Hardware Reference
In-Depth Information
Application of optimal controller design is explained in section 3.4. Several
advanced control strategies are explained in section 3.5, where HDD-specific
issues such as residual vibration, RRO, low sampling frequency etc receive spe-
cial attention. The specifications for HDD servomechanism are continuously
becoming more and more stringent to meet the demands for increasing track
density and diversification of applications. Application of a second actuator,
lighter and faster than the VCM, is gradually becoming a necessity. Section 3.6
discusses pros and cons of different micro-actuators suitable for application in
HDD. Different approaches to design controller for dual-stage actuators are
presentedinsection3.7.
3.2 PID-type Control
According to the Bode stability criterion (see references e.g. [55] or [144]),
typicalshapeoftheBodemagnitudeplot of a compensated open-loop transfer
function should have the following characteristics.
(a) Low-frequency band: high gain above 0 dB and decreases with increasing
frequency at a rate of -20N dB/decade where N is an integer and N ≥ 2;
(b) Crossover band: crosses the 0 dB with a slope of approximately -20 dB/decade
to ensure stability;
(c) High-frequency band: low gain under 0 dB and decreases with increasing
frequency at a rate of -20N dB/decade (N ≥ 2).
Figure 3.1: Block diagram representation of the control problem.
Any typical approach to design the controller for HDD head positioning
servomechanism attempts to meet the above mentioned requirements and to
achieve high servo bandwidth. This approach is explained in this chapter by
illustrating the design of controller for a typical control problem shown in
Figure 3.1. This illustration starts with a simple controller designed for the
nominal model of the actuator. Then the limitations of this simple controller
are explained, and solutions to overcome those problems are sought in the
subsequent design approaches.
