Disk drive determining frequency response of actuator near servo sample frequency
View Patent ↗A disk drive is disclosed comprising a servo control system configured to control an actuator for actuating a head over a disk. Servo sectors are sampled at a servo sample frequency f s to generate a position error signal PES(k) that is filtered with a compensator to generate a first control signal u 1 (k). A first discrete-time sinusoid comprising a sinusoid frequency of f s +Δf is added to the first control signal u 1 (k) to generate a second control signal u 2 (k). The second control signal u 2 (k) is applied to the actuator, and a frequency response of the actuator is determined at the frequency |Δf|. The frequency response of the actuator is determined at the frequency f s +Δf based at least in part on the frequency response of the actuator at the frequency |Δf| and a measured signal of the servo control system when applying the second control signal u 2 (k) to the actuator.
1. A disk drive comprising:
a head;
a disk comprising a plurality of servo tracks, wherein each servo track comprises a plurality of servo sectors;
an actuator configured to actuate the head over the disk, wherein the actuator comprises a frequency response;
control circuitry comprising a servo control system configured to control the actuator, the control circuitry configured to:
sample the servo sectors at a servo sample frequency f s to generate a position error signal PES(k);
filter the PES(k) with a compensator to generate a first control signal u 1 (k);
add a first discrete-time sinusoid comprising a sinusoid frequency of f s +Δf to the first control signal u 1 (k) to generate a second control signal u 2 (k);
apply the second control signal u 2 (k) to the actuator;
determine the frequency response of the actuator at the frequency |Δf|; and
determine the frequency response of the actuator at the frequency f s +Δf based at least in part on the frequency response of the actuator at the frequency |Δf| and a measured signal of the servo control system when applying the second control signal u 2 (k) to the actuator.
2. The disk drive as recited in claim 1 , wherein the frequency Δf is one of a positive value and a negative value.
3. The disk drive as recited in claim 1 , wherein the actuator comprises a voice coil motor.
4. The disk drive as recited in claim 1 , wherein the actuator comprises a microactuator.
5. The disk drive as recited in claim 1 , wherein the control circuitry is further configured to determine the frequency response of the actuator at the frequency f s +Δf based on a frequency domain representation of the PES(k) at the frequency |Δf| when applying the second control signal u 2 (k) to the actuator.
6. The disk drive as recited in claim 5 , wherein the control circuitry is further configured to determine the frequency response of the actuator at the frequency f s +Δf based on a second discrete-time sinusoid comprising a sinusoid frequency of |Δf|.
7. The disk drive as recited in claim 6 , wherein the control circuitry is further configured to determine the frequency response of the actuator at the frequency f s +Δf based on:
P
(
f
s
+
Δ
f
)
=
PES
(
Δ
f
)
S
2
(
Δ
f
)
(
1
+
P
(
Δ
f
)
C
(
Δ
f
)
)
where:
P(f s +Δf) represents the frequency response of the actuator at the frequency f s +Δf;
PES(|Δf|) represents the frequency domain representation of the PES(k) at the frequency |Δf| when applying the second control signal u 2 (k) to the actuator;
S 2 (|Δf|) represents a frequency domain representation of the second sinusoid;
P(|Δf|) represents the frequency response of the actuator at the frequency |Δf|; and
C(|Δf|) represents a frequency response of the servo compensator at the frequency |Δf|.
8. The disk drive as recited in claim 1 , wherein the control circuitry is further configured to determine the frequency response of the actuator at the frequency f s +Δf based on a frequency domain representation of the first control signal u 1 (k) at the frequency |Δf| when applying the second control signal u 2 (k) to the actuator.
9. The disk drive as recited in claim 8 , wherein the control circuitry is further configured to determine the frequency response of the actuator at the frequency f s +Δf based on a second discrete-time sinusoid comprising a sinusoid frequency of |Δf|.
10. The disk drive as recited in claim 9 , wherein the control circuitry is further configured to determine the frequency response of the actuator at the frequency f s +Δf based on:
P
(
f
s
+
Δ
f
)
=
U
1
(
Δ
f
)
S
2
(
Δ
f
)
[
1
+
P
(
Δ
f
)
C
(
Δ
f
)
C
(
Δ
f
)
]
where:
P(f s +Δf) represents the frequency response of the actuator at the frequency f s +Δf;
U 1 (|Δf|) represents the frequency domain representation of the first control signal u 1 (k) at the frequency |Δf| when applying the second control signal u 2 (k) to the actuator;
S 2 (|Δf|) represents a frequency domain representation of the second sinusoid;
P(|Δf|) represents the frequency response of the actuator at the frequency |Δf|; and
C(|Δf|) represents a frequency response of the servo compensator at the frequency |Δf|.
