How do engineers model any device as a system, and what is the difference between open-loop and closed-loop control?
The universal system model of input, process and output, the use of block diagrams to represent systems and sub-systems, and the difference between open-loop and closed-loop control with feedback.
An SQA Higher Engineering Science answer on the universal system model of input, process and output, representing systems with block diagrams, and the difference between open-loop and closed-loop control using feedback.
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What this key area is asking
The SQA wants you to model any engineered device as a system with an input, a process and an output, to draw and read block diagrams of systems and their sub-systems, and to tell apart open-loop control (no feedback) from closed-loop control (with feedback). This is the organising idea behind the whole course: the electronics area is full of input, process and output sub-systems, and recognising the structure tells you what each block does.
The universal system model
The power of the model is that it fits almost any engineered device, so you can analyse something unfamiliar by asking what each block does. In an automatic light: the input is a light sensor, the process is a switching circuit that decides when it is dark enough, and the output is the lamp. The same three-part structure underlies every electronic control circuit later in this course.
Block diagrams and sub-systems
A block diagram shows the system as labelled boxes joined by arrows that represent the signals. It hides the internal detail of each block so you can reason about the whole system. Real systems are built from sub-systems: an input sub-system (for example a potential divider with a sensor), a process sub-system (for example a comparator or logic), and an output sub-system (for example a transistor switching a motor). The arrows between them are the interfaces, and matching one block's output to the next block's input is a recurring design task.
Open-loop versus closed-loop control
The crucial distinction in control is whether the system checks its own output.
- Open-loop control sends the input through the process to the output and stops. It has no feedback, so it cannot tell whether the output is correct, and it cannot correct for disturbances. It is simple and cheap.
- Closed-loop control measures the actual output, feeds it back, and compares it with the desired value (the set point). The process acts on the error (the difference), so the system automatically corrects itself. It is more complex but accurate and self-correcting.
Feedback
The feature that makes a loop "closed" is feedback: a path that returns a measurement of the output to the input side. The controller then works on the error between the desired and the actual value. Most feedback in this course is negative feedback, meaning it acts to reduce the error and so stabilises the system at the set point. Recognising the feedback path on a block diagram is usually the quickest way to label a system as closed-loop.
Examples in context
A cruise control in a car is a textbook closed loop: the desired speed is the set point, a sensor measures the actual speed, and the controller adjusts the throttle to close the gap, so the car holds speed up and down hills. A basic sprinkler timer is open loop: it waters for a set time whether or not the soil is already wet, because it never measures soil moisture. Swapping the timer for a moisture sensor with feedback would turn it into a closed-loop system that waters only when needed, which is both the control idea and a sustainability improvement.
Try this
Q1. Name the three blocks of the universal system model in order. [1 mark]
- Cue. Input, process, output.
Q2. State the one feature that makes a control system closed-loop rather than open-loop. [1 mark]
- Cue. Feedback: the output is measured and returned to be compared with the desired value.
Q3. Give one advantage and one disadvantage of open-loop control. [2 marks]
- Cue. Advantage: simpler and cheaper. Disadvantage: cannot sense or correct for disturbances or errors in the output.
Exam-style practice questions
Practice questions written in the style of SQA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
SQA Higher (specimen)3 marksAn automatic greenhouse heater keeps the temperature at a set value. Draw or describe the system as input, process and output blocks, and name the quantity that is fed back.Show worked answer →
The system splits into three blocks with a feedback path.
Input: a temperature sensor (and a set-point) that measures the greenhouse temperature.
Process: a control circuit (comparator or controller) that compares the measured temperature with the set value and decides whether to switch the heater.
Output: the heater, which warms the greenhouse.
Feedback: the greenhouse temperature is sensed and returned to the input, so the controller acts on the difference between the actual and the set temperature.
Markers reward three correctly labelled blocks in the order input then process then output, and naming temperature as the quantity fed back from the output side to the input.
SQA Higher (specimen)4 marksExplain the difference between an open-loop and a closed-loop control system, and give one advantage of closed-loop control. Illustrate with a named example of each.Show worked answer →
Open loop: the system acts on its input only and does not measure its output, so it cannot correct for disturbances. Example: a basic toaster runs for a set time regardless of how brown the toast actually is.
Closed loop: the output is measured and fed back to be compared with the desired value, so the system corrects any error automatically. Example: a thermostat-controlled oven measures its temperature and switches the element to hold the set value.
Advantage of closed loop: it automatically corrects for disturbances and changing conditions, so it holds the desired output accurately even when the load or surroundings change.
Markers reward a clear contrast (no feedback versus feedback), one valid example of each, and a correct advantage of closed-loop control.
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Sources & how we know this
- SQA Higher Engineering Science Course Specification — SQA (2019)
- Higher Engineering Science Course Specification (PDF) — SQA (2019)