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1. The Excitement of Control Engineering

1.5.9 Disturbances and Uncertainty

One of the things that makes control science interesting is that all real-life systems are acted on by noise and external disturbances. These factors can have a significant impact on the performance of the system. As a simple example, aircraft are subject to disturbances in the form of wind gusts, and cruise controllers in cars have to cope with different road gradients and different car loadings. However, we will find that, by appropriate design of the control system, quite remarkable insensitivity to external disturbances can be achieved.

Another related issue is that of model uncertainty. All real-world systems have very complex models, but an important property of feedback control is that one can often achieve the desired level of performance by using relatively simple models. Of course, it is incumbent on designers to appreciate the effect of model uncertainty on control performance and to decide whether attention to better modeling would enable better performance to be achieved.

Both of the issues raised above are addressed, in part, by the remarkable properties of feedback. This concept will underpin much of our development in the book.