7.1.5+Describe+the+role+of+feedback+in+a+control+system.

What is feedback?
Wikipedia gives us the definition of feedback as being: "Information about reactions to a product, a person's performance of a task, etc., used as a basis for improvement."

This definition alone gives us an idea of what feedback does in a control system, and why it is in fact used, **Feedback occurs when the outputs from a control system change the inputs, usually to keep the system in a stable state.**

How does feedback work?
To better understand how feedback works in a control system the best way to do so is look at an example of it in action:

Consider a refrigeration system which is used to keep food frozen at below -20°C. A temperature sensor is used to measure the temperature inside the freezer. The system contains a refrigeration unit to cool the freezer. If the temperature is below -20°C then the refrigeration unit is turned off to save energy. If the temperature rises above -20°C then the refrigeration unit is turned on to cool the food down.

The cooling system inside the freezer will repeatedly follow this sequence of actions :

1. The input from the temperature sensor causes the refrigeration unit to turn on because the temperature is above -20°C.

2. The refrigeration unit is turned on so the temperature falls.

3. The temperature becomes so cold that the temperature sensor causes the refrigeration unit to be turned off.

4. The refrigeration unit is off so the temperature in the freezer rises slowly.

5. The system goes back to step 1.

In this system the inputs first affect the outputs (temperature sensor causes refrigeration unit to turn on). Then the outputs affect the inputs (refrigeration unit changes temperature reading). Because the outputs affect the inputs we can say that the system involves feedback. The feedback keeps the system in a stable (cold) state.

Feedback can occur in information-based systems as well. Often an output will have a result on further inputs. For example, the output of accepting an online booking for an air ticket will be to reduce the number of tickets available.

Feedback loop


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