Precision control of hard disk drives

The paper discusses the challenges and trends in the precision control of the head-positioning servomechanism of a hard disk drive. The read/write head is positioned through a voice coil motor (VCM) actuator to which the heads are attached using a thin stainless steel suspension. The servomechanism is expected to move the read/write head from one track to another in minimum time (track-seek mode) and regulate head position precisely on the center of the track during track-following mode. Near time-optimal control is widely used for the seek mode, while linear mode such as proportional-integral differential (PID) plus loop-shaping compensation is used for track following. An alternative to the servomechanism is a lightweight, secondary-stage actuator placed close to the point of control. For small step commands, the dual-stage set-up significantly performs better than a single-stage actuator in regulating the read/write head position. Maneuvers for large step commands are dominated by the VCM's response where the secondary stage provides negligible improvement.