Feedback control

Feedback control regulates a system by measuring its output and using that measurement to adjust its input. It can reduce the effects of disturbances and bring the output toward a desired value.

Control loop: A sequence in which a system’s output is measured, compared with a target, and used to adjust its input. It is the basic cycle that turns output measurements into corrective action.
System dynamics: The study of how a system’s state changes over time under internal processes and external inputs. Feedback must account for the system’s time-dependent response to control actions.
Thermostat: A device that regulates temperature by switching or adjusting heating and cooling in response to measured temperature. It is a familiar feedback controller that compares room temperature with a setpoint.
Open-loop control: Control that applies inputs according to a plan without using measurements of the resulting output. Unlike feedback control, it cannot correct deviations based on the system’s actual response.
Closed-loop transfer function: A mathematical relation between a system’s input and output when feedback connects its output to its control input. It describes how the complete feedback arrangement changes system response.
Setpoint: The desired value or trajectory that a control system aims to maintain. The controller compares measured output with this target to determine the correction.
Plant (control theory): The process or physical system whose behavior a controller seeks to regulate. The plant produces the output that feedback measures and the input that control changes.
Cruise control: A vehicle system that adjusts engine or motor power to maintain a selected speed. It corrects speed deviations caused by changing slopes and road loads.
Feedforward control: Control that adjusts inputs in response to measured disturbances or planned demands before their effects appear in the output. It anticipates some changes instead of waiting for output error, and can be combined with feedback.
Stability theory: The study of whether a system’s behavior remains bounded or returns toward equilibrium after perturbations. Feedback can stabilize a system, but poorly designed corrections can make it unstable.

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