Hardware Reference
In-Depth Information
chamber affects the flying height and can conversely be used as a thermal ac-
tuator to control the flying height. Additionally, the electrostatic force between
the slider and the disk can also be used to adjust the flying height [182].
A brief introduction to the piezo-electric or electrostatic microactuator is
presented in this section. These actuators can be used to move the suspension
or the slider or the read/write head for fine head positioning of the read/write
element and also for active control of the spacing between head and disk. These
actuators typically require a driving voltage of less than 40 Volts, generates
a displacement of around 1 µm for the head positioning control, and offer a
“clean” transfer function which is very useful for reducing the microactuator
plant model uncertainty and hence expanding the servo bandwidth.
In the case of the actuated suspension design, the resonant frequency of
the microactuator is limited by the suspension resonance which is about 10
kHz at this moment. Using the rule of thumb that open loop servo bandwidth
is about 1/3 of the critical resonant frequency which has been the case for
the dual-stage servos reported so far, actuated suspension can support a servo
bandwidth of about 3.3 kHz. The electrostatic actuated slider or head, on the
other hand, have frequency responses that resemble a spring-mass system to
about 50 kHz.
Given a suitable PES sampling, it is easy for such an actuator to support a
servo bandwidth 10 kHz and above, which can effectively reject the vibrations
due to disk, spindle and HSA to achieve accurate positioning. As such, actu-
ated slider or head is the actuator of choice from point of servo performance.
Accordingly, low overhead, accurate and very fast PES sampling is required
for such a microactuator. Nevertheless, in addition to the higher bandwidth
feature they offer, the microactuators must be reliable and low cost in order
to be useful in practical systems.
Addition of a secondary stage actuator brings in significant changes in the
design of controller as it turns out to be a double-input-single-output (DISO)
problem. Examination of the controllability matrix reveals that the system is
controllable from both inputs. However, care must be taken to coordinate the
movements of the two actuators in order to make use of the microactuator for
high bandwidth control.
The next section discusses the issues related to the design of controller for
dual-stage servo for achieving higher servo bandwidth. All examples of dual-
stage servo controller illustrated there use actuated suspension as the plant
model since it is more readily available.
3.7 Control of Dual-Stage Actuator
Though the motion of the read/write head slider can be generated by energiz-
ing either the VCM or the secondary actuator, the only available measurement
in the dual-actuated HDD servomechanism is the displacement of the slider.
 
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