A-Level Computer Science / Unit 3: Hardware, Memory and Control

3.1.7 Monitoring and Control Systems

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3.1.7 Monitoring and Control Systems

Computer systems can collect measurements from their surroundings. Some systems only record, display, or report those measurements. Others use the readings to change what is happening in the physical environment.

This lesson develops a complete model: sensor → data conversion → processor decision → actuator → new sensor reading. The final measurement is important because it shows whether the action had the intended effect.

By the end of this section, you should be able to:

  • Distinguish a monitoring system from a control system.
  • Explain how temperature, pressure, infrared, and sound sensors can be used.
  • Describe how sensor readings become data that a processor can use.
  • Explain the purpose of an actuator in a computer-controlled system.
  • Trace the stages of a closed control loop.
  • Explain why feedback is needed to check and adjust the system's response.
  • Design a monitoring or control solution for an unfamiliar situation.

Monitoring and control are not the same

Both types of system begin by collecting data. The key difference is what happens after the data has been processed.

Feature Monitoring system Control system
Main purpose Observe and report conditions. Maintain or change conditions automatically.
Typical output Stored readings, graphs, messages, or alarms. A command sent to an actuator.
Physical change required? No automatic physical response is required. Yes. The system acts on the environment.
Example A river station records water pressure and sends a warning when the level rises. A reservoir controller opens a valve when pressure indicates that the level is too high.
Monitoring system: a system that collects and processes measurements so that conditions can be observed, recorded, or reported.
Control system: a system that uses measurements to decide whether an actuator should change the physical environment.

Common mistake

An alarm alone does not necessarily make a system a control system. If the system only warns a person and the person decides what to do, it is monitoring. It becomes automatic control when the computer itself operates an actuator.

Sensors: converting physical conditions into signals

A sensor responds to a property in the physical world and produces a signal that represents the measured condition. The processor does not directly sense heat, pressure, movement, or sound; it receives data produced from the sensor signal.

Sensor What it detects or measures Original application
Temperature sensor Changes in temperature Checking whether a medicine cabinet remains within its safe storage range.
Pressure sensor Force applied over an area, or pressure within a gas or liquid Measuring pressure in an automated irrigation pipe.
Infrared sensor Infrared radiation or interruption/reflection of an infrared beam Detecting a person entering a restricted archive room.
Sound sensor Sound level or sound vibration Detecting unusual noise near a wildlife nesting enclosure.

Choosing a sensor

The correct choice depends on the physical quantity that must be measured. A sound sensor cannot directly measure pressure in a pipe, and an infrared sensor does not provide an accurate room temperature simply because warm objects emit infrared radiation.

Exam tip

Name the sensor and connect it to the exact condition: “a pressure sensor measures the fluid pressure” is stronger than “a sensor checks the pipe”.

From a sensor reading to usable data

Many sensors produce a continuously varying electrical signal. A computer-controlled system normally needs a digital representation before it can compare the reading with a stored value or use it in an algorithm.

  1. The physical condition changes.
  2. The sensor responds and produces an electrical signal.
  3. Signal-conditioning circuitry may amplify or filter the signal.
  4. An analogue-to-digital converter creates a numerical reading when required.
  5. The processor reads the value and applies its decision rules.
Analogue-to-digital converter (ADC): hardware that converts a continuously varying electrical signal into digital values.

Not every sensor interface is identical. Some modern sensor modules perform conversion internally and send digital readings directly. In either case, the processor ultimately receives a digital value or state that it can process.

Common mistake

A sensor and an ADC are not the same component. The sensor responds to the physical condition; the ADC converts an analogue electrical signal into digital data.

Actuators: turning decisions into physical action

An actuator receives an output signal from the controller and causes a change in the physical system. It is the part that allows software decisions to affect the real world.

Actuator Physical effect Possible control use
Motor Produces rotary or linear movement Moves a ventilation panel or conveyor gate.
Valve Changes the flow of a liquid or gas Reduces pressure in a pipe or releases water.
Heater Raises temperature Warms an equipment enclosure when the temperature is too low.
Fan or cooling unit Moves air or removes heat Cools a storage cabinet when the temperature is too high.
Speaker or buzzer Creates sound Produces an automatic warning when an unsafe condition is detected.
Light Produces visible output Shows system status or illuminates an area after infrared detection.
Actuator: an output component that converts a control signal into a physical action.

Exam tip

Do not stop at naming the actuator. Explain its effect: “the controller activates a motor, which closes the valve”.

Feedback closes the control loop

A single command does not prove that the environment has changed correctly. A fan may be blocked, a valve may open only partly, or the surrounding conditions may change again. The system therefore takes another sensor reading after acting.

Feedback: a new measurement returned to the controller so it can judge the effect of the previous action and decide what to do next.
  1. A sensor measures the current condition.
  2. The processor compares the reading with a target value or acceptable range.
  3. If action is needed, the processor sends a control signal.
  4. An actuator changes the physical environment.
  5. The sensor measures the changed condition.
  6. The processor repeats the comparison and adjusts or stops the actuator.
Without feedback With feedback
The controller assumes that its command worked. The controller measures the result of the command.
The actuator may continue for too long or stop too early. The actuator can be adjusted or stopped when the target is reached.
Changes in the environment may be missed. Repeated readings allow the system to respond to new conditions.

