Fixed order H∞ control design for dual-stage hard disk drives

Crucial to the increase in aerial densities of future hard disk drives (HDD) is the increase of the number of tracks per inch. In order to achieve very high track densities new high bandwidth and very accurate servo systems will be necessary. One possible solution is the use of dual-stage actuators, in which a high bandwidth and a highly accurate micro-actuator (MA) is used in combination with a traditional voice coil motor (VCM). In this case the nature of the servo controller becomes multivariable since both the VCM and the MA have to be controlled simultaneously. The design of servo controllers for track following with dual-stage actuators has been an active area of research in the last few years. Modern control methods were specifically developed for multivariable systems and offer significant advantages over classical control methods. On the down side, they usually lead to controllers which are of order equal to or greater than the plant they were designed for. The aim of the paper is to present a systematic approach for the design of a fixed order servo controller and to illustrate this design on a piezoelectric MA designed by Hutchinson Technologies Inc. The complexity of the servo controller is limited to address costs and implementation issues in a commercial HDD. The systematic approach for such a low order servo controller design is developed in the paper by utilizing H_∞-optimization with a choice of specific closed loop weighting functions and additional constraints on the order of the controller