u-Synthesis Control with PI Augmentation

The mu-synthesis approach has been successfully applied to both single-stage and dual-stage disk drive servo control. There is a problem, however, in that it is not easy to get tight control over the low frequency properties of the controller (such as integral action). Moreover, this problem is exacerbated by the fact that the plant is essentially a double integrator at low frequencies, which can cause significant numerical problems for the associated algorithms. In this paper we show how to address this problem, and develop a decoupled design approach which allows tight a-priori specification of the low frequency controller, while allowing for the mu-synthesis approach to handle the mid/high frequency regions. Moreover, by pulling the low frequency control out of the associated H_infin-synthesis problem, we alleviate the numerical issues described above