Multirate designs for faster control of dual-stage actuator in magnetic disk drives
Hiroshi Ishida, March 2003 (in Japanese)

Recently, technology of manufacturing magnetic disks has been developing rapidly. Areal density of the magnetic disks is increasing by more than 60\% a year, so that data tracks are placed at intervals of less than 0.5 micrometers. The Hard Disk Drive(HDD) employing such a magnetic disk of extremely high density is required to realize extremely accurate control of the head positioning. Since it is almost impossible to achieve the control objective of servo mechanism with a single traditional actuator, new servo mechanism consisting of two different actuators has been recently attracting a lot of attention of researchers. The mechanism called the dual-stage servo mechanism is believed to be capable of not only increasing accuracy, but also improving speedy positioning of the head moving between tracks.

This background motivates the author to pursue new ways for innovations of the dual-state servo mechanism. This thesis proposes proper employment of multirate control, and demonstrates the effectiveness of multirate control for designing more accurate and faster dual-stage servo mechanism. In order to make the most of characteristics of two different actuators, multirate feedback and feedforward controllers are designed to achieve frequency separation. It is described how to combine capabilities of feedback and feedforward controllers efficiently. The robustness in a broader frequency range and the swift recovery from positioning errors are due to the multirate feedback controller, while the quickness of moving toward target tracks is due to the multirate feedforward controller. Design methods of achieving such effective positioning are proposed for the multirate feedback controller and the multirate feedforward controller. The effectiveness of the proposed control strategy is carefully investigated by simulation. In addition, comparing simulation results of multirate and single-rate designs, the author illustrates the superiority of the multirate control over the single-rate control in robustness and quickness.

In order to enhance quickness of the positioning further, another method of designing feedforward control is proposed in this thesis. In the feedforward design, the quickness is put more emphasis on than the robustness. The method is based on generation of ideally cooperative trajectories of individual actuators. The usefulness of the feedforward control mechanism in achieving faster positioning is confirmed by simulation. Its robustness needs to be investigated deeply in the future research.