The big idea: An open loop does what it was told. A closed loop does what it was asked.
Open loop
- Acts on its input and stops there
- Nothing measures the result
Closed loop
- Measures its own output
- Feeds that measurement back
- Compares it with what was asked for
- Corrects the difference
The same block chain with and without the return path, then what each one does when something unexpected happens.
Interactive diagram
Explore the labelled diagram, charts and maps for this topic in full study mode.
Free preview
This is the free notes preview
You're reading the free notes. Aimnova Pro unlocks the full study experience — and you can try it with your first topic free to keep:
- FlashcardsLock in vocabulary and key terms with spaced repetition.
- Practice questionsAnswer exam-style questions and get instant AI marking.
- Mock exams & past-paper vaultSit full mocks and see exactly how examiners award marks.
- Personalised study planA daily plan built around your exam date and weak areas.
| 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.
Learn what examiners really want
See exactly what to write to score full marks. Our AI shows you model answers and the key phrases examiners look for.
| 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. A heater that swings between 15 °C and 25 °C is worse than one that sits steadily at 19 °C.
The usual answers: 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.
Model answer plan
See the answer broken into clear points, with guidance on how each point earns marks.