QFT design for uncertain non-minimum phase and unstable plants revisited

Design method for uncertain non-minimum phase and unstable plants in the Quantita- tive Feedback Theory (QFT) developed by Horowitz and Sidi (Horowitz and Sidi, 1978; Horowitz, 1992) is revisited in this paper. It is illustrated that the existing method may not work since some design rules have not been clearly speci¯ed by several exam- ples including non-minimum phase plants and unstable plants. Then stability of a new nominal plant is carefully examined and analysed, and an improved design method is presented. The result in this paper provides mathematical justi¯cation of the QFT de- sign procedure for non-minimum phase and unstable plants in Horowitz and Sidi (1978) and Horowitz (1992).