Feedback Loops and Closed-Loop Control — Automated Systems | IGCSE Computer Science

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 3% of your exam marks.

stable

Rare

Stable3%

Sensor to microprocessor to actuator control loop questions. Usually 3 to 4 marks.

An automated system that just keeps running forever is only useful if it can respond to changes in the environment. The key idea that makes this possible is the .

A feedback loop is the cycle by which the output of a system is fed back as an input, so that the system can compare its current state with the target state and adjust accordingly.

In an automated control system, the loop usually looks like this:

The sensor measures the current state of the environment.
The microprocessor compares the reading to the target.
The microprocessor adjusts the actuator to push the environment back towards the target.
The actuator's action changes the environment.
The sensor takes a fresh reading, and the cycle repeats.

The output of the actuator changes the very thing that the sensor is measuring; that change feeds back into the next reading. This makes the system self-correcting.

Open-loop vs closed-loop control

A common distinction:

	Open-loop control	Closed-loop control
Does the system use a feedback loop?	No	Yes
How does it decide what to do?	Follows a fixed sequence, regardless of what is actually happening	Continually checks the result and adjusts
Reacts to changes in the environment?	No

Almost every automated system in the syllabus is closed-loop, because the whole point of automation is that the machine adapts to its surroundings.

Exam tip

Describing the role of the microprocessor

What comes up: "Describe the role of the microprocessor in this automated system" (typically 3 marks). It is not enough to say "it processes data."

Write (three marks): Give all three steps in the microprocessor's job: (1) It receives data from the sensor. (2) It checks whether the data is within or outside a set range (compares to a stored threshold/target value). (3) Based on the result of that comparison, it sends signals to trigger the appropriate action (activates or deactivates the actuator).

Watch out: Only stating "it processes the data" earns zero marks. You must include the comparison step and the output signal step to collect all three mark points.