Monitoring, measurement and control systems
How sensors, microprocessors and actuators work together.
Review these first
Learning objectives
What you need to be able to do
Teacher-mapped phrasing — check against the official Cambridge syllabus for exact wording.
- 6.1.1Describe monitoring and control systems using sensors and microprocessors.
5 minute read
Monitoring, measurement and control systems
Both kinds of system read the physical world with sensors. The difference is what happens next.
Monitoring versus control
In a monitoring system the computer records and reports the readings, but takes no action — a weather station, or a hospital monitor displaying a patient's heart rate and sounding an alarm for a nurse. In a control system the computer compares readings to preset values and acts automatically through actuators — a central heating system switching a boiler on, or a greenhouse opening a window. The distinction is the exam question: monitoring reports; control changes something.
How a control system works
- Sensors continuously measure a physical quantity, e.g. temperature.
- The reading is analogue, so an ADC (analogue-to-digital converter) turns it into digital data the microprocessor can use.
- The microprocessor compares the value with the stored preset value.
- If the value is outside the acceptable range, the microprocessor sends a signal to an actuator — a heater, motor, valve or buzzer — to correct it.
- The process repeats continuously in a loop.
Why it is better than a person doing it
It runs continuously, day and night without tiring; it responds faster and more consistently; readings are more accurate; and it can operate in environments dangerous to humans. Against that, it is expensive to install, and a sensor fault or power failure can cause the whole system to fail.
Think of it like this
A monitoring system is a thermometer; a control system is a thermostat. One tells you the room is cold, the other does something about it — and only the second closes the loop.
Worked examples
Method, step by step
Describe how a computerised system keeps a greenhouse at a constant temperature.
- 1A temperature sensor continuously measures the air temperature inside the greenhouse.
- 2The analogue reading is converted to digital by an ADC and sent to the microprocessor.
- 3The microprocessor compares the value with the stored preset temperature.
- 4If the temperature is too high it signals an actuator to open a window or start a fan; if too low it switches on a heater.
- 5The cycle repeats continuously so the temperature is held near the preset value.
A temperature sensor reads the air temperature, an ADC converts it to digital, the microprocessor compares it with a preset value, and it signals actuators (heater, fan or window motor) to correct any difference — repeating continuously.
Common misconceptions
- Saying a monitoring system "controls" something. If no actuator acts on the result, it is monitoring only.
- Forgetting the ADC. Sensors produce analogue signals and computers need digital data, so the conversion step is a genuine marking point.
- Describing the process as happening once. Control systems run in a continuous loop, constantly re-reading and re-comparing.
In the exam
- Describe control systems as a numbered loop: sensor reads → ADC converts → microprocessor compares with preset value → actuator acts → repeat. Include the word "continuously".
- Name the specific sensor and the specific actuator for the scenario given — "temperature sensor" and "heater", not "a sensor" and "a device".