Quantitative feedback theory-reply to criticisms

Quantitative feedback theory (QFT) consists of a steadily growing body of design techniques for achieving prespecified system performance tolerances, despite prespecified large plant parameter and disturbance uncertainties. Since 1959, QFT has been extended to SISO and MIMO, linear and non-linear, time-invariant and time-varying, output feedback and internal variable feedback, lumped and distributed plants. Design examples in all the above classes have been described in great detail. In contrast, modern control theory almost completely ignored the uncertainty issue in feedback theory until about five years ago. There has since been much activity in this subject, which it denotes as the robustness problem. Despite this activity, hardly a single detailed design example involving large plant parameter uncertainty has been described. Nevertheless, researchers in robustness have ignored QFT. This conspiracy of silence has recently been broken with a list of criticisms by Doyle. These provide a very welcome means of explanation and elaboration of important QFT properties, including some new results.