The big idea: An open-loop system acts on its input and stops there. Nothing measures the result.
A closed-loop system measures its own output, feeds that measurement back, compares it with what was asked for, and corrects the difference.
An open loop does what it was told. A closed loop does what it was asked.
The same block chain with and without the return path, then what each one does when something unexpected happens.
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| Situation | Open loop — a toaster on a timer | Closed loop — an oven with a thermistor |
|---|---|---|
| Everything as expected | Works perfectly | Works perfectly |
| The input is unusual | Frozen bread comes out pale, because the timer ran regardless | Cold food simply takes longer, because the sensor keeps reporting |
| The environment changes | A cold kitchen gives a paler result every time | The element runs longer and the set temperature is still reached |
| Something disturbs it mid-cycle | Nothing happens — it cannot know | Opening the door drops the temperature and the element comes back on |
| A component drifts with age | The error grows unnoticed for years | The loop absorbs it, up to the point where the sensor itself drifts |
The purpose of feedback, in one sentence: It lets a system correct for everything the designer could not predict — a cold room, a heavy load, a slipping wheel, a worn part.
That is the sentence to write. "It makes it more accurate" is the consequence, not the purpose.
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| Open loop is right when | Closed loop is right when |
|---|---|
| The task is predictable and a small error is harmless | Accuracy matters, or being wrong is dangerous |
| Cost matters more than precision | Conditions vary in ways the designer cannot predict |
| A toaster, a washing-machine fill timer, a doorbell, fixed-cycle traffic lights | An oven, a cruise control, a lift, a drone, a powered wheelchair, an insulin pump |
A loop can also make things worse: A system that corrects too hard overshoots, then corrects back, then overshoots again — it oscillates, and a heater that swings between 15 °C and 25 °C is worse than one that sits steadily at 19 °C.
The usual answers are a dead band — a range within which it does nothing — and correcting gently rather than at full power.
How this is tested — comparing open- and closed-loop systems and explaining the purpose of feedback. It comes up two ways:
Paper 1 — multiple choice
- Classify a named product as open or closed loop.
- Identify the feedback device in a described system.
Paper 2 — analysing a product
- Explain the purpose of the feedback in a named product.
- Evaluate an open- and a closed-loop version of the same product.
The trap: Calling a timer feedback. A timer counts, whatever happened. Feedback requires something that measures the OUTPUT.
A greenhouse can be ventilated either by a fan on a time switch or by a fan controlled from a temperature sensor. Analyse the two systems against each other.
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