Hardware
Monitoring and control systems
What you need to be able to do
Syllabus 3.1 Computers and their components
- 3.1Explain the difference between monitoring and control systems, and justify which a given system is.
- 3.1Describe the use of sensors (temperature, pressure, infra-red, sound …) and actuators in a given system.
- 3.1Explain the importance of feedback in a control system.
You need: Embedded systems (most control systems are embedded).
Monitoring or control?
| Monitoring system | Control system | |
|---|---|---|
| Purpose | observes / records data and may warn (display, alarm, message) | acts autonomously to change the environment to keep it within a set range |
| Actuators | none | uses actuators (motors, valves, heaters, pumps) |
| Feedback | no feedback: its output does not affect the next input | uses feedback: its output affects the next input |
| Examples | hospital patient monitor, weather station, burglar alarm, bridge weight warning sign | air conditioning, greenhouse window, automatic doors, car automatic braking, central heating |
- Ask: does the system's output change the thing the sensor measures?
- Yes (doors open, so the person walks through and the sensor reading changes; brakes slow the car, so the distance changes) → control.
- No (a warning sign lights, a message is sent, the camera records - but the sensor reading is unaffected) → monitoring.
- In the answer, justify: no marks for the word “control”; the marks are for the reasons (feedback, actuator, output affects input).
Feedback
Feedback ensures that the system operates within set criteria / constraints by enabling the system's output to affect subsequent (later) input, so that conditions can be automatically adjusted.
- The sensor measures a physical quantity (e.g. temperature) and sends the reading (via an ADC) to the processor.
- The processor compares the reading with the stored set point / range.
- If it is out of range, the processor sends a signal to an actuator (e.g. switch on the heater, open the window).
- The actuator's action changes the environment, so the next sensor reading is different: the loop repeats continuously.
Feedback here is not a user survey or a review (November 2022 report): it is the system's own output affecting its next input.
Sensors and actuators
| Sensor | Measures / detects | Example uses (with the reason) |
|---|---|---|
| Temperature | how hot or cold something is | greenhouse, fridge, 3D printer nozzle (prevent overheating), central heating |
| Pressure | force / weight on a surface | vehicle weight on a bridge, someone sitting in a car seat (alarm), item lifted off a shop shelf, robot bumping into a table |
| Infra-red | a beam being broken / heat from a body / distance | counting products on a conveyor, vehicle height at a bridge, a person near automatic doors, obstacles for a robot, intruders |
| Sound | noise level | car alarm (glass breaking), speech for a voice assistant / robot |
| Light | brightness | street lights, security floodlights at dusk |
| Proximity / ultrasonic | distance to an object | parking sensors, automatic braking |
| Humidity, gas, pH | moisture, gas level, acidity | greenhouses, air-quality monitors, rivers |
An actuator converts an electrical signal into physical action / mechanical force: a motor, valve, pump, heater, solenoid. E.g. it pushes an arm to remove a chocolate bar with the wrong weight, opens a trap door, opens a window.
When asked for a sensor in a context, name it AND say exactly what it detects in that system - “a pressure sensor detects any pressure on the vehicle” was too vague (November 2022).
“A pressure sensor in the driver's seat detects an intruder sitting down.”
Mark-scheme focus
Explain the importance of feedback in a control system (2 marks)
![(a) Explain the importance of feedback in a control system. [2]](/notes-content/9618/hardware/img/9618_w23_11_q09_a.png)
Credited
- Feedback ensures that a system operates within set criteria / constraints B1
- … by enabling system output to affect (subsequent) system input // … allowing conditions to be automatically adjusted B1
Not credited
- feedback from users / reviews
- “feedback tells the computer what happened” (no output → input link)
From: 9618/11/O/N/23 Q9(a)
Differences between a monitoring system and a control system (3 marks)
![Describe the differences between a monitoring system and a control system. [3]](/notes-content/9618/hardware/img/9618_w25_11_q09.png)
Credited
- Monitoring systems do not take any action; control systems act autonomously to change the environment if values are out of range B1
- Control systems use actuators; monitoring systems do not B1
- Control systems use feedback; monitoring systems do not // output from a control system affects the next input B1
Not credited
- one-sided statements
- “control systems control things”
From: 9618/11/O/N/25 Q9
- Monitoring vs control in most papers: identify + justify for a given system (2-3), differences (3), feedback (2-3), sensors in context (1-2), actuator role (2).
