A1.3.6 · Operating systems and control systems
Control systems and their parts
By the end you can
Describe the input–process–output model, open-loop and closed-loop feedback, and what controllers, sensors, actuators, transducers and control algorithms do.
A keeps something in the physical world the way we want it: a room at 20 °C, a lift level with the floor, a car at 50 km/h, a field watered but not flooded. It does this automatically, many times a second, without a person watching. Every control system, simple or complex, can be described the same way.
Input, process, output, feedback#
- Input: what the system is told and what it measures. The (the goal, such as 20 °C) and readings from sensors (the room is now 17 °C).
- Process: the controller compares the two. The difference is the error (3 °C too cold), and the control algorithm decides what to do about it.
- Output: signals to the actuators (switch the heater on), which change the physical system (the room warms).
- : the sensors measure the result and pass it back to the controller, which compares again. The loop repeats until the error is gone, and keeps running to catch anything that disturbs it.
Open loop and closed loop#
An system has no feedback. It carries out a fixed plan and never checks the result: a toaster heats for the time you set, whether the bread is pale or burnt; a basic washing machine runs its programme step by step. Open loop is cheap, simple and fine when conditions are predictable.
A system measures its output and corrects itself. A thermostat notices the room cooling and turns the heating up; cruise control notices the car slowing on a hill and opens the throttle. Closed loop costs more (sensors, a controller) but copes with the unexpected. See for yourself:
Strip: heater power each minute (darker = more). Green line: the 20 °C target.
After warming up: 0.6 °C from the target on average; lowest 18.6 °C
The sensor measures the drop, the controller sees the error, and the heater works harder until the window shuts: feedback at work.
The components#
Controller
The is the brain: a microcontroller, a programmable logic controller (PLC) in a factory, or a full computer in a self-driving car. It reads the sensors, compares them with the set point, runs the control algorithm and sends commands to the actuators.
Sensors
are the system's eyes and ears. Each measures one physical quantity and turns it into an electrical signal: temperature (a thermistor or thermocouple), light, pressure, humidity, movement (infrared), distance (ultrasonic), position, speed. The controller can only be as good as its measurements, so a sensor's accuracy and placement matter.
Actuators
are the muscles: they turn the controller's signals into action in the physical world. Motors (lift, door, drum), valves (water, gas), pumps, heaters, relays that switch bigger machines on and off, lights, locks and sirens.
Transducers
A converts energy or a signal from one form into another. It is the link that lets physical quantities and electronics talk to each other: in a tyre-pressure sensor it turns pressure into a voltage; in a loudspeaker it turns an electrical signal into sound; in a motor, electrical energy into movement. Most sensors and many actuators contain a transducer. You will often see actuators called output transducers (and sensors input transducers), especially in older exam papers: “sensors, processor and output transducers” means the same loop as “sensors, controller and actuators”.
The signal a transducer produces is usually analogue: a voltage that rises and falls smoothly with the temperature or pressure. A digital controller can only work with numbers, so an analogue-to-digital converter (ADC) samples the voltage and turns it into a binary value (the same sampling as sound in A1.2.2). On the way out, a digital-to-analogue converter (DAC) turns the controller's numbers back into a voltage for an actuator that needs one, such as a motor whose speed depends on it. The full chain is: physical quantity → transducer → ADC → controller → DAC → actuator → physical change.
Control algorithm
The is the rule the controller follows to turn the error into a command:
- On/off: full power below the set point, none above it. Simple, but the output swings around the target, so a small band (hysteresis) stops the heater switching on and off every second.
- Proportional: the bigger the error, the bigger the correction. Smoother, but it can settle slightly away from the target.
- PID (proportional, integral, derivative): also looks at how long the error has lasted and how fast it is changing, so it reaches the target quickly without overshooting. Used in cruise control, drones and industrial processes.
Check yourself
Control system components
Quick questions, made fresh every time and marked here. Do a few until they feel easy.
Exam practice
Paper 1-style questions
6 questions · 20 marks
Write your answer as you would in the exam, then show the mark points and tick each one your answer made. These are our own questions in the style of the exam, not past-paper questions.
1.State the four stages of the model used to describe a control system.
[2]State: Give a short answer, no explanation needed.
2.Distinguish between an open-loop and a closed-loop control system.
[2]Distinguish: Make the differences between two things clear.
3.A car’s cruise control keeps it at the speed the driver sets. Identify the input, process, output and feedback in this system.
[4]Identify: Pick out the answer from the options or the context.
4.Explain how sensors and actuators work with the controller in a closed-loop system.
[4]Explain: Give reasons: say what happens AND why, or how one thing causes another.
5.Outline the role of a transducer in a control system, giving an example.
[2]Outline: A brief account: the main points, without much detail.
6.A school heats its classrooms with heaters on timers that switch on at 07:00 and off at 16:00. Evaluate replacing the timers with closed-loop control.
[6]Evaluate: Weigh the strengths against the weaknesses (or the benefits against the costs) and reach a judgement that you back up.
Past papers
Old exam questions on this topic
From HL Paper 1, May 2014 to November 2025. These papers were set on the previous syllabus, so the wording may differ from today's exam, but the topic is the same. Find the questions and markschemes in your school's past-paper pack.
| Paper | Question | Marks | About |
|---|---|---|---|
| November 2025 | 9 | 3 | The microprocessor in a home alarm |
| May 2024 TZ1 | 11 | 4 | Feedback in room temperature control |
| May 2023 TZ1 | 12(a) | 5 | Sensor, processor and output transducer for home heating |
| May 2023 TZ1 | 12(b) | 2 | The role of feedback |
| May 2022 | 13(b) | 6 | A car’s distance-keeping feedback loop |
| November 2022 | 12(b) | 4 | Doors in a smart building |
| May 2021 | 15(e) | 4 | Sensor data deciding when lights switch on |
| November 2020 | 11(b) | 3 | An output transducer in a robot |
| May 2019 | 16(c) | 5 | Holding a constant temperature |
| November 2019 | 11(c) | 6 | Sensors, microprocessors and output transducers |
| May 2018 | 14(b) | 3 | Feedback in control systems |
| November 2018 | 10(d) | 4 | Sensors, processor and output transducers |
| May 2017 | 16(c) | 6 | Steps in a motor’s feedback loop |
| November 2017 | 12(c) | 2 | What an output transducer does |
| November 2016 | 10(b) | 5 | The control process in a greenhouse |
| May 2015 | 12(a) | 4 | Sensors and a microprocessor at traffic lights |
| November 2015 | 9(a) | 5 | Parts of an automatic-door control system |
| November 2014 | 13(a) | 2 | Sensors for a perimeter alarm |
In a nutshell
- A control system manages a device or process automatically: input → process → output, with feedback in a closed loop.
- Open loop: acts without measuring the result (a toaster timer). Cheap and simple, but cannot correct for disturbances.
- Closed loop: sensors measure the output and the controller corrects any error from the set point (a thermostat). Accurate and adaptable, but more complex.
- Controller: compares measurement with set point and decides. Sensor: measures a physical quantity. Actuator: acts on the world (motor, valve, heater).
- Transducer: converts one form of energy or signal into another, linking sensors and actuators to the controller. An ADC turns a sensor’s analogue voltage into a number; a DAC turns numbers back into a voltage.
- Control algorithm: the rule for turning the error into a command: on/off (with a small band), proportional, or PID.