The big idea: A control system keeps something physical in a desired state without a person watching it.
The control loop
1
Sensors measure
They read the current state.
2
Microprocessor decides
It compares the measurement with the target and decides.
3
Actuators act
They change the world.
4
Measure again
The change is measured again.
That loop is the whole idea.
| Component | Job | Examples |
|---|---|---|
| Sensor | Turns a physical quantity into a signal | Thermistor, light sensor, pressure pad |
| ADC | Converts that analogue signal to a number | Built into most microcontrollers |
| Microprocessor | Compares with the target and decides | The controller itself |
| Actuator | Turns a signal back into physical change | Motor, valve, heater, relay |
| Feedback path | Carries the result back to be measured | The loop closing |
Two more parts the guide names
| Part | What it is | Example |
|---|---|---|
| Transducer | Any device that converts one form of energy into another. Sensors (heat to electrical signal) and actuators (electrical signal to movement) are both transducers | A microphone; a loudspeaker |
| Control algorithm | The rules the controller runs to turn the input and the target into an output | "If temperature < 19.5 °C, heater on; if > 21 °C, heater off" |
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Open loop
- Acts and never checks the result
- A heater on a timer — on for an hour regardless
- Cannot correct for anything unexpected
- Simple and cheap, and blind
Closed loop
- Measures the result and acts on the difference
- A thermostat: heat until the target is reached, then stop
- Corrects for disturbances nobody predicted
- An open window is handled without being anticipated
The difference
- Open loop follows a plan
- Closed loop pursues a goal
- Only the second can respond to the unexpected
Negative feedback stabilises: The action opposes the error: too cold, heat; too hot, stop. That is what settles the system on its target.
Positive feedback would amplify the error instead, and the system would run away.
Hysteresis, and why it is deliberate: Switching exactly at the target makes a heater flick on and off constantly around it, wearing the relay out.
So a real thermostat heats to 21°C and does not restart until 19.5°C. That deliberate gap is hysteresis — it trades precision for far fewer switching cycles.
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What makes control software different
- A correct answer too late is a wrong answer — braking 200 ms after it was needed is a crash
- So the system must respond within a guaranteed time, not merely an average one
- Hard real-time: missing a deadline is a failure — an airbag, an engine controller
- Soft real-time: missing one degrades quality — a video stream dropping a frame
- Which is why control systems use simple, predictable code over clever, variable code
Sensors are not perfect: A sensor reading carries noise, drifts over time, and has a range outside which it is meaningless.
Good control software smooths readings, checks them for plausibility, and has a defined behaviour when a sensor fails — because a failed sensor reading zero looks exactly like a genuinely cold room.
How this is tested — you must name the components in the loop and explain what feedback specifically adds. It comes up two ways:
Paper 1 Section A
- Describe the use of control system components, 3-4 marks
- Name the component doing a described job
- State the difference between open and closed loop
Paper 1 Section B — case study
- Explain feedback in a described system
- Justify a design choice such as hysteresis
The classic trap: Describing sensors and actuators but never the feedback path. Without the result being measured again it is an open loop, and the system cannot correct anything it did not anticipate.
IB-style questionDescribe[5 marks]
Describe the components of an automatic greenhouse temperature system, and explain why it uses closed-loop control.
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See the answer broken into clear points, with guidance on how each point earns marks.