A historical review of robust control

A historical review of recent results in robust control is presented. The robust control problem, i.e., the problem of designing accurate control systems in the presence of significant plant uncertainties, is classical. However, over the past 15 years, significant new theory has been developed for the solution of this problem, especially with respect to linear multivariable systems characterized in the frequency domain, and the term robust control for this classical problem is only of recent vintage (1972). Some of the major contributions of modern robust control theory include the development of synthesis techniques for robust stabilization, and H
²
and
H^{\infty}
sensitivity optimization of multivariable systems. In this review, we confine the term robust control to the design of fixed controllers. Thus, only nonadaptive or nonself-tuning solutions to the problem of controlling uncertain systems are reviewed. Finally, it should be noted that the review is largely limited to the literature published in IEEE journals and conference proceedings, and some English-language journals. It is, of course, recognized that many significant related contributions have appeared elsewhere.