Hardware Reference
In-Depth Information
VCM actuator is:
1.715 × 10 12
s 2 +153.9s +4.836 × 10 4
1.668 × 10 9
s 2 + 816.8s +1.668 × 10 9 .
G p (s)=
(3.133)
The discretized feedback control law is:
G c (z)= .107z 6 + .181z 5 .033z 4 .221z 3 .092z 2 + .067z 1 + .046
z 6 +2.376z 5 +1.463z 4 − .916z 3 −1.626z 2 − .692z 1 − .056 , (3.134)
with a sampling frequency of 12.64 kHz.
Figure 3.54 shows the open-loop transfer function of the servo system. The
phase margin is 52 degrees, the gain margin is about 8 dB, and the cross
frequency is about 1 kHz. Figure 3.55 shows the shock transfer functions
measured in the setup (solid line) and derived from simulation model (dashed
line).
60
40
20
0
−20
10 2
10 3
0
−100
−200
−300
−400
10 2
10 3
Frequency [Hz]
Figure 3.54: Open-loop frequency response of servo system.
Due to the design of the air bearing surface, the slider flies above the rotat-
ing disk surface. The flying height is about 10 nm and variation in flying height
is also in the nanometer scale regardless of the vertical disk flutter which is in
µm scale [132]. Therefore the measured vertical displacement of the slider is a
fairly accurate representation of vertical movement of the disk. It is observed
that the variation of the slider in vertical direction has a repeatable component
synchronous to the spindle rotation measured by the clock signal. Therefore,
the measurement output from the LDV with the synchronized averaged value
removed from it can be considered as the disk flutter signal.
Figures 3.56 and 3.57 show the power spectral densities of the measured disk
flutter and the nonrepeatable components of the PES, respectively. Figure 3.56
clearly shows the modes of disk flutter, especially for those at 504 Hz, 598 Hz,
648 Hz, and 696 Hz. These are the four dominant modes in disk flutter. All
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