11. The disk drive as recited in claim 9 , wherein the control circuitry is further configured to determine the frequency response of the actuator at the frequency f s +Δf based on:
P
(
f
s
+
Δ
f
)
=
U
1
(
Δ
f
)
S
2
(
Δ
f
)
P
(
Δ
f
)
where:
P(f s +Δf) represents the frequency response of the actuator at the frequency f s +Δf;
U 1 (|Δf|) represents the frequency domain representation of the first control signal u 1 (k) at the frequency |Δf| when applying the second control signal u 2 (k) to the actuator;
S 2 (|Δf|) represents a frequency domain representation of the second sinusoid; and
P(|Δf|) represents the frequency response of the actuator at the frequency |Δf|.
12. A method of operating a disk drive comprising a servo control system for controlling an actuator for actuating a head over a disk, wherein the actuator comprises a frequency response, the method comprising:
sampling servo sectors on a disk at a servo sample frequency f s to generate a position error signal PES(k);
filtering the PES(k) with a compensator to generate a first control signal u 1 (k);
adding a first discrete-time sinusoid comprising a sinusoid frequency of f s +Δf to the first control signal u 1 (k) to generate a second control signal u 2 (k);
applying the second control signal u 2 (k) to the actuator;
determining the frequency response of the actuator at the frequency |Δf|; and
determining the frequency response of the actuator at the frequency f s +Δf based at least in part on the frequency response of the actuator at the frequency |Δf| and a measured signal of the servo control system when applying the second control signal u 2 (k) to the actuator.
13. The method as recited in claim 12 , wherein the frequency Δf is one of a positive value and a negative value.
14. The method as recited in claim 12 , wherein the actuator comprises a voice coil motor.
15. The method as recited in claim 12 , wherein the actuator comprises a microactuator.
16. The method as recited in claim 12 , further comprising determining the frequency response of the actuator at the frequency f s +Δf based on a frequency domain representation of the PES(k) at the frequency |Δf| when applying the second control signal u 2 (k) to the actuator.
17. The method as recited in claim 16 , further comprising determining the frequency response of the actuator at the frequency f s +Δf based on a second discrete-time sinusoid comprising a sinusoid frequency of |Δf|.
18. The method as recited in claim 17 , further comprising determining the frequency response of the actuator at the frequency f s +Δf based on:
P
(
f
s
+
Δ
f
)
=
PES
(
Δ
f
)
S
2
(
Δ
f
)
(
1
+
P
(
Δ
f
)
C
(
Δ
f
)
)
where:
P(f s +Δf) represents the frequency response of the actuator at the frequency f s +Δf;
PES(|Δf|) represents the frequency domain representation of the PES(k) at the frequency |Δf| when applying the second control signal u 2 (k) to the actuator;
S 2 (|Δf|) represents a frequency domain representation of the second sinusoid;
P(|Δf|) represents the frequency response of the actuator at the frequency |Δf|; and
C(|Δf|) represents a frequency response of the servo compensator at the frequency |Δf|.
19. The method as recited in claim 12 , further comprising determining the frequency response of the actuator at the frequency f s +Δf based on a frequency domain representation of the first control signal u 1 (k) at the frequency |Δf| when applying the second control signal u 2 (k) to the actuator.
20. The method as recited in claim 19 , further comprising determining the frequency response of the actuator at the frequency f s +Δf based on a second discrete-time sinusoid comprising a sinusoid frequency of |Δf|.
21. The method as recited in claim 20 , further comprising determining the frequency response of the actuator at the frequency f s +Δf based on:
P
(
f
s
+
Δ
f
)
=
U
1
(
Δ
f
)
S
2
(
Δ
f
)
[
1
+
P
(
Δ
f
)
C
(
Δ
f
)
C
(
Δ
f
)
]
where:
P(f s +Δf) represents the frequency response of the actuator at the frequency f s +Δf;
U 1 (|Δf|) represents the frequency domain representation of the first control signal u 1 (k) at the frequency |Δf| when applying the second control signal u 2 (k) to the actuator;
S 2 (|Δf|) represents a frequency domain representation of the second sinusoid;
P(|Δf|) represents the frequency response of the actuator at the frequency |Δf|; and
C(|Δf|) represents a frequency response of the servo compensator at the frequency |Δf|.
22. The method as recited in claim 20 , further comprising determining the frequency response of the actuator at the frequency f s +Δf based on:
P
(
f
s
+
Δ
f
)
=
U
1
(
Δ
f
)
S
2
(
Δ
f
)
P
(
Δ
f
)
where:
P(f s +Δf) represents the frequency response of the actuator at the frequency f s +Δf;
U 1 (|Δf|) represents the frequency domain representation of the first control signal u 1 (k) at the frequency |Δf| when applying the second control signal u 2 (k) to the actuator;
S 2 (|Δf|) represents a frequency domain representation of the second sinusoid; and
P(|Δf|) represents the frequency response of the actuator at the frequency |Δf|.