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Section 1: What is a System?

Engineers and designers are in the business of solving problems. Whether designing a new bridge, a home electronic device, or a satellite communications system, engineers use a common way of thinking called the systems approach.

A system is a collection of components that work together to perform a function. All systems — no matter how simple or complex — can be described using the same basic structure:

This is called the universal system diagram. The word "universal" means it can be used for any situation at all.

• Input — what goes into the system to make it work (e.g. electricity, a signal, a physical force)
• Process — what the system does to the input
• Output — what comes out of the system (desired outputs and unwanted outputs)

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Section 2: Sub-Systems Diagrams

The universal system diagram tells us what goes in and what comes out — but not how it works inside. The next step is to divide the process box into its sub-systems, showing how each part of the system interacts.

Key terms

• System boundary — a dashed line enclosing all the sub-systems. Real-world inputs and outputs are outside the boundary.
• Input device (sensor) — converts a real-world input into an electrical signal. Examples: thermistor, microphone, switch, light sensor.
• Output device (actuator) — converts an electrical signal into a real-world output. Examples: motor, loudspeaker, LED, heater.
• Output driver — an electronic component that boosts a low-power control signal from the control unit to provide the higher current or voltage required to operate an output device such as a motor or heating element. Every output device needs a driver (e.g. transistor, relay).

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Section 3: Open-Loop and Closed-Loop Control

Any system must have some way to control its operation. Systems can also be described as open-loop or closed-loop.

Open-loop control

In an open-loop system, the output simply switches on or off when a certain input happens. There is no feedback — the system does not monitor its output. Once activated, it stays on until switched off.

Closed-loop control

In a closed-loop system, the output is continually monitored and fed back to adjust the process. This makes the system far more accurate. The feedback signal is compared to the desired value and the system corrects itself automatically.

Open-loop vs closed-loop: summary

Type	Feedback?	Example
Open-loop	No	Street light activated by a light sensor — switches on when dark, no monitoring of output
Closed-loop	Yes — sensor monitors the output	Car cruise control, central heating thermostat, oven temperature control

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Section 4: Case Study — Domestic Appliances

Many everyday appliances use systems principles. The washing machine is a good example of a system with multiple levels of control complexity.

Washing machine

A washing machine is a complex system with both open and closed-loop elements. The wash programme (timer, spin speed) is essentially open-loop — it runs a fixed sequence regardless of outcome. However, the water heating uses closed-loop control — a temperature sensor monitors the water temperature and feeds back to the control unit, which switches the heater on or off to maintain the correct washing temperature.

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Section 5: Case Study — The Furby Electronic Toy

The Furby is a good example of a mechatronic system — one that uses an electronic circuit to control a number of mechanisms. It contains several sensors so it can react to changes in its environment (e.g. being placed in a dark room or turned upside down).