Closed-Loop Control
A control structure in which the output is measured, compared with a reference, and the resulting error influences subsequent control action.
A closed-loop controller does not merely issue a command and assume that the plant followed it. The output is measured, compared with the desired reference, and the difference is fed back into the control decision.
For a basic negative-feedback loop:
error = reference - measured_outputthe controller computes a new action from that error.
Feedback can improve disturbance rejection and reduce sensitivity to model mismatch, but feedback alone does not guarantee stability. Delay, loop gain, plant poles, sampling, and controller dynamics all matter.
The defining feature of closed-loop control is therefore corrective action based on observed output. The engineering problem is to decide how fast, how stable, and within what error bounds that correction should occur.
See: Automatic Control.