Worked examples
Example 1 · Why the fridge is a control system (Paper 1, 2 marks) ★ Easy
![(c) The refrigerator's temperature must be kept between 4 and 6 °C. The microprocessor turns on the cooling if the temperature is too high and turns it off if too low. Explain why the system in the refrigerator is a control and not a monitoring system. [2]](/notes-content/9618/hardware/img/9618_s21_11_q05_c.png)
1 Read it with me what is actually happening?
Don't repeat the question (“it turns the cooling off…”); name the features that make it control.
2 What is it asking for? the target, part by part
- (c)2 reasons
3 What do we know? everything the question gives us
4 Plan the route work backwards from what we want
FindKey ideaAlready have itWork out firstTrap
Does switching the cooling on change the next reading?
Yes: the temperature falls - that is feedback.
5 Now solve it have a go on paper first, then open the worked solution
- The system uses feedback B1
- The system causes the temperature to change // produces an action B1
6 Check it does the answer make sense?
The report: many answers only repeated the question ✓.
Example 2 · Bridge warning: sensors and system type (Paper 1, 4 marks) ★★ Medium
![A road bridge has a weight limit (10 000 kg) and a height limit (3 m). If a vehicle exceeds a limit, a sign warns the driver to stop. (a) Identify two different sensors and describe how each is used by the system. [2] (b) Explain whether the bridge warning system is an example of a monitoring system or of a control system. [2]](/notes-content/9618/hardware/img/9618_w24_12_q09.png)
1 Read it with me what is actually happening?
(b): does the sign change the vehicle's weight or height?
2 What is it asking for? the target, part by part
- (a)2 sensors + uses
- (b)justify
3 What do we know? everything the question gives us
4 Plan the route work backwards from what we want
FindKey ideaAlready have itWork out firstTrap
- (b)Monitoring
- Whysign is only a warning; no actuator; output doesn't change the input
5 Now solve it have a go on paper first, then open the worked solution
- (a) Infra-red: measure / check the height of the vehicle B1; pressure: measure / check the weight B1
- (b) Monitoring (no mark): no feedback - the warning sign is only an indicator B1; the output does not affect the input from the sensors // no actuators B1
6 Check it does the answer make sense?
✓
Example 3 · Automatic braking is a control system (Paper 1, 3 marks) ★★ Medium
![A car has an automatic braking system: a sensor measures the distance to objects in front; the car brakes if an object is too close or the distance is decreasing too quickly; the closer the object, the harder it brakes. Explain the reasons why the automatic braking system is a control system. [3]](/notes-content/9618/hardware/img/9618_w24_13_q03.png)
1 Read it with me what is actually happening?
Show the loop: input → brake → new distance → input.
2 What is it asking for? the target, part by part
- Whole3 reasons
3 What do we know? everything the question gives us
4 Plan the route work backwards from what we want
FindKey ideaAlready have itWork out firstTrap
5 Now solve it have a go on paper first, then open the worked solution
- The system uses feedback and acts independently B1
- Input data causes the car to brake … braking changes the distance to the object B1
- … this new distance is used to decide whether / how hard to brake B1
6 Check it does the answer make sense?
✓
Example 4 · A counting sensor and a reject actuator (Paper 1, 3 marks) ★ Easy
![A chocolate factory uses a conveyor belt. (a) An automated system counts the bars at the end of production using a sensor above the belt. Identify one appropriate type of sensor. [1] (b) A second system removes bars with an incorrect weight. Describe the role of an actuator in this second system. [2]](/notes-content/9618/hardware/img/9618_w23_12_q01_ab.png)
1 Read it with me what is actually happening?
(b): what the actuator converts, and what physical job it does here.
