Robust internal stability in multi input/output quantitative feedback theory

It is shown that, for robust stability design, the recent multi-input multi-output (MIMO) quantitative feedback theory (QFT) technique is essentially a composition of two frequency-domain design approaches: the single-input single-output (5150) QFT and the sequential procedure for Nyquist arrays developed in the 1970s. This unification allows straightforward adaptation of results on generalization of classical design procedures to multivariable fixed plants and takes full advantage of the SISO QFT unique approach for achieving robust stability and robust performance while minimizing controller bandwidth in the presence of plant uncertainties. Using results from both approaches, a necessary and sufficient condition is derived for robust internal stability of the sequential MIMO QFT procedure