A linear function between the input signal and stroke is the best way to ensure maximum control precision. Control valves with pneumatic actuators, however, are subject to friction, media pressure, and high flow forces, which means that this linearity is not intrinsic in the system. Only a positioner can eliminate positioning errors by comparing the input signal with the actual stroke and by yielding the actuating pressure for the actuator.
Positioner mounting to IEC 534 (NAMUR)
The standard mounting method (to IEC 534) is based on manufacturer-neutral mechanical interfaces with the actuator lantern and stem. A bracket is normally used to secure the positioner, while the feedback lever comprises a lever with a spring element. The intake air is connected to the positioner, while the pneumatic connection with the actuator is realized by means of a pipe or hose.
Direct, integrated mounting
The positioner can be easily mounted on the actuator lantern by means of two screws to ensure that it is robust and protected against vibrations. The feedback lever is located within the lantern, which offers better protection than the NAMUR mounting method. The actuating pressure is channeled from the positioner through the actuator lantern to the actuator without the need for additional piping, thereby preventing any leakage. The intake air is connected directly to the positioner.
Integrated mounting to VDI/VDE 3847
As with the NAMUR mounting method, this method uses standardized, mechanical interfaces with the actuator lantern and stem. The actuator interface, however, is also a pneumatic interface, which means that, as with the direct mounting method, the actuating pressure is channeled to the actuator through the actuator lantern. In addition, the rear of the lantern is equipped with an interface for a solenoid valve, which is pneumatically connected between the positioner and actuator, thereby enabling a safety-oriented shutdown of the unit. The intake air is connected directly to the lantern, which means that no work needs to be carried out on the piping if the positioner is replaced.