How do we describe an engineered system using inputs, processes and outputs?
The input-process-output model, the role of feedback in a control system, and how systems are drawn as block diagrams.
A focused answer to AQA GCSE Engineering on the input-process-output (IPO) model, open and closed loop systems, the role of feedback, and drawing systems as block diagrams.
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What this dot point is asking
AQA wants you to describe any engineered system using the input-process-output model, explain the role of feedback, tell open from closed loop systems, and draw a system as a block diagram. The most examined idea is feedback and how a closed loop corrects itself.
The input-process-output model
For an automatic door: the input is a person detected by a sensor, the process is the controller deciding to open, and the output is the motor moving the door. Breaking a product into these three parts is the first step in analysing or designing it, because it forces you to identify what is sensed, what decision is made, and what happens.
Feedback and loops
The key idea is the comparison: in a closed loop the actual output is compared with the desired value, and the difference (the error) drives the output until the error is removed. An open loop simply assumes the output is correct, so it is simpler and cheaper but cannot cope with disturbances. A washing machine on a fixed timed cycle is largely open loop for its sequence of steps, but the water heater inside it is closed loop, because a temperature sensor feeds back to hold the water at the set value. This shows that one product often contains both kinds of loop: open loop where a fixed sequence is acceptable, and closed loop where a quantity must be held steady despite changing conditions.
Block diagrams
A system is drawn as labelled boxes joined by arrows showing the flow of signals. The input box leads to the process box and then the output box; in a closed loop a feedback arrow runs from the output (often via a sensor) back to the process, where it is compared with the target. The arrows show the direction the signal travels, which makes the structure of the system clear at a glance.
Try this
Q1. Name the three parts of the IPO model. [3 marks]
- Cue. Input, process, output.
Q2. State what feedback does in a closed loop system. [1 mark]
- Cue. It sends output information back to the input so the system self-corrects.
Exam-style practice questions
Practice questions written in the style of AQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
AQA 20194 marksExplain how feedback allows a thermostat-controlled heater to keep a room at a set temperature.Show worked answer →
A good answer describes the closed loop and the role of feedback.
The room temperature is the input that is sensed; the thermostat processes it by comparing it with the set temperature; the heater is the output.
Feedback means the sensor constantly measures the actual room temperature and feeds it back to the thermostat. If the room is below the set value the heater turns on; once the set value is reached the thermostat turns the heater off. This is a closed loop system, so the output (heat) affects the input (sensed temperature), keeping the room near the target.
Markers reward identifying the closed loop and explaining that feedback compares the actual value with the target and adjusts the output.
AQA 20226 marksCompare an open loop system and a closed loop system, and describe how each would control the speed of an electric fan. Recommend which is better for keeping a steady speed.Show worked answer →
A good answer contrasts the two and applies them to the fan, then judges.
An open loop system has no feedback: the controller sets the fan voltage and assumes a speed, but does not measure it. If the load or supply changes, the speed drifts and the system cannot correct itself.
A closed loop system measures the actual fan speed (for example with a tachometer), feeds it back, compares it with the target, and adjusts the voltage to remove the error. If the fan slows under load, the controller increases the drive automatically.
Recommendation: for a steady speed, the closed loop system is better because feedback corrects disturbances; the open loop is simpler and cheaper but cannot hold the speed if conditions change. Markers reward the no-feedback versus feedback contrast, the fan application, and a justified recommendation.
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Sources & how we know this
- AQA GCSE Engineering (8852) specification — AQA (2017)