Disk drive computing repeatable run out while actuator arm is pressed against crash stop
View Patent ↗A disk drive is disclosed comprising a voice coil motor (VCM) for rotating an actuator arm about a pivot in order to actuate a head over a disk. The disk is rotated by a spindle motor, and a feed-forward compensation value is computed that compensate for a non-centric alignment of the disk with respect to the spindle motor. During a calibration mode, the VCM is controlled to press the actuator arm against a crash stop, and the feed-forward compensation value is computed in response to a position error signal (PES). The PES is generated in response to embedded servo sectors recorded on the disk, wherein each embedded servo sector comprises a track address for coarse alignment and servo bursts for fine alignment.
1. A disk drive comprising:
(a) a disk comprising a plurality of data tracks, wherein each data track comprises a plurality of data sectors and a plurality of embedded servo sectors;
(b) an actuator arm;
(c) a head coupled to a distal end of the actuator arm;
(d) a voice coil motor (VCM) for rotating the actuator arm about a pivot in order to actuate the head over the disk;
(e) a crash stop;
(f) a spin die motor for rotating the disk; and
(g) control circuitry for generating a control signal applied to the VCM in response to a position error signal (PES) generated from reading the embedded servo sectors and a first feed-forward compensation value that compensates for a repeatable runout (RRO) due to a non-centric alignment of the disk with respect to the spindle motor, wherein the control circuitry computes the first feed-forward compensation value if the PES exceeds a predetermined threshold and the control circuitry computes the first feed-forward compensation value by:
controlling the VCM to press the actuator arm against the crash stop; and
computing the first feed-forward compensation value in response to the PES.
2. The disk drive as recited in claim 1 , wherein the PES is generated from a track address recorded in the embedded servo sectors.
3. The disk drive as recited in claim 1 , wherein the PES is generated from a plurality of servo bursts recorded in the embedded servo sectors.
4. The disk drive as recited in claim 1 , wherein the control circuitry for computing the first feed-forward compensation value from coefficients {a,b} of a sinusoid;
a *cos (2 □k/N )+ b *sin (2 □k/N )
wherein k is an index representing one of N servo sectors.
5. The disk drive as recited in claim 4 , wherein the control circuitry for computing the coefficients {a,b} according to:
[
a
b
]
=
(
M
T
*
M
)
-
1
*
M
T
*
PES
→
wherein:
PES
→
=
[
PES
1
,
PES
2
,
PES
3
…
PES
m
]
T
,
and
M
=
[
cos
(
2
π
k
1
/
N
)
sin
(
2
π
k
1
/
N
)
cos
(
2
π
k
2
/
N
)
sin
(
2
π
k
2
/
N
)
cos
(
2
π
k
3
/
N
)
sin
(
2
π
k
3
/
N
)
⋮
cos
(
2
π
k
m
/
N
)
sin
(
2
π
k
m
/
N
)
]
.
6. The disk drive as recited in claim 1 , wherein the control circuitry computes the first feed-forward compensation value when the disk drive is powered on.
7. The disk drive as recited in claim 1 , wherein the control circuitry determines that the PES exceeds the predetermined threshold if the average PES exceeds a predetermined threshold.
8. The disk drive as recited in claim 1 , wherein the disk drive receives a command from a host to enable the control circuitry to compute the first feed-forward compensation value.
9. The disk drive as recited in claim 1 , wherein the control circuitry for generating the control signal applied to the VCM in response to a second feed-forward compensation value that compensates for additional RRO, including RRO due to errors in writing the embedded servo sectors to the disk.
10. A method of operating a disk drive, the disk drive comprising a disk comprising a plurality of data tracks, wherein each data track comprises a plurality of data sectors and a plurality of embedded servo sectors, an actuator arm, a head coupled to a distal end of the actuator arm, a voice coil motor (VCM) for rotating the actuator arm about a pivot in order to actuate the head over the disk, a crash stop, and a spindle motor for rotating the disk, the method comprising the steps of:
(a) determining whether a position error signal (PES) generated from reading the embedded servo sectors exceeds a predetermined threshold
(b) if the PES exceeds the predetermined threshold, controlling the VCM to press the actuator arm against the crash stop and computing a first feed-forward compensation value in response to the PES generated from reading the embedded servo sectors with the actuator arm against the crash stop, wherein the first feed-forward compensation value compensates for a repeatable runout (RRO) due to a non-centric alignment of the disk with respect to the spindle motor; and
generating a control signal applied to the VCM in response to the PES and the first feed-forward compensation value.
11. The method as recited in claim 10 , wherein the PES is generated from a track address recorded in the embedded servo sectors.
12. The method as recited in claim 10 , wherein the PES is generated from a plurality of servo bursts recorded in the embedded servo sectors.
13. The method as recited in claim 10 , wherein the first feed-forward compensation value is computed from coefficients {a,b} of a sinusoid:
a *cos (2 □k/N )+ b *sin (2 □k/N )
wherein k is an index representing one of N servo sectors.
14. The method as recited in claim 13 , wherein the coefficients {a,b} are computed according to:
[
a
b
]
=
(
M
T
*
M
)
-
1
*
M
T
*
PES
→
wherein:
PES
→
=
[
PES
1
,
PES
2
,
PES
3
…
PES
m
]
T
,
and
M
=
[
cos
(
2
π
k
1
/
N
)
sin
(
2
π
k
1
/
N
)
cos
(
2
π
k
2
/
N
)
sin
(
2
π
k
2
/
N
)
cos
(
2
π
k
3
/
N
)
sin
(
2
π
k
3
/
N
)
⋮
cos
(
2
π
k
m
/
N
)
sin
(
2
π
k
m
/
N
)
]
.
15. The method as recited in claim 10 , wherein the actuator arm is pressed against the crash stop and the first feed-forward compensation value computed when the disk drive is powered on.
16. The method as recited in claim 10 , wherein the step of determining whether the PES exceeds a predetermined threshold comprises determining whether the average PES exceeds a predetermined threshold.
17. The method as recited in claim 10 , further comprising the step of receiving a command from a host to enable the step of pressing the actuator arm against the crash stop and computing the first feed-forward compensation value.
18. The method as recited in claim 10 , wherein the control signal applied to the VCM is further computed in response to a second feed-forward compensation value that compensates for additional RRO, including RRO due to errors in writing the embedded servo sectors to the disk.