How does feedback turn an open-loop system into a self-correcting closed-loop one, and what is PID control?
Open- and closed-loop control, feedback and the error signal, and proportional, integral and derivative (PID) control.
A CCEA A-Level Technology and Design answer on open- and closed-loop control systems, the role of feedback and the error signal, and the proportional, integral and derivative (PID) control modes.
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What this dot point is asking
CCEA expects you to distinguish open-loop from closed-loop control, explain the role of feedback and the error signal, and describe the proportional, integral and derivative (PID) control modes. The open/closed contrast and the meaning of the error signal are core; PID terms are commonly tested.
The answer
Open- and closed-loop control
The error signal
PID control
Worked example: comparing control of an oven
Examples in context
Example 1. Cruise control. The car measures its speed, forms the error against the set speed, and adjusts the throttle (PID) to hold speed up hills and down, a closed loop you experience directly.
Example 2. 3D printer hot end. A thermistor feeds the temperature back to a PID controller that holds the nozzle at the set point; without the integral term it would sit slightly below target, the offset PID removes.
Try this
Q1. State the key difference between open-loop and closed-loop control. [2 marks]
- Cue. Closed-loop measures the output and feeds it back to correct itself; open-loop does not measure or feed back the output.
Q2. Write the equation for the error signal in a closed-loop system. [1 mark]
- Cue. error = set point - measured value.
Q3. What does the integral (I) term in PID control achieve? [2 marks]
- Cue. It acts on the accumulated error over time, removing the steady-state offset that proportional control alone leaves, so the output reaches the set point exactly.
Exam-style practice questions
Practice questions written in the style of CCEA exam questions on this dot point, with worked answer explainers. The year tag is the paper they imitate, not the source.
CCEA 20206 marksExplain the difference between open-loop and closed-loop control, using a heating system as an example, and explain the role of the error signal in a closed-loop system.Show worked answer →
In an open-loop control system the output is not measured or fed back; the system simply applies an input and assumes the result. Example: a heater on a fixed timer - it runs for a set time regardless of the actual room temperature, so it cannot correct for a cold day or an open window.
In a closed-loop control system the output is measured by a sensor and fed back and compared with the desired value (the set point). Example: a thermostat-controlled heater - a temperature sensor feeds the actual temperature back, it is compared with the set point, and the heater is switched (or modulated) to keep the room at the set temperature, correcting for disturbances.
The error signal is the difference between the set point (desired value) and the measured output (actual value):
The controller acts on this error to drive it towards zero: a large error calls for strong action, and as the error shrinks the action reduces, so the system settles at the set point. Without the error signal there is no self-correction.
Markers reward the not-measured (open) vs measured-and-fed-back (closed) contrast with valid examples, and the error = set point minus measured value definition with its role in driving correction.
CCEA 20214 marksExplain the proportional and integral terms in PID control, and state one benefit each provides.Show worked answer →
PID control computes the control action from the error using three terms.
- Proportional (P): the control action is proportional to the size of the error (bigger error, bigger correction). Benefit: it gives a fast response that scales with how far off the system is. (On its own it can leave a small steady-state offset.)
- Integral (I): the control action depends on the accumulated error over time (the sum/integral of past error). Benefit: it eliminates the steady-state error/offset that proportional control alone leaves, by continuing to push until the error is truly zero.
- (Derivative (D), for completeness: responds to the rate of change of the error, damping overshoot and improving stability.)
Markers want proportional = action proportional to current error (fast response) and integral = action from accumulated error (removes steady-state offset), each with its benefit.
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Sources & how we know this
- CCEA GCE Technology and Design specification — CCEA (2016)