How do control systems use feedback to hold an output at a desired value, and what is the difference between open and closed loop?
Control systems described by the system model, the difference between open-loop and closed-loop control, and the role of feedback, the error signal and the components of a closed-loop system.
An SQA Advanced Higher Engineering Science answer on control systems, covering the system model, open-loop versus closed-loop control, negative feedback and the error signal, and the comparator, controller, actuator and sensor of a closed-loop system.
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What this key area is asking
The SQA wants you to describe a control system with the system model (input, process, output), to distinguish open-loop from closed-loop control, and to explain how feedback and the error signal let a closed-loop system hold its output at the desired value. You must name the parts of a closed-loop system, the comparator, controller, actuator and sensor, and say what each does.
The system model and the set point
Many systems are described first by the universal system model: input, process, output, with feedback added for closed-loop control. The set point is the value the system is asked to achieve, set by a dial, a program or another circuit, and the whole design is judged on how closely and how steadily the output follows it.
Open-loop versus closed-loop control
An open-loop system, such as a toaster on a timer or a simple sprinkler, is cheap and simple because it needs no sensor, but it cannot react to anything unexpected: if the bread is frozen, the toaster still runs for the same time. A closed-loop system, such as a thermostatically controlled oven or a cruise control, is more complex but far more accurate, because it continually feeds the output back and works to drive the error to zero.
Feedback and the error signal
The feedback is negative: the measured output is subtracted from the set point, so the controller always acts to reduce the error. As the output approaches the set point the error shrinks, the correcting action eases off, and the system settles. This is the same principle as the negative feedback that stabilises an op-amp, applied to a whole physical system.
Components of a closed-loop system
A closed-loop control system has four functional parts arranged in a loop:
- Comparator - subtracts the fed-back measurement from the set point to form the error signal.
- Controller - decides the correcting action from the error (it may simply switch, or it may vary the drive in proportion to the error).
- Actuator - the device that changes the output, such as a heater, motor or valve.
- Sensor (feedback element) - measures the actual output and feeds it back to the comparator.
Examples in context
A room thermostat is closed-loop: it measures temperature, compares it with the dial setting, and switches the heater to drive the error to zero. Cruise control measures road speed and adjusts the throttle to hold the set speed up hills and down. A servo positions a control surface by comparing the commanded angle with a measured angle. By contrast, a microwave on a timer is open-loop, running for a set time regardless of how hot the food actually gets. Recognising whether the output is measured and fed back is what classifies the system.
Try this
Q1. State the relationship for the error signal in a closed-loop system. [1 mark]
- Cue. .
Q2. State the key feature that distinguishes closed-loop from open-loop control. [1 mark]
- Cue. The output is measured and fed back to be compared with the set point.
Q3. Name the part of a closed-loop system that measures the output. [1 mark]
- Cue. The sensor (feedback element).
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 AH style4 marksA heating system holds a room at using a thermostat. The measured temperature is . State whether this is open-loop or closed-loop control, calculate the error signal, and state what the controller does in response.Show worked answer →
Because the output (room temperature) is measured and fed back, it is closed-loop control.
The error signal is the difference between the set value and the measured value.
Relationship: .
Substitution: .
Response: the positive error tells the controller the room is too cold, so it switches the heater on to raise the temperature towards the set value.
Markers reward identifying closed-loop, the error of , and the correct response of turning the heater on to reduce the error.
SQA AH style4 marksCompare open-loop and closed-loop control, giving one advantage of each.Show worked answer →
Open-loop control sends a fixed command to the output without measuring the result, so it does not correct for disturbances.
Closed-loop control measures the output and feeds it back, comparing it with the desired value and adjusting to reduce the error.
Advantage of open-loop: it is simpler and cheaper, with no sensor or comparator needed.
Advantage of closed-loop: it automatically corrects for disturbances and load changes, so it is far more accurate and stable.
Markers reward a correct description of each, an advantage of open-loop (simplicity or cost) and an advantage of closed-loop (accuracy or disturbance rejection).
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