An H_∞ design method for a multi-rate servo controller and applications to a high density hard disk drive

This paper proposes a method of designing a multi-rate servo controller for a hard disk drive that operates at a higher sampling frequency than the position error signal (PES). It is desirable to increase the servo performance without increasing the capacity overhead caused by the tracking information on a disk. A zero-interpolator is introduced as an up-sampling scheme to convert the plant to a higher sampling rate. Then a conventional H∞ method is applied to optimize the controller performance as well as to reduce the effects of aliasing noise caused by the up-sampling scheme. This method is also applied to the design of a dual-stage actuator system of a hard disk drive. The performances are compared with single-rate controllers and improvements of 15-50% were obtained. Experiments using a 2.5 inches hard disk drive were carried out and the effectiveness was confirmed.