ABSTRACT
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 position 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 performance 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.
