A control system changes an input or actuator command to make a dynamic process follow a reference or reject disturbances. Open-loop control acts without measuring the result. Closed-loop control compares measured output with a reference and uses the error to adjust the plant input.
The articles below cover feedback, mathematical models, stability and transform methods. Laplace-domain transfer functions support continuous-time linear analysis, while Fourier methods describe frequency content and response. A controller design must account for stability margins, transient response, steady-state error, actuator limits, sensor noise, delay, uncertainty and sampling. PID control is one option, not a universal solution.
