Hardware Reference
In-Depth Information
Bode Diagram
45
0
−45
−90
−135
−180
−225
10
0
10
1
10
2
10
3
10
4
Frequency (Hz)
Figure 3.52: Phase of shaped plant O(z) O
−1
(z). Solid line: pseudo inverse
scheme; dash-dot line: dual rate inverse scheme.
Figure 3.52 compares the phase of the shaped plant O(z) O
−1
(z) when using
the two inverse schemes. When using the pseudo inverse scheme, the phase
drops down exponentially when the frequency increases while zero phase is
assured over the whole frequency range when using multirate inverse scheme.
The multirate scheme does not have this problem of phase drop.
3.5.5 Multisensing Servo
Vibration of disk and suspension, repeatable runout (RRO), windage (shown as
i
d
in Figure 3.31) etc are the major contributing factors to track misregistration
(TMR) in an HDD servo system [47]. According to the discussions presented
so far, peak filters can be used to deal with some narrow band disturbances
and, in general, high bandwidth servo for broad band disturbances.
Another source of disturbance in HDD servomechanism comes from disk
flutter which refers to the vertical motion of disk contributed mainly by spindle
and air-disk interaction. Although disk flutter is a vertical motion, it can
have an in-plane component that can cause shift in the track center. Thus
it has direct impact on the position error signal (PES) [66]. Such vibration
is not phase locked to the disk rotation and, therefore, is a contributor to
nonrepeatable runout (NRRO) [66] [185]. Compensation methods for RRO
has no influence on this source of disturbance.
But the energy of disk vibration is concentrated in certain frequencies, and
therefore, one common approach to suppress these vibrations is to increase the