2 What is it asking for? the target, part by part
- (a)sensor
- (b)actuator role
3 What do we know? everything the question gives us
4 Plan the route work backwards from what we want
FindKey ideaAlready have itWork out firstTrap
5 Now solve it have a go on paper first, then open the worked solution
- (a) infra-red / proximity B1
- (b) Actuator converts electrical energy / a signal into a mechanical force / causes an action B1 … to push an arm / open a trap door to remove the bar with the wrong weight B1
6 Check it does the answer make sense?
✓
Example 5 · Automatic doors (Paper 1, 3 marks) ★★ Medium
![An automated system opens doors when a person is detected within 2 metres and closes them when there is no longer a person within 2 metres. Identify whether it is a monitoring or a control system. Justify your choice. [3]](/notes-content/9618/hardware/img/9618_w25_12_q11.png)
1 Read it with me what is actually happening?
Opening the doors lets the person walk through, which changes what the sensor detects.
2 What is it asking for? the target, part by part
- Wholejustification
3 What do we know? everything the question gives us
4 Plan the route work backwards from what we want
FindKey ideaAlready have itWork out firstTrap
5 Now solve it have a go on paper first, then open the worked solution
- Control (no mark)
- Uses an actuator to open / close the doors B1
- Opening the doors lets people move, which changes the input to the sensor B1
- Acts autonomously on the feedback from the sensor B1
6 Check it does the answer make sense?
✓
Summary
- Monitoring: observes / warns; no actuators; output doesn't affect input. Control: acts on the environment through actuators; feedback.
- Feedback: output affects later input, keeping the system within set limits automatically.
- Sensors measure (temperature, pressure, infra-red, sound, light…); actuators turn signals into movement / action.
Can I do this?
- I can say whether a system is monitoring or control and justify it.
- I can explain feedback and the control loop.
- I can choose a sensor for a context and say what it detects; describe an actuator's role.
Anything unticked: revise that section first. All ticked: take the quiz.
Quiz
The multiple-choice and guided questions need JavaScript. The concept checks and exam practice below work without it.
Concept checks
Answer each one in your head first, then open it.
1Can a monitoring system have an alarm?Show answer
Yes: an alarm is an output that warns people, but it doesn't change what the sensor measures, so it is still monitoring.
2Why does a control system need an ADC?Show answer
Sensors produce analogue signals; the processor works with digital data.
Exam-style practice
New questions, not used above. Do them on paper, then mark your work against the official mark scheme.
![(a) Explain the importance of feedback in a control system. [3] (b)(i) Identify one sensor that could be used in a car alarm system. Justify your choice. [2]](/notes-content/9618/hardware/img/9618_w22_13_q10_abi.png)
Show the mark scheme
| Part | Answer / working | Marks | Guidance |
|---|---|---|---|
| (a) | To ensure the system operates within the given criteria | B1 | |
| … by enabling system output to affect subsequent system input | B1 | ||
| … thus allowing conditions to be automatically adjusted | B1 | ||
| (b)(i) | e.g. sound / infra-red / pressure sensor | B1 | |
| matching justification: a sound inside the car activates the alarm // senses the heat of a person / beams broken // an intruder sits in the driver's seat | B1 | ||
| Total: 5 marks | |||
![A video doorbell uses a motion sensor; when activated the camera records and a message is sent to a smartphone; a button plays the doorbell sound on the phone. (b) State whether the video doorbell is a monitoring system or a control system. Justify your choice. [2]](/notes-content/9618/hardware/img/9618_s24_11_q02_b.png)
Show the mark scheme
| Part | Answer / working | Marks | Guidance |
|---|---|---|---|
| (b) | No mark for the choice. Monitoring: turning on the camera / sending the video / ringing does not affect the input to the sensor / button | B2 | max 2 |
| or Control: the data is processed, generating a signal to start the camera recording // button / motion causes a signal to be sent to the smartphone | |||
| Total: 2 marks | |||
![A shop's new system automatically detects items customers take from a shelf. (b) The system uses sensors to identify the items taken from a shelf. Identify one type of sensor and state how it can be used. [2]](/notes-content/9618/hardware/img/9618_s25_11_q04_b.png)
Show the mark scheme
| Part | Answer / working | Marks | Guidance |
|---|---|---|---|
| (b) | Pressure | B1 | |
| Detects when the pressure of an item is removed from / put back on a shelf | B1 | or infra-red: beam broken when an item is removed / added | |
| Total: 2 marks | |||
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