A valve Positioner is a device used to increase or decrease the air load pressure driving the actuator until the valve’s stem reaches a “POSITION” balanced to the output SIGNAL from the process variable instrument controller.Valve positioners are used on controlling valves where accurate and rapid control is required without error or hysterises.Positioners are generally mounted on the side-yoke or top casing of the pneumatic actuator for linear sliding stem control valves, and at/near the end-of-shaft for rotary control vales. For either basic design type, “mechanical feedback linkage” connected directly to the valve’s stem provides feedback to controller. The process controller tells the positioner to “change” position; the feedback linkage reports back to the positioner confirming that a change has occurred and gives a “sense” of the magnitude of the change in position.Principle of OperationThe Figure shows a valve positioner.

The valve positioner is a force-balanced instrument, with pneumatic module installed on a double-acting actuator for air to open action. Positioning is based on a balance of two forces; one proportional to the instrument signal and the other proportional to the stem position.A downward force is activated as the signal pressure acts upon the diaphragms in the instrument signal capsule, through the follower arm and cam, the motion of the actuator stem is transmitted to the top end of the feedback spring resulting in the varying of tension in feedback spring as stem position changes.The system will be in equilibrium and stem will be in the position called for by the instrument signal when these opposing forces balance exactly. The balance will move up or down and by means of the spool valve, will change the output pressures and flow rate if these opposing forces are not in balance. This will lead to the piston to moving until the tension on the feedback spring opposes exactly the instrument signal pressure.The detailed sequence of positioner operations are as follows:.

Controllers are available from the VRC PRO e-shop. You MUST select and calibrate your controller in the. Control options dialog before to go.

An increase in the instrument signal forces the instrument signal capsule and balance beam downward. This motion of the balance beam also pulls the pilot valve spool downward from its equilibrium position.

This opens the pilot valve ports, supplying air to port 1 and exhausting air from port 2. Super toss the turtle hacked. This causes the actuator piston upward.