Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
2000-10-10
2003-11-11
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
Reexamination Certificate
active
06646824
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a positioning control method and device for positioning the head of the disk device at a target position, and more particularly to a positioning control method and device for compensating for non-linear detection characteristics of the head.
2. Description of the related Art
Disk devices that read a disk medium with a head are widely used. For example, magnetic disk drives that are used as a storage device for a computer comprise a magnetic disk, a spindle motor for rotating the magnetic disk, a magnetic head for reading from and writing to the magnetic disk, and a VCM actuator for positioning the magnetic heat at a track on the magnetic disk. The recording density of this kind of disk drive is rapidly increasing, as well as is the track density of the magnetic disk. Particularly, by using an MR head for the magnetic head, high density storage have become possible. Therefore, high-precision positioning at high speed is becoming necessary.
When the disk drive receives a read or write command from the computer, it moves the magnetic head from the current position to the target position. This is called the seek operation. This seek operation is a transition operation which moves to following control after coarse control.
Feedback control is used for moving to the target position and for following at the target position. Feedback control detects the current position of the head, calculates the position error between the target position and the current position, and controls the actuator for the head in order to do away with the position error.
In order to detect the current position of the head, position information is recorded on the disk. The head reads the position information from the disk and detects the current position from the position information that is read. In addition, it calculates the position error between the target position and the current position. In the disk device, the current position is demodulated from a position signal that is read by the head, so the detection characteristics of the head affect the accuracy of detecting the current position. Therefore, it is necessary to compensate for the detection characteristics.
FIG. 22
is a schematic drawing of the feedback control system of the prior art. The position information (servo information) of the magnetic disk comprises a track number and a servo (burst servo) signal. The servo signal, for example, is a 4-phase servo signal that is 90-degrees out of phase. The position information that is read by the head is demodulated by a demodulation circuit, to obtain the track number and offset signal. The offset signal is obtained from the amplitude of the servo signal, and its magnitude is proportional to the amount that it has shifted from the center of the track for that track number.
The output level of the magnitude of this offset signal changes according to the characteristics of the magnetic head and the track position. Therefore, it is necessary to correct this change, and to convert the value of the offset signal to a value in actual track units. A method for doing this is known (for example, as disclosed in Japanese Unexamined published Patent No. H8-195044) where gains for the magnetic head and each track position (called the position sensitivity) are set, and the offset value is corrected by referencing the gain that corresponds to the head and track position.
In this method, it is assumed that the detection sensitivity of the head is uniform within the width of the track. However, as the track width becomes narrow, it is not possible to assume that the detection sensitivity of the head is uniform within the same track, due to the core width of the head. Especially, there is a strong tendency for this in the case of a MR head or GMR head. Therefore, it is not possible to accurately convert the demodulated offset value to the offset value of the track position. This error causes fluctuation in the loop gain of the feedback control system, and reduces the positioning accuracy.
A first method for correcting the non-linear characteristics of the head has been proposed (for example, as disclosed in Japanese Unexamined published Patent No. H10-222942). In other words, as shown in
FIG. 22
, there is a correction table
90
for storing the conversion gains (called position sensitivity) for the head, and each track position and offset position. The correction table
90
is referenced according to the demodulated offset value, in order obtain the corresponding correction gain. For example, as shown in
FIG. 24
, the correction table
90
stores correction gains (for example, for each 0.05 track) for each offset position indicated by a real position (Real Position).
A multiplier
91
multiplies the demodulated offset value by the correction gain to obtain a decoded offset position. As shown by the dotted line in
FIG. 25
, when this gain is a suitable gain, the decoded offset position indicates the real offset position. Furthermore, by adding this offset position and the track number with an adder
92
, the current position is obtained. In addition, a computing element
93
subtracts the target position from the current position to calculate the position error. The position error is input to a servo-control unit
94
. As mentioned above, the servo-control unit
94
, performs coerce control, integral control or following control, depending on the value of the position error, and outputs the control amount. The head actuator (not shown in the figure) is driven according to this control amount, to position the head at the target position (target track).
On the other hand, another non-linear correction method, as shown in
FIG. 23
, is known (for example, Japanese Unexamined published patent H8-249844). In
FIG. 24
, the loop gain
97
of the controller
94
can be changed. Also, the gain of a demodulation circuit (position detection circuit)
96
for the each offset position from the track center is measured by a measurement circuit
98
, and stored in memory. The gain
97
of the controller
94
is controlled by referencing the gain of the measurement circuit
98
according to the demodulated current position.
In this method, the position error is not corrected, however, by controlling the gain of the controller
94
, the fluctuation of the loop gain of the control system due to the non-linearity of the head output is suppressed, and stable positioning is performed even at the offset position.
However, the prior art had the following problems.
(1) In the first non-linear correction method, it is difficult to obtain an accurate correction gain (correction curve) from the correction table
90
. In other words, it is necessary to find the correction gain with the absolute position as a reference. Therefore, the correction gain for the absolute position is measured by the STW (servo track writer). However, the value measured by the STW is not necessarily the optimum value for each drive (device). In other words, the detection sensitivity changes due to the solid difference or setting of the demodulation circuit. Therefore, the relationship between the decoded position and the real position shifts, as shown by the solid line in
FIG. 26
, even though the gain measured by the STW is used, and this makes it impossible to correct to the accurate offset position for each individual drive.
(2) Also, the detection sensitivity of the head changes due to wear of the core end of the head or changes in characteristics of the MR element, and there is no guarantee that the detection sensitivity of the head will not be permanently changed. For this reason as well, there is the problem that it is not possible to correct to the accurate offset position for each individual drive. To solve this problem, it is necessary to measure the correction gain for the drive itself, however, since the drive controls positioning by using the position distorted by the effect of the head sensitivity distribution, it is difficult to find the accurate absolute position by
Saito Shunji
Takaishi Kazuhiko
Fujitsu Limited
Greer Burns & Crain Ltd.
Hudspeth David
Slavitt Mitchell
LandOfFree
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