Performance enhancement of modular control systems using µ synthesis

For complex dynamic systems, a modular control design process is often employed, wherein the overall design is partitioned into smaller modules. The designers of each module only possess a model for a particular subset of the entire plant as well as closed loop performance specifications for the other module(s). In this paper, we will examine a common modular control strategy in which an outer loop controller computes a desired virtual control input and the inner loop computes real control inputs in order to achieve this desired virtual control input as closely as possible. The outer loop design is based on a specification for the inner loop, which may not always be achieved. We propose a modular control error compensator that is aimed at mitigating the performance degradation caused when the inner loop specifications are not achieved. We show that this compensator can be designed using μ synthesis and propose an iterative procedure to optimize performance based on two concrete worst-case metrics. The effectiveness of the proposed compensator is shown through an automotive example.