Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
1999-03-11
2003-02-04
Sniezek, Andrew L. (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
C360S077020
Reexamination Certificate
active
06515818
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a servo control method of a disk drive mechanism. More particularly, the present invention relates to a method ofproviding for track follow of a read/write head at a quarter track position and a method of quarter track seeking using the track follow technique.
BACKGROUND OF THE INVENTION
In a conventional disk drive system, the disk drive operates using two successive servo modes in accordance with the moving distance of a head. The first servo mode is concerned with a seek mode in which a head accesses tracks of a disk to search for and settle upon a desired target track. The second servo mode relates to a track following mode in which the head is accurately positioned on a data line of the target track for reading and writing data on the target position.
Conventional track follow methodology uses servo bursts that are typically designated as the “A”, “B”, “C” and “D” bursts. As shown in
FIG. 1
, the A, B, C and D bursts have varying amplitudes with respect to a track center line (TK center), such that when the head is positioned exactly over the center line of the target one of an odd or even track, approximately one-half of the amplitude of the A and B bursts will be read and when said head is positioned over the center line of the other of an odd or even track, approximately one-half of the amplitude of the C and D bursts will be read. As the head moves off the data line of the target track (ie, the center), the amplitude of one burst decreases while the amplitude of the other burst increases, depending on the direction of the misalignment. In this manner, a position error signal (PES) can be derived from the relative amplitudes of the A and B bursts, which are read in a timed displaced manner as the head passes over the bursts. Accordingly, ifthe head is properly centered with respect to the track, the difference of the A and B bursts (i.e., PES) equals zero.
An example of a conventional system within a disk drive to generate the PES is described in U.S. Pat. No. 4,415,939, entitled “Head Positioning Servo for Disk Drive”, issued on Nov. 15, 1983, which is incorporated herein by reference in its entirety. This system is illustrated in
FIG. 2
, which shows a control circuit
10
for controlling the position of the read/write head
12
with respect to the disk track. Window counter logic
30
is provided, which sequentially enables A and B detectors
32
and
34
, respectively, upon initialization by the supply of the timing mark to the window counter logic
30
. This input, together with the read clock signal from the data channel, indicates at what times the A and B bursts are expected to be present. The outputs of the detectors
32
and
34
are summed in a summing node
36
. This A+B signal is compared with a constant value K in an automatic gain control loop to control the amplification provided by a voltage controlled amp
38
which amplifies the read back signal supplied to the detectors
32
and
34
. This maintains a constant value of A+B independent of head flying height variations and the like, and allows the amplitudes of the A and B bursts to be compared reliably to one another for position error determination. The difference of the outputs of the detectors
32
and
34
is determined in a differential amplifier
40
, thus providing the A−B signal which is then stored in a sample and hold circuit
42
. As noted above, when the head is following the center of the disk track, A−B is zero. Hence, the position error signal is zero. It is passed to a comparator and compensator
44
, where it is compared with a command signal. If the command signal is also zero, the output of the comparator and compensator
44
to the power amplifier
46
is zero. The output of the power amplifier
46
is supplied to the servo motor
18
, which moves the servo arm
14
, which carries the read/write head
12
, which in turn supplies new servo position information to the voltage-controlled amplifier
38
.
By applying a non-zero DC level to the command input of
FIG. 2
, the servo system may locate the read/write head at a position offset from track center. With the proper DC level applied to the command input, the read/write head may be moved to the quarter track position. However, as the head width becomes much narrower than the track width, the usable offtrack range is reduced. Referring again to
FIG. 1
, when the heads are moved beyond the quarter track position, there is a point at which the A and B signals reach a maximum and/or minimum level and then do not change further as the heads are moved further offtrack. At this point, the servo system becomes unstable, since A−B is constant, regardless of changes in track position. The quarter track position margin “M” as indicated in
FIG. 1
, is herein defined as the additional position offset which may be applied beyond quarter track before the servo system becomes unstable.
FIG. 1
illustrates that as the head width becomes narrower with respect to the track width, the position margin M is reduced. If the head width approaches ½ the track width, the position margin approaches zero and the system becomes unstable. Also note that as the head width changes, the slope of the A and B amplitudes versus track position also changes. The amplitude of PES (A−B) versus track position is dependent on the head width. Likewise, the amount of DC offset required at the command input of
FIG. 2
to locate the heads at quarter track will vary depending on the head width. Thus, if a constant pre-programmed voltage is used to command the quarter track offset, drive to drive variations in head width will create a DC positioning error about the quarter track point for the read/write head.
In view of the above, it can be recognized that conventional servo systems have a number of drawbacks when performing track follow at the quarter track position. As such, a need exists for providing a disk drive that implements a technique of utilizing servo patters that overcomes the drawbacks of the prior art.
SUMMARY OF THE INVENTION
In accordance with an aspect of the invention, a method of performing track follow at quarter track positions is provided that comprises reading servo bursts designated as A, B, C and D servo bursts located on the disk; deriving a position error signal from relative amplitudes of the servo bursts; and positioning the read/write head over the quarter track position in accordance with the position error signal. The method may include reading the servo bursts in a time-displaced manner, and the position error signal may be determined from the relative amplitudes of the servo bursts. The position error signal may be based on one of C−A and D−B when track following at −¼ track positions and one of A−D and B−C when track following at +¼ track positions
In accordance with another aspect of the invention, there is provided a servo controller for controlling a position of a read/write head with respect to a track on a disk within a disk drive wherein the disk preferably includes servo bursts being designated as A, B, C and D servo bursts having varying amplitudes with respect to a center line of the track. The servo controller may comprise a digital control/timing circuit that controls timing of events within the servo controller and generates an offset signal; a servo demodulator that receives a read back signal from the read/write head and determines a difference signal; a summing node that receives the difference signal and the offset signal; and a programmable gain stage that generates a position error signal based on an output of the summing node.
According to a feature of the invention, the servo demodulator may receive the read back signal and, in accordance with a track position, demodulate selected servo bursts to determine a difference signal. The difference signal for even tracks may be determined as C−A for −¼ track positions and A−D for +¼ track posi
Iomega Corporation
Sniezek Andrew L.
Woodcock & Washburn LLP
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