In the last article, I cover the Force balance type of positioner. Here, I will cover article on Motion balance type pneumatic positioner.
Following is the diagram of the internal component of the motion balance type positioner.
It operates like with the concept of flapper-nozzle but instead of controlling force, it uses motion balance principle.
Following photograph of actual D-shaped positioner which operates on motion balance principle.
In this mechanism, an increasing signal pressure cause beam to move towards the nozzle, which increases backpressure which then pneumatic amplifying relay to send more air pressure to lift up the valve.
As valve stem lifts up upward motion is imparted toward right hand of beam counter's previous motion towards the nozzle.
When equilibrium condition is reached the beam will be an angled position with the bellow motion balance by the valve stem.