Hardware Reference
In-Depth Information
Chapter 3
Design of Actuator Servo
Controller
The head positioning servomechanism of an HDD uses the VCM actuator
to move read-write head from one track to another and to regulate its posi-
tion over the center of the track. Position feedback used by the servo loop is
extracted from the readback signal obtained by the read head scanning the
servo sector. Design objectives of the servo controller are fast movement of
read/write head from one track to another during track seek and minimum
variance regulation of the position of the head during track following. Various
sources of disturbances and noise have detrimental influences on the perfor-
mance of the servomechanism and the objectives of the servo controller must be
fulfilled in presence of those influences. Moreover, a cost effective realization of
the controller puts additional constraints on designer's choice of sophisticated
and complex algorithms. To make such cost effective realization possible the
designer must avoid a complex model of the process, leaving significant parts
of plant dynamics unmodeled. The HDD servo controller is often implemented
using fixed point digital signal processors (DSP), and low cost analog-to-digital
converter (ADC) and digital-to-analog converter (DAC). This chapter explains
issues related to design of digital controller, highlights factors to be considered
while implementing such controllers, and provides different algorithms to solve
problems specific to HDD servomechanism. Design considerations for track
seek mode are already presented in chapter 2. Primarily the issues relevant to
track-following controller are focused in this chapter, but some of these issues
are equally important for the seek mode.
When the plant model, the disturbance model and design objectives are
known, the first step towards the realization of a servomechanism is to select
the structure of the controller. Since the objectives of the track seek mode are
quite different from those of the track-following mode, two different controllers
can easily be identified, each of which independently satisfies the requirements
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