What is automatic control

Control theory was developed to support the emergent activity of control. It is therefore a useful motivation to turn our attention to it. Historically, the discipline of automatic control was concerned with the replacement of the human worker of Figure 7 by the automatic controller of Figure 8.

Although automatic control is nowadays a complex discipline, no longer primarily concerned with the replacement of human operators, it is a useful starting point to consider what sort of skills are necessary to move from an existing, manually controlled situation to a new automatically controlled situation, as in Figure 8.

(1) A central idea of control theory is the control loop. All control loops have the same basic form, regardless of the particular application area. Thus, control theory uses an application-independent notation to convert all control problems into the same standard problem. We can consider that control theory concentrates on studying the universal situations that underlie all applications of quantitative control.

In broadest form a control loop appears as in Figure 9. The decisions govern actions that are taken. The effect of the actions is reported back by the information channel. Further decisions are taken and the loop operates continuously as described. A control loop provides an extraordinarily powerful means of control but. at the same time, the existence of the loop always brings the possibility of the potentially very destructive phenomenon of instability.

(2) All control loops are error-driven, where error is defined as the difference between the behaviour that is desired and the behaviour that is measured.

(3) An important performance measure for a control system relates to rate of error reduction. Often, performance is quoted in terms of the highest frequency that the control system can follow, when required to do so.

(4) All control loops tend to become unstable as higher and higher performance is sought. A good understanding of the topic of stability is central to understanding control theory.

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