Multirate Control Design of Frequency Separative Dual-Stage Actuator in Magnetic Disk Drives
Shuji Higuchi, March 2001 (in Japanese)

The demand for larger storage capacity of magnetic disk drives is is accelerating. Indeed, the areal density has been increasing by more than 60% every year. The track pitch has reached 1\mu m recently. The performance of positioning control which is achievable by conventional single-stage actuators is becoming unsatisfactory in such situations and dual-stage actuators has attracted much attention by engineers.

The thesis is concerned with how to exploit the potential of dual-stage actuators in the design of positioning control. The thesis demonstrates the effectiveness of cooperative multirate control strategy for designing swift and precise servo mechanism in magnetic disk drives. The operational ranges of two individual actuators in the dual-stage structure are separated in frequency using sampled-data ${\cal H}_\infty$ criteria. The proposed strategy is amazingly useful for accomplish faster and more accurate positioning by feedback controllers with less frequent acquisition of head position information.