Servo Mechanism Control Valve Positioner Principle


#1

This positioner is kind of servo control mechanism for air operated automatic control system.

Typical usage and features, can be used as a double acting positioner as well as a single acting positioner because this model has a double acting pilot valve.

Dynamic characteristic are far better than any other positioner as a large capacity pilot valve is employed in the nozzle flapper circuit.

Application covers direct operation and reverse operation of both actuator and positioner pilot.

The operation ofsingle acting positioner as picture above, the signal pressure from controller enters the input chamber and exerts a force on diaphragms. Difference of the pressurized area of the two diaphragms creates a force to push the flapper.

As the clearance between the flapper and the nozzle increase, back pressure in the nozzle decrease and the supply pressure actuated pilot diaphragms shift the pilot stem rightward to open the pilot valve.

Outlet pressure passed through the pilot valve and the bypass cock enters the diaphragm motor to operate the valve stem. The movement of the valve stem is converted into rotary movement by the effect of the lever, and further transmitted to the feedback lever through the transmission lever and pin.

Movement of the feedback lever varies the force of the feedback spring. The valve stem moves on until the spring force balance with the force of the input chamber. Consequently the valve stem is always controlled precisely in proportion to the signal pressure.

Stroke Adjusment

Stroke can be adjusted by changing the effective number of turns of the feed-back pring. The span adjuster is for this adjustment. For shorter stroke, decrease the number of effective turn.

Zero Adjusment

Turn zero adjuster to vary the tensile strength of the feedback spring.

Sensitive Adjustment

Use zero adjuster to adjust hunting control system and the sensitivity of positioner. However the sensitivity is preset before shipment from plant and re-adjustment is not necessary unless it is not good enough on your actuator.

Another type of positioner is such a positioner that is added the feedback controlling mechanism to type standard pneumatic type positioner.

This type of positioner has been so design as to provide the regulating function of fully pressurizing (DA valve) or completely discharging (RA valve) supply air to/from the actuator separately from ordinary zero adjustment and span adjustment of various control valve and control supply and exhaust air of the positioner against full-close signal to the control valve so that the full close operation can be performed surely and easily…