Feedback
Elementary
Think of a system that can see the results of what it does, like a person riding a bike. Feedback is when the system watches its own results and adjusts.
High school
Consider a system with an input and an output. Feedback is using information about the system’s output to decide its input.
Undergraduate
Consider a system with an input $u$ and an output $y$ (or state $x$). Feedback is the use of measurements of the output (or state) to determine the input, as in $u = k(y)$.
Examples
Examples
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A thermostat
Elementary
A thermostat is closed-loop feedback control because it measures the temperature and switches the heater accordingly.
- Steering a bike to keep from tipping over Elementary
- Cruise control that uses both the speed error and a map of upcoming hills High school
Non-examples
Things that may be mistaken for examples.
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A toaster that runs for a fixed time
Elementary
A toaster that runs for a fixed time is not closed-loop or feedback control, because it does not measure how brown the toast is.
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A precomputed optimal input $u(t)$, applied without measurements
High school
A precomputed input is not feedback control, so any disturbance or model error goes uncorrected.
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Raising heater power when an outdoor thermometer reports a cold snap
High school
Raising heater power on a cold-snap report is not feedback control, because it does not use the room temperature.
Properties
- Feedback can stabilize unstable systems and reduce sensitivity to disturbances and model errors.
Distinctions involving this term
- Types of control: A controller can decide its input with or without measuring what the system is doing. Open-loop control computes the input without measuring the output, and closed-loop (feedback) control uses the measured output to correct errors. Feedforward acts on a measured disturbance or a known reference before it affects the output, and is usually combined with feedback.