The systems approach & control

Input, process, output

Any system can be shown as a universal system diagram: input → process → output. A sub-system diagram breaks the process into blocks (e.g. sensor → control → driver → motor), with arrows showing the signal or energy passing between them.

In exam diagrams use the general words input, process and output where asked. The 2025 report noted that specific labels such as "user input" or "output energy" scored nothing on a universal system diagram.

Finding inputs and outputs

To fill in a universal system diagram, ask two questions:

• Input: what do I have to do to make it work? (e.g. switch it on, add fuel, press a button)
• Output: what does it do? (e.g. hot water, rotary movement, light)

You can also show the energy going in and out: a kettle turns electrical energy into heat (thermal) energy; a car turns chemical energy into kinetic energy.

Sub-system diagrams, boundary and driver

A sub-system diagram breaks the process into parts. Draw a dashed box round it: this is the system boundary. Anything inside is part of the system; anything outside is the outside world (the user, the room).

Most Engineering Science systems follow the same pattern:

• Control unit: usually an electronic circuit that makes decisions.
• Driver: amplifies the small signal from the control unit so it can power the output (e.g. a transistor or relay). A driver is always needed before a motor, lamp or heater.
• Output: usually a motor, lamp or heater.
• Feedback sensor (closed loop only): usually a light, temperature or speed sensor.

Always draw proper arrowheads to show the direction of each signal.

Open and closed loop control

• Open loop: no feedback. The system does what it is set to do and doesn't check the result, e.g. a toaster on a timer.
• Closed loop: a sensor measures the actual output and feeds it back; the controller compares the actual level with the set (desired) level and adjusts the output, e.g. a thermostat-controlled heater.

When describing a closed-loop system, include all three: the sensor measuring the actual output and feeding back; the control sub-system comparing set and actual; and the specific action on the output ("the heater switches on when the temperature is below the set level and off when above"). Write "closed loop control", not just "closed".

Describing a closed loop system: four steps

1. Say whether it is open or closed loop (most exam systems are automatic and closed loop).
2. The user sets the required value (the set value, or reference value), e.g. 21 °C, and the system starts producing its output.
3. The feedback sensor measures the actual value and feeds it back to the control unit.
4. The control unit compares the actual value with the set value and adjusts the output to match, e.g. switches the heater off when 21 °C is reached.

Name the real sensor and output in the question: "the thermistor measures the room temperature", not just "the sensor". A speedometer only displays speed; a speed sensor measures it.

Design challenge: automatic greenhouse vent

The brief. A garden centre in Perthshire wants roof vents that open when the greenhouse gets too hot and close when it cools.

1. Draw a universal system diagram for the vent system.
2. Draw a closed-loop control diagram, labelling the sensor, controller, actuator and feedback.
3. Describe how the system keeps the temperature near 25 °C.
4. Suggest one reason a closed-loop system is better here than a timer.

Feedback, error and types of control HIGHER

• Negative feedback: the feedback signal is subtracted from the set value at a comparator (summing point) to give an error signal. The controller acts to reduce the error to zero.
• Two-state (on/off) control: the output is either fully on or fully off. Simple and low cost, but the output overshoots and oscillates about the set point, giving uneven motion or temperature.
• Proportional control: the output is proportional to the size of the error, so the system slows as it approaches the set point, giving a smoother response with less overshoot.
• A response graph for proportional control shows a quick initial response, the error reducing over time, and the system settling at (or near) the set point.

Control diagrams may include a gain: e.g. output = gain × (set − feedback). Rearrange to find the feedback voltage: 8.0 = 4 × (3.2 − V_f) gives V_f = 1.2 V.

Positive feedback and gain HIGHER

• Positive feedback adds to the error instead of reducing it, so the output runs away, e.g. the howl when a microphone is held near its speaker. Control systems use negative feedback.
• Two-state (comparator) control has a time lag: the heater stays hot after switching off, so the temperature overshoots and then undershoots the set level.
• Proportional control uses a difference amplifier: a big error gives a big output and a small error a small one, so the output eases off as it approaches the set level.
• The gain matters: low gain responds slowly; high gain responds quickly but overshoots before settling. Engineers adjust it for a fast response with little overshoot.

Key terms

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