control systems, combinations of components (electrical, mechanical, thermal, or hydraulic) that act together to maintain actual system performance close to a desired set of performance specifications. Open-loop control systems (e.g., automatic toasters and alarm clocks) are those in which the output has no effect on the input. Closed-loop control systems (e.g., thermostats, engine governors, automotive cruise-control systems, and automatic tuning control circuits) are those in which the output has an effect on the input in such a way as to maintain the desired output value. A closed-loop system includes some way to measure its output to sense changes so that corrective action can be taken. The speed with which a simple closed-loop control system moves to correct its output is described by its damping ratio and natural frequency. A system with a small damping ratio is characterized by overshooting the desired output before settling down. Systems with larger damping ratios do not overshoot the desired output, but respond more slowly. See feedback.
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