A valve positioner is basically a device that senses both the signal of an instrument (controller) as the position of a valve stem. Its main function is to ensure that the position of this rod corresponds to the output signal of the controller or regulator.
For example, if the pneumatic positioner receives a signal of 35%, should provide sufficient air pressure to the actuator to cause the piston stroke is 35% of its overall range. Can indeed be described as a closed loop controller, whose input signal to the instrument, its output goes to the actuator diaphragm and the feedback signal from the valve stem.
It is used in operating valves in split range, to reverse the action of a control valve to overcome the frictional forces within a valve and control applications that require fast and accurate. Usually mounted on the control valve.
The positioners can be divided and electro-pneumatic. Regarding the former, in turn are subdivided into those driven by a balance of movement. It basically consists of a bellows which receives the control signal, a horizontal bar to the bellows on one side and a pneumatic relay system which forms a nozzle-actuator toben with the bar.
As the bellows moves in response to signal change of the instrument, the arrangement moves nozzle-shutter, aperture admitting or expelling air to air it, until the position of the rod corresponds to the signal sent by the controller. At that time the position will be back in balance with the control signal.
Electropneumatic positioners arose growing use of electronic control systems that act on pneumatic control valves. Essentially consist of a combination of a current to pressure converter (I / P) and a positioner. It is a balance of power device and can be used to direct action or reverse action.
Figure 12 valves with positioners
With the emergence of electronic controllers jy the need to control pneumatic valves, appeared the so-called current to pressure converters (1 / P). These become standard signals 0 / 4 -20 mA signals to act on the pneumatic control valve or the respective positions. Shortly after positioners are manufactured with built-in converter thus achieving the so-called electro-positioners.
The further advance in digital communication tools has forced many manufacturers to develop positions that can communicate with digital output drivers. Thus we now have digital positioners (also called "intelligent") for various protocols. This clearly allows the integration of control valves in an industrial network with the benefit of a greater amount of information about them and process control.
Figure 13 digital positioner and configurator
