GB728339A - Improvements in or relating to pneumatic control systems - Google Patents

Abstract

728,339. Fluid-pressure servomotor control systems; valves. FOXBORO CO. May 28, 1953 [May 29, 1952], No. 14887/53. Class 135. A pneumatic servomotor for actuating the stem of a valve is controlled by a pilot valve comprising a nozzle and baffle mounted on a support capable of rotary adjustment in a plane perpendicular to an arm which actuates the pilot valve, the arm being subject to an input signal and to a repeat back signal indicative of the position of the stem of the firstmentioned valve. In an embodiment, the pressures on the two sides of a restriction in a flow line controlled by the main valve are applied to a controller which supplies to a bellows 30, Fig. II, a signal pressure indicative of the pressure differential across the restriction. Pivoted to the bellows at 39 is a springy U- shaped arm 35, the free end of which is ballshaped and contacts a baffle plate 45 controlling the leak from a nozzle 44. The free end of arm 35 is carried by the end of a shaft 34 which can be rotated by a lever 26 connected to the stem of the main valve so that, while the feedback movement imparted to arm 35 will cause it to swing in a vertical arc about pivot 39, the movement due to the input signal applied to bellows 30 will cause it to move in a horizontal plane. Baffle plate 45 is connected by a spring to a rotary support plate 46 which also carries the nozzle 44, and rotation of plate 46 by means of gearing 48 adjusts the operative relation between the pilot valve and arm 35 so that a proportional, on-off, or reversed control can be obtained. Air under pressure is supplied to the nozzle from a line 20, Fig. IV, past a restriction 20a and through a line 51 passing through the arbor of plate 46. The back-pressure from the nozzle of the pilot valve acts on a diaphragm 53 of a relay valve constituted by inlet and exhaust valves 54 connected to the diaphragm. The control line 21 from the chamber between the valves 54 passes to a spring-loaded singleacting power device to which the stem of the main valve is connected. The connection to this diaphragm power device can also be changed from one end to the other so that a direct or reverse acting power device is obtained. Specifications 670,427 and 682,841 are referred to.