A 20-100 kPa controller signal travelling perhaps a long distance to a control valve to exercise the required control action, may possibly not be powerful enough to quickly move a large valve to its new position.
It may therefore be advantageous to allow the controller signal to control the air supply locally at the valve rather than to operate the valve by itself.
The device that may be added to enhance the controller signal to a valve, is called a valve positioner.
A valve positioner is essentially a pneumatic controller that senses the valve stem position, compares it to the incoming controller signal and adjusts the pressure to the actuator until the stem position corresponds to the controller signal.
Positioners can be used to overcome stem friction, high fluid pressure, viscous or dirty fluids, or to improve slow system dynamic response.
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