Introduction

Abstract

Before 1960, control theory was composed of several classical theorems such as the Routh-Hurwitz stability criterion, Nyquist stability theorem, Bode’s dispersion relations, Wiener’s realizability criterion and factorization theory on the one hand, and a set of design algorithms such as lead-lag compensation, Smith’s prediction met hod, Ziegler-Nichols ultimate sensitivity method, Evans’ root locus method, and the like on the other h and. Although the preceding theoretical results were highly respected, they were not closely related to each other enough t o form a systematic theory of control. However, classical design algorithms such as lead-lag compensation were not clearly formalized as a design problem to b e solved, but rather as a tool of practices largely dependent on the cut-and-try process. They were u su ally explained through a set of examples.