Using Open- Or Closed-Loop Control

All processes can be operated using either an open- or a closed-loop control system. Deciding on which to use is usually the responsibility of a control engineer but it pays to understand how these systems operate in any setting. Fermentations, ovens etc. are usually controlled in some way with a closed-loop system but there always times when the open-loop system is needed.

The Open-Loop System

The main difference between the two is that in a closed system there is feedback whilst the open system has no feedback. The open system is often known as the non-feedback system and can only act on the nature of the input. The output has absolutely no effect on the control action. Thus, an open-loop system cannot be disturbed.

Examples of an Open-Loop Control system include an electric drum drier where the output of the drier is hot air. The operation of the drier continues until it is switched off but the level of dryness of the object being dried has no impact on the amount of hot air being used. A light bulb when switched on from a light switch will continue to shine whether the light is required or not.

The benefit of this system is that it requires human intervention meaning there is a conscious decision to intervene.

The Closed-Loop System

The closed-loop system is one that produces an output. The current situation with the output is altered to a more desirable situation using a feedback system. The control action in such systems is based solely on the output. It all depends on the type of control situation required and is usually the system most employed in automatic control systems.

The advantages of the closed-loop system are associated with measurement. Controlling a system automatically requires measurement. This can be direct or indirect, and inferred. The closed-loop system is preferred when measurement is actually possible. The process being controlled also must be predictable meaning there is some prior knowledge about its performance. There should be a known and approximate response to any input control.