Hardware Reference
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properties, efforts must be distributed properly between the two actuators. Ac-
cording to the design published in , , and , the following guidelines
should be adhered to while designing the dual-stage servo controller:
• The VCM responds to low frequency components of error while the sec-
ondary stage responds to high frequency error signals.
• Hand-off:Hand-off frequency is the frequency where the magnitudes of
the compensated VCM branch and the compensated secondary actua-
tor branch are equal. The phase difference between the outputs of the
secondary stage and that of the VCM should be ≤ 120 at the hand-off
frequency to avoid destructive interference. Since the resultant displace-
ment of the slider is sum of displacements contributed by two actuators,
they cancel each other when they are out of phase. Typical hand-off
frequency reported so far is about 400 Hz.
• The gain of the VCM should exceed the secondary stage's gain by 20 dB
for frequencies below 60 Hz to avoid saturation of the secondary stage
actuator.
The basic concepts of shaping the responses of two actuators are explained
here using few simple examples of dual-stage servo system with PZT actuated
suspension. For simplicity, when introducing various design concepts, we as-
sume that the resonances of both VCM and PZT actuator are compensated
using filters in series so that the VCM can be represented approximately by
thesimplemodelofk/s 2 whereas the PZT actuator is described by a pure gain
g m .WeuseP V ,P M ,C V ,C M to denote the transfer functions of the VCM, the
secondary actuator, and their respective controllers.
3.7.2 Parallel Structure
The parallel structure dual-stage control is shown in Figure 3.72. The open
loop transfer function O p (s), closed-loop transfer function T p (s), sensitivity
transfer function S p (s) are given as 
O p = C V P V + C M P M ,
(3.146)
C V P V + C M P M
1+C V P V + C M P M ,
T p =
(3.147)
1
1+C V P V + C M P M ,
S p =
(3.148)
respectively if the actuators are not saturated.  Search WWH ::

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