Hardware Reference
In-Depth Information
for a wide range of frequencies. If resonant modes of such actuator are at very
high frequencies, a simple gain can model the actuator fairly accurately [134].
In such case, the secondary stage actuator loop can bring down the peak of
the sensitivity transfer function to practically less than 3 dB [46].
Control structures for dual-actuated HDD head positioning servomech-
anism and the basic concepts of the dual-stage actuation loop shaping are
introduced in this section. Three different strategies, namely, the decoupled-
master slave structure, the parallel structure and PQ method, for the design
of controller for dual-stage servo are illustrated. A simple lag-lead compen-
sators plus notch filters for high frequency resonances, if present, produces
fairly satisfactory results for the dual-stage servomechanism if the actuators
are not saturated. This fact is proven with the help of simulation of control
loop. In reality, however, the microactuator saturation is unavoidable, and in
such case, the response of the closed loop shows more oscillation when a large
step is applied at the reference input. Stability of the loop is also deteriorated
when there is saturation in the microactuator.
The performance and stability of the dual-stage servomechanism can be
greatly improved even in the presence of microactuator saturation through a
simple modification to the decoupled master-slave configuration. In this modi-
fication, a nonlinear model of the secondary stage actuator is used to estimate
its output. Results presented here show that the proposed nonlinear controller
retains all the properties of the original linear controller when the secondary
stage actuator is not saturated and, when the secondary stage is saturated, the
proposed design is found to show better stability of the dual actuator servo
loop. The design is also robust in presence of uncertainties in the microactuator
model. Additionally, the design method can be applied to other dual-actuator
systems with a saturating secondary stage actuator such as power systems with
flexible AC transmission devices [203]. A more generic anti-windup design ap-
proach is proposed in [78], however, the controller presented above is simple
and yet effective. Simple and yet effective system is of great importance in the
design and implementation of HDD servomechanism and many other practi-
cal servo systems. Dual-stage actuators coupled with multisensing and other
advanced controls offer additional avenue for improvement of control perfor-
mance. Interested readers may refer to the article by Pang et al [158] and the
references therein for more information.
From the examples given so far illustrating different design approaches
for ideal VCM model and PZT based microactuator model, it is easily com-
prehended that each method has its advantages as well as disadvantages. It
depends on the designer to decide on the trade-off and choose the parameters
accordingly so that the design goals are achieved.
