Control : Feedback

Most useful control systems use feedback. Feedback occurs when the previous outputs from a control system change the inputs, usually to keep the system in a stable state.

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 previous outputs affect the inputs we can say that the system involves feedback. The feedback keeps the system in a stable (cold) state.

GCSE ICT Companion 04 - (C) P Meakin 2004