Performance issues in the individual channel design of 2-input 2-output systems. Part 2. Robustness issues

In this second part of a three part series of papers examining the performance issues raised by the introductory paper (O'Reilly and Leithead 1991) on individual channel design for multivariable control, attention is focused on robustness: that is, on the meeting of performance specifications—in particular closed-loop system stability—in the face of plant uncertainty. The main results are that the use of phase margins and gain margins to assess robustness of 2-input 2-output multi-variable systems is justified. Further, it is shown that controllers for uncertain 2-input 2-output multivariable systems can be designed by classical SISO Nyquist-Bode local loop shaping using stable minimum phase controllers. Several additional results are also described in the paper.