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Proportional control is a type of linear feedback control system in which a correction is applied to the controlled variable which is proportional to the difference between the desired value (SP) and the measured value (PV). Two classic mechanical examples are the toilet bowl [[Ballcock|float proportioning valve]] and the [[Centrifugal governor|fly-ball governor]].
The proportional control system is more complex than an [[On–off control|on–off control system]] but simpler than a [[PID controller|proportional-integral-derivative]] (PID) control system used, for instance, in an automobile [[cruise control]]. On–off control will work for systems that do not require high accuracy or responsiveness but are not effective for rapid and timely corrections and responses. Proportional control overcomes this by modulating the manipulated variable (MV), such as a [[control valve]], at a gain level that avoids instability, but applies correction as fast as practicable by applying the optimum quantity of proportional correction.
A drawback of proportional control is that it cannot eliminate the residual SP–PV error, as it requires an error to generate a proportional output. A [[PI controller]] can be used to overcome this. The PI controller uses a proportional term (P) to remove the gross error, and an integral term (I) to eliminate the residual offset error by integrating the error over time.
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