Hardware Reference
In-Depth Information
Designing the controller for a dual-stage actuator is a problem of designing con-
troller for a dual-input single-input (DISO) system. If the relative displacement
of the secondary stage with respect to the VCM can be somehow measured,
it would make the actuator a dual-input dual-output (DIDO) system and the
controllers for the two actuators can be designed independently. Various meth-
ods of designing controller for dual-actuated HDD servomechanism have been
reported in the published literature. Commonly used configurations are,
• parallel loop (Figure 3.72),
• master-slave, and
• decoupled master-slave (DMS, Figure 3.90) ([173], [65]).
The controller parameters for each of these structures can be found either by
classical loop shaping method or using optimal control in state space. Some
very simple methods for parameter selection are introduced in the next few
pages to illustrate the design considerations for dual-stage servo system. This
design problem has been researched by many researchers and designers, and
they can be found in published literatures such as [173], [212], [65], [125], [123],
[113], [62], [106], [128], and the references therein. Interested readers may refer
to these works to get better understanding of each of these approaches.
3.7.1 Control Design Speci fi cations
Following the the shape of the magnitude of a compensated open-loop transfer
functiongiveninSection3.2,atypicaldesignspecification for dual-stage HDD
servomechanism is summarized in Table 3.7.
Tabl e 3 . 7 : Sp ec ifications for Dual-stage Control Design Example
Open-loop bandwidth f o : >2000 Hz
Disturbance attenuation: >40 dB below 100 Hz
Phase margin (PM): > 40 degree
Gain margin (GM): > 6dB
Rise time: < 0.2 ms
Overshoot: < 20%
Peak in the sensitivity transfer function: < 10 dB
In the dual-stage actuator, the primary stage, i.e., the VCM actuator has
larger inertia and lower bandwidth but provides a larger range of movement.
The secondary stage, on the other hand, is lighter, has greater bandwidth but
supports a very small range of displacement. Because of these contradictory
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