Common mistake

Feedback is not merely a message shown to a user. In a control loop, feedback is the new sensor data used by the controller to evaluate the physical result.

Worked example: protecting a medicine cabinet

A refrigerated medicine cabinet should remain between 3.0°C and 5.0°C. A temperature sensor is read every ten seconds.

Reading Controller decision Actuator action Feedback meaning
5.8°C Temperature is above the safe range. Switch the cooling unit on. The next reading will show whether cooling is effective.
4.6°C Temperature has returned to the safe range. Switch the cooling unit off. The action has achieved the target for now.
2.7°C Temperature is below the safe range. Keep cooling off and activate a warning. Further readings show whether the cabinet recovers or needs attention.

Possible control logic

READ Temperature

IF Temperature > 5.0 THEN
    CoolingUnit ← ON
ELSE
    CoolingUnit ← OFF
ENDIF

IF Temperature < 3.0 THEN
    WarningBuzzer ← ON
ELSE
    WarningBuzzer ← OFF
ENDIF

The algorithm is repeated. This repetition turns a one-off reading into a feedback-based control process.

Exam tip

In a scenario answer, identify: sensor → comparison → condition → actuator → new reading.

Interactive bridge: sound sensor and sound actuator

The existing sound widget has been retained because it demonstrates both sides of a physical computing system. In microphone input mode, it shows how a sound sensor can create digital data. In speaker output mode, it shows how an actuator can turn a control signal into a physical sound.

〰 Sound wave Physical vibration in air
→
◌ Diaphragm Moves with the sound
→
∿ Analogue signal Changing electrical signal
→
ADC Analogue → Digital Creates data for processing
→
101 Digital data Used by the controller

Sound sensor input

Sound moves the diaphragm and creates an analogue electrical signal. An ADC converts the signal into digital values that the monitoring or control program can process.

Important distinction

This widget illustrates signal conversion. A complete control system still needs a decision rule and feedback from a later sensor reading.

Designing a monitoring or control system

Begin with the required outcome rather than choosing hardware at random.

Design question Example answer for an archive room
What condition must be observed? Entry into the room outside authorised hours.
Which sensor measures or detects it? An infrared movement sensor.
Is the system monitoring or controlling? It is monitoring if it only logs and reports movement.
What would make it a control system? Automatically locking an internal gate or switching on security lighting.
Which actuator is required? A lock solenoid, motor, light, or warning buzzer.
What feedback could confirm the result? A second sensor reports that the gate is closed or the light has switched on.

Design-answer structure

Write: condition → suitable sensor → data processing → decision → actuator → feedback.

Exam support

Monitoring or control?

Look for automatic physical action. Collecting, storing, displaying, or warning is monitoring. Operating an actuator to alter the environment is control.

Explaining feedback

State that the sensor takes a new reading after the actuator acts. The controller uses this new value to decide whether to continue, change, or stop the action.

Using precise vocabulary

Prefer sensor reading, target value, comparison, control signal, actuator, physical change, and feedback over vague phrases such as “the computer checks it and fixes it”.

Common mistakes and misconceptions

  • Calling every sensor-based system a control system.
  • Using “sensor” and “actuator” as interchangeable terms.
  • Forgetting that some analogue sensor signals require conversion.
  • Describing feedback as a message to the user instead of a new measurement.
  • Naming an actuator without explaining the physical change it causes.
  • Giving a single reading and action instead of describing the repeated loop.

Practice

Monitoring or control?

  1. A sound sensor records noise levels near a construction site and uploads a daily graph.
  2. An infrared sensor detects movement and automatically switches on a light.
  3. A pressure sensor sends an alert to an engineer when a pipe exceeds its safe limit.
  4. A pressure sensor causes a valve to open automatically until the pressure falls.

Classify each system and justify your decision.

Original system-design task

A conservation centre needs to protect a temperature-sensitive insect habitat.

  1. Identify a suitable sensor.
  2. Describe how the reading reaches the processor.
  3. Choose an actuator that could lower the temperature.
  4. Explain how feedback would prevent continuous unnecessary cooling.

Explain the complete loop

An infrared sensor controls an automatic door. Describe the full process from detecting a person to confirming that the door has closed again.

Evaluate a claim

A student writes: “The temperature sensor cools the cabinet when it becomes too hot.” Explain what is technically incorrect and rewrite the statement accurately.

Review

Concept Key idea
Monitoring Collects and reports data without necessarily changing the environment automatically.
Control Uses processed sensor data to operate an actuator automatically.
Sensor Responds to a physical condition and produces a corresponding signal.
Actuator Converts a control signal into physical action.
Feedback A new reading used to judge the result of the previous action.
Control loop Measure, compare, act, measure again, and adjust.