Which device is commonly used to automatically regulate system variables such as speed or pressure?

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Automatic regulation of a variable in a machine relies on a feedback loop that keeps that variable at a desired value without human intervention. The governor is the classic device built for this purpose. It senses the current speed and continuously adjusts the energy input to the prime mover (such as fuel supply or steam supply) to hold the speed at the set point, even as load changes. This automatic adjustment is achieved through a mechanical or hydraulic feedback mechanism that links speed measurement to an actuator controlling the input.

Sensors, by themselves, only measure and provide data and do not enforce regulation. Valves are actuators that change flow or pressure but require a control signal from another device to operate automatically. A controller processes a set point and a measurement, but the packaged, purpose-built device that directly maintains a variable like speed in many machines is the governor.