Legacy processing units may use pneumatic pressure signalling as the control set point to the control valves. In a common pneumatic positioner design, a pneumatic input signal is received from a control device. It modulates the supply pressure to the control valve actuator, providing an accurate valve stem or shaft position proportional to the pneumatic input signal.
Most modern processing units use a 4 to 20-mA DC signal to modulate control valves. This introduces electronics into the positioner design and requires that the positioner convert the electronic current signal into a pneumatic pressure signal (current-to-pneumatic or I/P). In a typical analog I/P positioner, the converter receives a DC input signal and provides a proportional pneumatic output signal through a nozzle/flapper arrangement. The pneumatic output signal provides the input signal to the pneumatic positioner.
