zones with the data rate incrementally increasing from the inner to outer diameter zones in order to exploit the maximum storage capacity of the recording surface.
Typically the magnetic disks
2
also comprise servo sectors
18
which are recorded at a regular interval and interleaved with the data sectors
12
as shown in
FIG. 1B. A
servo sector, as shown in
FIG. 1C
, typically comprises a preamble
20
and sync mark
22
for synchronizing to the servo sector; a servo data field
24
comprising coarse position information, such as a Gray coded track address, used to determine the radial location of the head with respect to the plurality of tracks; and a plurality of servo bursts
26
recorded at precise intervals and offsets from the track centerlines which provide fine head position information. When writing or reading data, a servo controller performs a “seek” operation to position the head over a desired track; as the head traverses radially over the recording surface, the Gray coded track addresses in the servo data field
24
provide coarse position information for the head with respect to the current and target track. When the head
4
reaches the target track, the servo controller performs a tracking operation wherein the servo bursts
26
provide fine position information used to maintain the head over the centerline of the track as the digital data is being written to or read from the recording surface.
The servo sectors
18
are written to the recording surfaces as part of the manufacturing process to enable the seek and tracking operations necessary to write and read the data sectors
12
. A common mechanism for writing the servo sectors to the recording surfaces is an external servo track writer which uses the write preamplifier electronics and heads within the HDA, but which uses separate control circuitry and servo mechanics for radially positioning the heads using well known techniques such as a laser interferometer. A significant cost reduction can be achieved by a “self-servowriting” method which can use circuitry in the disk drive for writing the servo sectors.
It is desirable to expedite the process of writing the servo sectors
18
to the array of recording surfaces within each disk drive to maximize manufacturing throughput. It is known to write the servo sectors
18
to all of the recording surfaces simultaneously by using a technique referred to as “bank servo writing” wherein the write current generated by the preamplifier is applied to all of the heads to simultaneously write the servo sectors to all of the recording surfaces rather than one surface at a time. This is illustrated by the prior art preamplifier shown in
FIG. 4
wherein a register
28
is loaded with a digital write current setting converted into an analog write current setting
30
by a digital-to-analog converter (DAC)
32
. The analog write current setting
30
adjusts the output current of driver circuits (
34
0
-
34
N
) which supply the respective write currents (
36
0
-
36
N
) to the heads
4
. Head select circuitry
38
within the preamplifier enables the output of the appropriate driver circuit (
34
0
-
34
N
) over line
40
during normal operation of the disk drive, and it enables the output of all the driver circuits (
34
0
-
34
N
) during servo track writing in order to write the servo sectors to all of the recording surfaces simultaneously. The digital write data
42
to be recorded to the surface of the disk
2
modulates the operation of the driver circuits (
34
0
-
34
N
) by alternating the polarity of the write current
36
; for example, a digital “1” bit may modulate a positive write current and a digital “0” may modulate a negative write current.
Noise in the disk drive (electronic noise, media noise, intersymbol interference, etc.) may induce errors when reading the track addresses and/or servo bursts which will degrade the performance of the disk drive by increasing seek times as well as increasing the bit error rate if the head is unable to maintain proper centerline tracking. Therefore, when the servo sectors are written to the recording surfaces, it is important that enough write current is supplied to each head to saturate the magnetic material on the recording surface so as to maximize the signal power during read back. Prior art servo track writers that perform a bank servo write to all of the recording surfaces simultaneously would set the write current high enough to ensure that each head would be driven by enough current to saturate the recording surfaces. Setting the write current higher than the minimum required to saturate the recording surface does not significantly reduce the signal-to-noise ratio when using a conventional inductive head which comprises a single coil for both writing and reading the magnetic transitions. This is because the poles in a conventional inductive head are essentially the same width which results in minimal fringing fields emanating from the periphery of the write gap even if the write current is set higher than necessary. This is not the case, however, with magneto-resistive (MR) heads which comprise an inductive write element (write coil) and a MR read element integrated into one head. In typical MR heads ha
Dunbar Gary L.
Tanner Brian
Turner David P.
Davidson Dan I.
Hudspeth David
Shara, Esq. Milad G
Western Digital Technologies, INC
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