Most processors are not in PCs at all: they are hidden inside cars, ovens, thermostats and traffic lights. Learn what makes a system embedded, weigh up its benefits and drawbacks, and see how sensors, actuators and feedback let a computer monitor or control the real world.
An embedded system is a computer system, with its own microprocessor or microcontroller, memory and input/output, built into a larger device to perform one dedicated function. A monitoring system uses sensors to measure physical quantities and records, displays or warns, but does not change the conditions. A control system also uses actuators to change the conditions. Analogue sensor readings pass through an ADC; the processor compares each reading with a pre-set value. In a control system the output affects the next input: this feedback lets the system keep conditions within limits automatically.
Fig. 1: a closed feedback loop in a heating control system
Section 01
What makes a system embedded.
Click the devices in the lab to see the embedded system inside each. Notice that each one does one job, and the user cannot install new programs on it.
Embedded system: a microprocessor (often a microcontroller with its memory and I/O on one chip) built into a larger device to carry out a specific, dedicated task; its program is usually firmware stored in ROM or flash.
Examples: washing machine, microwave oven, car engine management and anti-lock brakes, central heating controller, set-top box, traffic lights, smart watch, factory robot.
Many embedded systems are real-time: they must respond to sensor input within a fixed time.
Benefits
Drawbacks
Small, so it fits inside the device
Difficult or impossible to upgrade; often the whole unit is replaced
Cheap to mass-produce
Faults can be hard to diagnose and need a specialist to repair
Low power consumption, so it can run on batteries
If connected to the internet, it can be hacked (for example a smart camera or thermostat)
Fast and reliable, as it runs one dedicated program
Cannot be reprogrammed for other tasks
Can be controlled remotely if networked
Encourages a throw-away culture when units cannot be repaired
Examiner trap
A laptop is not an embedded system: it is general purpose. The test is whether the computer is part of a larger device and does one dedicated job.
Section 02
Monitoring against control.
Both kinds of system read sensors continuously. The difference is what the processor does with the readings.
Monitoring system
Control system
What it does
Measures, records, displays or analyses; may sound a warning
Measures and then changes the conditions using actuators
Changes the environment?
No
Yes
Examples
Patient monitoring in hospital, weather station, pollution monitoring, intruder alarm
Central heating, greenhouse, traffic lights, automatic doors, chemical process control
Sensor
Measures
Example use
Temperature
How hot or cold
Central heating, greenhouse, patient monitoring
Pressure
Force on a surface, or gas/liquid pressure
Car detected at traffic lights, intruder on a mat, tyre pressure
Infra-red
Infra-red radiation, or a broken infra-red beam
Detecting people at automatic doors, intruder alarms
Actuator: a device that turns an electrical signal from the processor into physical movement or action, such as a motor, pump, valve or a relay switching a heater or light on and off.
Most sensors give an analogue signal, so an ADC converts it to digital before the processor can use it. Some actuators need a DAC.
Your turn
A hospital system reads a patient's heart rate and temperature every second and sounds an alarm if either goes outside a range. Is it monitoring or control? Explain.
Monitoring: it measures, displays and warns, but it does not act to change the patient's heart rate or temperature. A nurse makes any change.
Examiner trap
A sensor does not decide anything. Sensors only send readings; it is the processor that compares the reading with the stored value and decides whether an actuator is switched.
Stage 1 · Learn
Check what the sim just showed you
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Which is an embedded system?
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In the lab, what is a typical drawback of an embedded system?
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What is the key difference between a monitoring system and a control system?
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Why does a sensor's output usually pass through an ADC?
Section 03
Feedback: closing the loop.
In a control system, what the actuators do changes the next sensor reading. Feedback is what makes a control system automatic and stable.
Feedback: the output of the system affects its next input. The processor keeps measuring, so it can see the effect of its own actions and adjust them.
Why it matters: without feedback, a heater switched on would never be switched off. With feedback, conditions stay within set limits automatically, without human intervention, and the system responds to changes such as a door being opened.
The loop runs continuously: read sensor, convert, compare with pre-set values, signal actuators, repeat.
CONSTANT LowLimit = 20.5
CONSTANT HighLimit = 21.5
DECLARE Temp : REAL
WHILE TRUE
Temp ← ReadTemperature() // reading from the sensor, via the ADC
IF Temp < LowLimit THEN
CALL HeaterOn() // actuator: relay switches the heater on
ELSE
IF Temp > HighLimit THEN
CALL HeaterOff()
ENDIF
ENDIF
ENDWHILE
Worked example Describe a control system
Describe how a microprocessor keeps the soil in a greenhouse moist. [5]
A moisture sensor continuously measures the moisture level in the soil.
The analogue reading is converted to digital by an ADC and sent to the microprocessor.
The microprocessor compares the reading with a pre-set value stored in memory.
If the reading is below the pre-set value, it sends a signal to an actuator to open the water valve; if it is at or above the value, the valve is closed.
This repeats continuously, so the effect of watering is fed back as new readings (feedback).
Answer: Sensor, ADC, compare with stored value, actuator, continuous loop
Examiner trap
"The sensor turns the heater on" earns nothing. Name the chain: sensor sends reading, ADC, processor compares with pre-set value, processor signals actuator.
Stage 2 · Exam
Exam-style questions
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What is meant by feedback in a control system?
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Which sensor is most suitable for detecting a car waiting at traffic lights?
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Which is a benefit of an embedded system?
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An intruder alarm sounds a siren when an infra-red sensor detects movement. Why is it usually classed as monitoring rather than control?
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