Monitoring and control
Learn the core concepts and decision-making in Cambridge 9626 Topic 3, then practise recall, application and transfer with fictional scenarios.
Exam scope and assessment notes
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Before you begin
- Check whether your route is AS Level (Topics 1–11) or full A Level (Topics 1–21).
Work on paper. Try each question before revealing help, and explain why your method works.
Quick readiness check
Before Topic 3: monitoring and control
What you will learn
- Select sensors for environmental and patient measurement, including light/UV, temperature, pressure, humidity, pH, gases, sound, infrared, magnetic field, proximity and touch
- Explain one-, two- and multi-point calibration
- Explain actuators and microprocessor control in greenhouses, heating, air conditioning, alarms, traffic, barriers, sensor networks and smart homes
- Trace sensor input, processing, decision, actuator output and feedback
A sensor turns a condition into data
Different sensors detect different physical properties: a thermistor measures temperature, a pH probe acidity, a humidity sensor moisture in air, an infrared sensor emitted heat or movement, and an ultrasonic sensor distance. A monitoring system records or displays readings; a control system uses readings to change something. Select the sensor based on range, precision, location and response time.
Calibration and feedback make control dependable
Calibration compares a device with a known reference. One point adjusts an offset at one known value; two points can correct offset and scale across a range; multipoint calibration checks non-linearity or more complex response. A controller compares input with a target and directs an actuator. Feedback lets later measurements show whether the action achieved the target; poor calibration, lag or faulty actuators can destabilise control.
Sources for this lesson
Pause and explain
Which approach best demonstrates secure, reliable IT practice in monitoring and control?
Worked example
Design the logic for a greenhouse that opens a vent when temperature is too high and closes it after cooling.
Show the worked solution
- Read and calibrate a temperature sensor.
- Compare the measurement with upper and lower thresholds.
- Use a motor actuator and re-read to confirm the temperature response.
Answer The controller repeatedly reads the calibrated temperature. Above the upper threshold it drives the vent motor open; below a lower threshold it closes it; between them it holds the current state. Using two thresholds avoids rapid switching near one boundary. The system should detect sensor failure and provide a safe manual override.
Try a topic test
Choose Easy, Normal or Difficult. Save your answers and open worked review after finishing.
Test this topic →From guided practice to a new situation
Write your answer and reasoning first. Use a hint only if you are stuck. The model solution is for self-checking; your written work is not automatically marked.
A patient monitor must detect oxygen concentration and alert staff if it is outside limits. Outline suitable system stages.
Give me a hint
Trace measurement to a human-readable warning and avoid treating an alert as a diagnosis.
Compare my reasoning
- Select and calibrate an oxygen sensor.
- Sample and compare readings against clinically set limits.
- Display/alert and record the reading; define failure handling.
A calibrated oxygen sensor supplies readings to a processor, which checks the range and persistence against clinician-configured thresholds. It can display the measured value and trigger an audible/visual alert for staff. Sensor errors, communication loss and false alarms need clear detection. The monitor flags a reading; it does not independently diagnose the patient.
Look for these in your work
- Links sensor and processor
- Explains threshold action
- Includes human and failure handling
A moisture sensor reads 35% when dry and 48% when a reference shows 50%. What could two-point calibration improve?
Give me a hint
Two points help detect both offset and scale differences.
Compare my reasoning
- Record the known dry and wet reference points.
- Compare device output with both references.
- Adjust the response and retest at an intermediate value.
A two-point calibration can correct both an offset and an incorrect scale between the dry and wet reference values. A third intermediate reference can reveal non-linearity that two points miss. Calibration values and dates should be recorded so later readings can be interpreted.
Look for these in your work
- Uses known references
- Explains offset and scale
- Mentions retesting/recording
A car-park barrier must recognise an arriving vehicle, lift, then lower after it passes. Select sensors and an actuator.
Give me a hint
Use one sensor to detect presence and another signal to know the vehicle has cleared the barrier.
Compare my reasoning
- Use an inductive loop or proximity sensor to detect a vehicle.
- Use a second loop/beam or safety sensor to detect passage and clearance.
- Use a motor actuator with obstruction checks and safe control logic.
An inductive loop embedded in the approach can detect a vehicle; a second loop or safety beam after the barrier can indicate when it has cleared. The controller checks access permission, commands a rotary electric motor to lift, and lowers only when the danger zone is clear. A safety sensor must prevent the barrier closing on an obstruction.
Look for these in your work
- Matches sensors to detection
- Names an actuator
- Includes a safety interlock
Common mistakes
- Calling an actuator a sensor
- Ignoring calibration error
- Using a single threshold that causes rapid switching
What can you explain now?
A patient monitor must detect oxygen concentration and alert staff if it is outside limits. Outline suitable system stages.
Compare with the explanation
A calibrated oxygen sensor supplies readings to a processor, which checks the range and persistence against clinician-configured thresholds. It can display the measured value and trigger an audible/visual alert for staff. Sensor errors, communication loss and false alarms need clear detection. The monitor flags a reading; it does not independently diagnose the patient.
Trace measurement to a human-readable warning and avoid treating an alert as a diagnosis.
After trying it yourself, choose your next review. This is your self-assessment.
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Make it stick
What does calibration establish before a sensor is used in a control loop?
If you needed the solution, close it and solve the problem again from a blank page. A correct answer is stronger when you can explain the reason for each step.