How to use it. Choose a greenhouse or a central-heating system, and Control or Monitoring, in the title bar. Press Run and watch one reading go round the loop: the box lit in the diagram is the one working, and the box lit in the flowchart is the step of the program it's on. Move the outside weather slider and the set point, and watch the graph. Step goes one stage at a time.
The parts
- A sensor is an input device: it measures a physical quantity (temperature, humidity, light) and sends a signal. It does not decide anything and does not change anything.
- Most sensors give an analogue signal, so an analogue-to-digital converter (ADC) turns it into a binary number the microprocessor can process.
- The microprocessor (or computer) runs the control program: it compares each reading with the set point (the value the user wants, stored in memory) and decides what to do.
- An actuator is an output device that makes something physically happen: a heater, a motor opening a window, a fan, a sprinkler's valve, lights. An actuator that needs an analogue signal (a motor whose speed or position varies) is driven through a digital-to-analogue converter (DAC); one that is just switched on or off (a heater, through a relay) needs no DAC.
Control and monitoring
In a monitoring system the computer reads the sensors, stores the readings and warns people (a message or an alarm) when a reading is out of range, but it changes nothing: a hospital patient monitor, a weather station, pollution monitoring. In a control system it also drives actuators, and what they do changes the next readings: this is feedback. The loop never stops: read, compare, act, wait, read again.
Why it's real-time
A control system is a real-time system: it must respond to each reading straight away, before conditions change, so that the output affects the very next input. A greenhouse heater switched on an hour late is no use. Readings are taken at a set interval (here, once a simulated minute), fast enough for what is being controlled.
Common exam mistakes: saying the sensor "controls" the heater (the microprocessor does); saying the sensor only sends data "when the temperature changes" (it is read continuously, at intervals); forgetting the ADC or putting it on the output side; describing an actuator as an input; describing monitoring as if it switched things on; and leaving out the loop back ("the process repeats").
Objective: Cambridge International AS & A Level Information Technology (9626), monitoring and control systems: sensors, actuators, ADC and DAC, set points, feedback, real-time processing and control flowcharts. Also useful for IGCSE ICT (0417) measurement and control.
Where this fits
- Cambridge: Cambridge IGCSE Information and Communication Technology (0417); Cambridge IGCSE Computer Science (0478); Cambridge A Level Computer Science (9618); Cambridge AS Level Computer Science (9618); Cambridge A Level Information Technology (9626); Cambridge AS Level Information Technology (9626) Goes beyond Cambridge IGCSE Information and Communication Technology (0417): 0417 has sensors, actuators and ADC; the feedback loop, set point and control flowchart go further.
- NCEA Level 2 Digital Technologies: 91894 Use advanced techniques to develop an electronics outcome; 91894 Use advanced techniques to develop an electronics outcome
- NCEA Level 3 Digital Technologies: 91904 Use complex techniques to develop an electronics outcome; 91904 Use complex techniques to develop an electronics outcome
