Dynamic magnetic information storage or retrieval – Automatic control of a recorder mechanism – Controlling the head
Reexamination Certificate
2001-11-07
2004-07-20
Hudspeth, David (Department: 2651)
Dynamic magnetic information storage or retrieval
Automatic control of a recorder mechanism
Controlling the head
Reexamination Certificate
active
06765747
ABSTRACT:
FIELD OF THE INVENTION
The invention relates in general to transducer positioning in a magnetic data storage system and, more particularly, to compensation for low-frequency repeatable run-out (RRO) created by relatively high actuator arm bearing pivot friction in a magnetic disk drive.
BACKGROUND OF THE INVENTION
A simplified diagrammatic representation of a disk drive, generally designated
10
, is illustrated in FIG.
1
. The disk drive
10
comprises a disk stack
12
(illustrated as a single disk in
FIG. 1
) that is rotated by a spindle motor
14
. The spindle motor
14
is mounted to a base plate
16
. An actuator arm assembly
18
is also mounted to the base plate
16
.
The actuator arm assembly
18
includes a transducer
20
(or head) mounted to a flexure arm
22
which is attached to an actuator arm
24
that can rotate about a pivot bearing assembly
26
. The actuator arm assembly
18
also includes a voice coil motor
28
which moves the head
20
relative to the disk
12
. The spin motor
14
, and actuator arm assembly
18
are coupled to a number of electronic circuits
30
mounted to a printed circuit board
32
. The electronic circuits
30
typically include a digital signal processor (DSP), a microprocessor-based controller and a random access memory (RAM) device.
Referring now to the illustration of
FIG. 2
, the disk stack
12
typically includes a plurality of disks
34
each having a pair of disk surfaces
36
,
36
. The disks
34
are mounted on a cylindrical shaft
37
and are designed to rotate about axis
38
of the shaft
37
. The shaft
36
has clamps
40
which are used to secure each disk
34
to the shaft
37
. The spindle motor
14
as mentioned above, rotates the disk stack
12
.
Referring now to the illustration of
FIG. 3
, the actuator arm assembly
18
includes a plurality of transducers
20
, each of which correspond to a disk surface
36
. Each transducer
20
is mounted to a corresponding flexure arm
22
which is attached to a corresponding portion of the actuator arm
24
that can rotate about the pivot bearing assembly
26
. The VCM
28
operates to move the actuator arm
24
, and thus moves the transducers
20
relative to their respective disk surfaces
36
.
Although the disk stack
12
is illustrated having a plurality of disks
34
, it may also contain a single disk
34
, with the actuator arm assembly
18
having a corresponding single actuator arm
24
. A recent trend of many disk drive manufacturers is to move toward a single disk, single head, low cost design. This helps to reduce costs associated with the disk drive, as fewer components are required. Additionally, as is typical with many high volume manufacturing process, costs can be further reduced by using common components for a number of different products. Thus, it would be advantageous to have common components for both hard disk drives having multiple hard disks, and hard disk drives having a single disk. By having common components, the volume of the components required is increased, which typically results in a lower unit cost for each component.
Data is read from or written to a track on the disk surface using the transducer
20
that is held close to the track while the disk
34
spins about its center at a substantially constant angular velocity. The transducer
20
, located at the end of the actuator arm
24
, is positioned in close proximity to the track using the VCM
28
. When a disk drive
10
initially receives a request to read or write data to a specific track, the disk drive determines the current location of the transducer
20
(i.e. the starting track) and the location of the track where data is to be read or written (i.e. the destination track). The distance from the starting track to the destination track is commonly known as the seek length.
The electronic circuits
30
within the disk drive
10
determine a seek velocity profile which is used to supply current to the VCM
28
in order to move the actuator arm
24
, and thus the transducer
20
from the starting track to the destination track. Once the transducer
20
has reached the destination track, the disk drive
10
enters a settle state, where the position of the transducer
20
is settled close to the center of the destination track. When the transducer
20
has settled, the disk drive
10
enters a track following operation.
To properly locate the transducer
20
near the target track during a read or write operation, a closed-loop servo scheme is generally implemented that uses feedback from servo data read from the disk surface
36
to align the transducer
20
with the target track. The servo data is commonly written to the disk surface
36
using a servo track writer (STW), but may also be provided in other ways, such as through pre-printed media. The servo data is commonly written as radially aligned servo sectors, or servo wedges, which extend between the inner diameter and outer diameter on each disk surface
36
.
In an ideal disk drive system, the tracks of the data storage disk are non-perturbed circles situated about the center of the disk. As such, each of these ideal tracks includes a track centerline that is located at a known constant radius from the disk center. In an actual system, however, non-perturbed circular tracks on the data storage disk are rare. That is, problems, such as inaccuracies in the STW and disk clamp slippage, can result in tracks that are not ideal non-perturbed circular tracks. Positioning errors created by the perturbed nature of these tracks are known as written-in repeatable run-out (RRO). The perturbed shape of these tracks complicate the transducer positioning function during read and write operations because the servo system needs to continuously reposition the transducer during track following to keep up with the constantly changing radius of the track centerline with respect to the center of the spinning disk.
A number of methods are currently used to compensate for RRO, with a common method being a feedforward circuit. The RRO is measured by using a single-point discrete Fourier transform (DFT) to generate a runout coefficient which is stored in the memory of the digital signal processor (DSP). When compensating for the RRO, the runout coefficient is retrieved from the DSP memory. The runout coefficient is stored in the DSP memory in an index-synchronized sine and cosine value for the particular track and sector. The runout coefficient is adjusted with the gain and phase change by the controller, adjusted by the cylinder skew, and added back to the control output. The control output is used to actuate the VCM to reposition the transducer with respect to the disk surface and help keep the transducer centered over the data track.
The feedforward circuit generally uses one of two schemes to generate the runout coefficient. The first scheme calibrates the RRO at the power up and adaptively modifies it at the first one or two revolutions after the seek according to the following equations:
Runout_Sin
_Coef
k
=
Runout_Sin
_Coef
k
-
1
+
g
*
2
/
N
*
∑
k
=
0
N
-
1
perr
(
k
)
*
sin
(
2
π
*
k
/
N
)
[
1
]
Runout_Cos
_Coef
k
=
Runout_Cos
_Coef
k
-
1
+
g
*
2
/
N
*
∑
k
=
0
N
-
1
perr
(
k
)
*
cos
(
2
π
*
k
/
N
)
.
[
2
]
In the above equations, N is the number of servo wedges in one revolution, and g is the adaptation gain. In an ideal case, g is equal to one, which implies a one revolution cancellation of runout, however, since accurate cancellation requires precise knowledge of the servo system transfer function (gain and phase), g is generally less than one to ensure stability of the servo loop due to variation of the system. The position error signal, perr, is generated from the servo information located on the disk surface. The magnitude of perr corresponds to the distance between the transducer and the track centerline. The runout coefficient is stored in the memory of the DSP in an inde
Brunnett Don
Sun Yu
Hansra Tejpal S.
Hudspeth David
Maxtor Corporation
Slavitt Mitchell
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