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proportional control

Contributor(s): Matthew Haughn

Proportional control is a control system technology based on a response in proportion to the difference between what is set as a desired process variable (or set point) and the current value of the variable.

Proportional control is used where maintaining a process variable to a tighter tolerance and timely responsiveness are required. Control systems in many industrial settings as well as some smart devices use proportional control.

Proportional control involves fine-grained control through a feedback mechanism. In comparison to the on-off control (also known as bang-bang control) of a traditional bi-metallic strip-switched domestic thermostat, devices that use proportional control are more complex. The variable response is linearly proportional to the differential between a process variable and the desired optimum set point. This means that the response of the controller and the mechanism for affecting the variable make a greater response for a greater differential and a smaller change for a smaller differential.

The variability of the response to the process variable means tighter tolerances can be maintained. When automating dangerous machinery, handling of sensitive materials and chemical processes, proportional control makes processes and production safer and more reliable.

Adaptive cruise control is an example of proportional control. Throttle input is adjusted variably to react to both decreasing slopes and increasing momentum. As well, throttle is adjusted variably to the proximity of other vehicles in front of the controlled car. A separate proportional control system acts on the brakes gradually if nearing a followed vehicle too closely or dramatically in the case of emergency collision avoidance.

This was last updated in December 2017

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