A comparison of multirate robust track-following control synthesis techniques for dual-stage and multi-sensing servo systems in hard disk drives

This paper presents the design and analysis of multirate robust track-following controllers for a dual-stage servo system with a MEMS microactuator (MA) and an instrumented suspension. Two major categories of controller design methodologies are considered. The first is based on sequential single-input single-output (SISO) design techniques, and includes the sensitivity decoupling (SD) method and the PQ method. High rate inner loop damping control is implemented followed by a low rate outer loop controller. The second category is based on multirate, multi-input multi-output (MIMO) design techniques, including mixed H₂/H_infin, mixed H₂/_mu and robust H₂ synthesis. In this case, a set of controllers are designed all at once by explicitly considering plant uncertainty and hence robust stability. Comparisons are made between these design techniques in terms of nominal H₂ performance, robust stability, and robust performance. The advantages and disadvantages of each of these methods are discussed, as well as the guidelines for practical implementation