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1. The Excitement of Control Engineering

1.3 Historical Periods of Control Theory

We have seen above that control engineering has taken several major steps forward at crucial events in history (e.g., the industrial revolution, the Second World War, the push into space, economic globalization, and shareholder-value thinking). Each of these steps has been matched by a corresponding burst of development in the underlying theory of control.

Early on, when the compelling concept of feedback was applied, engineers sometimes encountered unexpected results. These then became catalysts for rigorous analysis. For example, if we go back to Watt's fly-ball governor, it was found that, under certain circumstances, these systems could produce self-sustaining oscillations. Toward the end of the 19th century, several researchers (including Maxwell) showed how these oscillations could be described via the properties of ordinary differential equations.

The developments around the period of the Second World War were also matched by significant developments in Control Theory. For example, the pioneering work of Bode, Nyquist, Nichols, Evans, and others appeared at this time. This resulted in simple graphical means for analyzing single-input, single-output feedback control problems. These methods are now generally known by the generic term Classical Control Theory.

The 1960's saw the development of an alternative state space approach to control. This followed the publication of work by Wiener, Kalman (and others) on optimal estimation and control. This work allowed multivariable problems to be treated in a unified fashion. This had been difficult, if not impossible, in the classical framework. This set of developments is loosely termed Modern Control Theory.

By the 1980's, these various approaches to control had reached a sophisticated level, and emphasis then shifted to other related issues, including the effect of model error on the performance of feedback controllers. This can be classified as the period of Robust Control Theory.

In parallel, there has been substantial work on nonlinear control problems. This has been motivated by the fact that many real-world control problems involve nonlinear effects.

There have been numerous other developments, including adaptive control, autotuning, and intelligent control. These are too numerous to detail here. Anyway, our purpose is not to give a comprehensive history but simply to give a flavor for the evolution of the field.

At the time of writing this book, control has become a mature discipline. It is thus possible to give a treatment of control which takes account of many different viewpoints and to unify these in a common framework. This is the approach we will adopt here.