A controller is any comparative device in a process that can sense a difference between a predetermined value (a control or set point) and a measured value received as an input signal from a variable process condition. The controller determines the difference between the two values, and adjusts the amount of output signal required by the final control element in the process. Pneumatic controllers are designed specifically for use with compressed air equipment and processes.
Pneumatic Controllers measure differences in air pressure or temperature, and adjust them to a set point using a corrective air signal transmitted to a control valve or other final control element. The controller has a panel indicating the set point, process measurement and output air signal, usually mounted on a range plate. Pneumatic Controllers are designed in either proportional or simple On/Off control, where proportional control is more suitable for a greater degree of control precision or unstable load conditions. On/Off control is more suitable for stable load conditions, and can be operated by a fast opening control valve.
Proportional pneumatic controllers react to the degree of deviation from the control point and send a corrective signal that can be adjusted by changing the band width. On/Off pneumatic controllers only recognise the existence of a deviation, and respond with a full corrective signal in every instance.
Pneumatic controllers are generally considered safer than electronic and electrical controllers, especially when used in hazardous environments such as chemical or explosive atmospheres.