Pneumatic positioner

Abstract

A force balance instrument providing precision control of pneumatic actuators in response to command pressure signals independent of external forces on the actuator shaft. The instrument includes a high gain signal detection circuit in combination with one or two pilot operated output valves. In the case of two output valves inverse 'push-pull' control of a double acting pneumatic actuator is provided. Full supply pressure differential can be developed across the piston of the actuator, permitting to minimize the size of the actuator for any specified output force. The positioner proper controls the position of valve actuator system through a stem connector arm and a feed-back spring. A force balance condition is initially established between the feedback spring and the force of a rolling diaphragm controlled piston assembly for the range of signal pressure P.sub.1 to be used. Any change in this force balance condition because of a change in signal pressure P.sub.1 produces a change in clearance between the above referred-to rolling diaphragm controlled piston assembly and a pilot nozzle. Air under pressure is fed from a chamber which may be referred-to as output chamber through a passageway having a restriction to a chamber which will be referred-to as a servo chamber. The servo chamber is vented through the orifice of the aforementioned nozzle and affixed to the aforementioned movable nozzle support which is, in effect, a rolling diaphragm controlled piston assembly. The above nozzle controls the exhaust rate of the serve chamber. In double acting positioners a reversing valve is connected to the positioner proper.