The loop that every automated system is built from
An automated system combines sensors, a microprocessor and actuators to control a physical process without human intervention. Every example on the syllabus — heating, greenhouses, traffic lights, production lines — is the same four-stage loop with different components plugged in.
A sensor measures continuously. An analogue-to-digital converter turns that reading into a number. The microprocessor compares it with a stored preset value. If action is needed, it signals an actuator, which changes the physical quantity — and the sensor reads it again.
Step to Compare. That stage is what makes this a control system: the same sensor and processor without the comparison would be monitoring and recording, and nothing would ever be switched on.
Monitoring and control are different systems
A monitoring system measures and reports — a weather station, or a patient monitor that displays a heart rate. A control system measures and then acts to change what it measured. The difference is the actuator and the comparison that drives it, and exam questions distinguish the two deliberately.
Why feedback is what makes it work
The loop is closed: the action changes the quantity, and the sensor measures the changed quantity. That is feedback, and it is what allows the system to hold a value steady without anyone watching.
A heater that simply switched on for ten minutes whenever it was cold would overshoot on a mild day and undershoot on a freezing one. A system with feedback keeps checking, so it switches off at exactly the right moment regardless of conditions. The response is determined by the actual state rather than by an assumption about it.
| Application | Sensor | Actuator |
|---|---|---|
| Central heating | temperature | boiler / valve |
| Greenhouse | moisture, temperature, light | water valve, heater, blinds |
| Automatic doors | infrared / proximity | motor |
| Street lighting | light | lamp switch |
| Anti-lock brakes | wheel rotation speed | brake pressure valve |
| Production line | proximity, weight, camera | robot arm, conveyor motor |
Robotics
A robot is an automated system with physical movement and, usually, some programmability. The syllabus identifies three characteristics: it has a mechanical structure, it has electrical components including sensors and actuators, and it contains a program that determines its behaviour.
Robots are used where a task is dull, dirty or dangerous — repetitive assembly, paint spraying, bomb disposal, work in reactors or deep water. They are also used where precision beyond human capability is needed, as in surgery or microchip manufacture.
- Advantages: consistent quality with no fatigue; can work continuously; can operate in environments that would harm a person; higher precision; lower long-term cost.
- Disadvantages: high initial cost; job displacement for existing workers; inflexible when the task changes; and they cannot handle an unfamiliar situation that was not anticipated in their programming.
Describe how an automated greenhouse keeps the temperature at 24 °C.
- A temperature sensor takes continuous readings of the air inside.Continuous, not occasional — the system must respond to changes as they happen.
- An ADC converts each analogue reading into a digital value.The microprocessor cannot use the analogue signal directly.
- The microprocessor compares the value with the stored preset of 24 °C.This comparison is what makes it a control system.
- If the reading is below the preset, it signals the heater to switch on; if above, it opens the vents.Two actuators, chosen according to the direction of the difference.
- The temperature changes, the sensor reads it again, and the process repeats continuously.The feedback loop is what holds the value steady without supervision.
Sensor reads, ADC converts, processor compares with 24 °C, actuator heats or vents, and the loop repeats.
Before you leave this chapter
- Sensor → ADC → microprocessor → actuator, then back to the sensor.
- The comparison with a stored preset is what makes it control rather than monitoring.
- Monitoring reports; control acts. The actuator is the difference.
- Feedback means the action changes what the sensor measures, holding a value steady.
- Robots suit dull, dirty or dangerous work, and work needing precision beyond human ability.
Describing a system for full marks
Every question in this topic follows the same pattern: a situation is described and you must explain how the system works. Answers lose marks by describing the outcome rather than the mechanism.
The reliable structure is one sentence per stage, naming the component and what it does. Say what is measured, say that it is converted, say what it is compared with, say what acts, and say that the loop repeats.
Explain how an automated system keeps a swimming pool at 28 °C.
- A temperature sensor in the water takes continuous readings.Name the sensor and say it measures continuously — occasional sampling would allow the temperature to drift between readings.
- An analogue-to-digital converter turns each reading into a digital value.A mark of its own, and the stage most often omitted.
- The microprocessor compares that value with the stored preset of 28 °C.Naming the preset explicitly is what shows this is a control system.
- If the reading is below the preset, it sends a signal to switch the heater on; if at or above, it switches it off.Both directions, so the system does not simply heat for ever.
- The water temperature changes, the sensor measures it again, and the cycle repeats indefinitely.Stating that the loop repeats is usually the final mark.
Sensor → ADC → processor compares with 28 °C → actuator heats or stops → sensor reads again.
Do not skip the converter
The ADC is the stage students leave out most often, because it does nothing visible. It is nearly always worth a mark, and its presence is the reason a sensor cannot simply be wired to a processor. Include it in every description of a system containing a sensor.