GB700441A - Improvements in pneumatically-actuated devices for controlling, indicating or recording a variable condition - Google Patents

Abstract

700,441. Fluid-pressure servomotor control systems. FOXBORO CO. March 6, 1952 [March 14, 1951], No. 5891/52. Class 135 A pneumatically-actuated device for controlling, indicating, or recording a variable condition includes a nozzle-and-flapper valve wherein changes in the variable cause movement of the flapper, and the ensuing changes in the flow through the nozzle produce changes in pressure in the fluid supply, which changes, besides being available for control &c. purposes, are applied to the flapper by means of a ball-piston unit in opposition to the movements due to changes in the variable condition. In an embodiment, the temperature in a process tank 10, Fig. 1, is sensed by a bulb 11 which produces a corresponding pressure in a Bourdon tube 13. Movement of the latter due to a change in temperature acts through a link 14 to pivot a T-lever 16 about its fulcrum 17, and an abutment 18 on the lever will cause, or allow, a composite frame 19, 20 to tilt about its pivoting axis 24. This composite frame comprises a trident 19 with its centre limb 19a capable of flexing and a superposed triangle 20 with its apex carrying a screwed abutment 23 by which the tension on the limb 19a can be adjusted. Limb 19a constitutes the flapper controlling the flow of air from a nozzle 26, the latter being connected through a chamber 31 and a restriction 32 to a supply 29 of air under pressure. The varying pressure produced in chamber 31 due to movements of flapper 19a acts on a diaphragm 33 which actuates inlet and exhaust valves 35 to control the connection of a chamber 34 to the supply 29 or to exhaust at 38. Valve 35 is loaded by a spring 36. Chamber 34, besides being connected to a motor-actuated valve 41 which controls the flow of heating or cooling medium to tank 10, is also connected to a cylinder 27 containing a piston 45, Fig. 2, in the shape of a ball, the latter being in contact with flapper 19a and serving to neutralize the latter after a controlling movement thereof due to a change in the temperature in tank 10. The amplification produced by the unit can be varied by adjusting the position of the nozzle along the length of the flapper by means of a slide 43, and when the nozzle is positioned immediately under the base of frame 19, the device becomes a simple " onoff " unit. Specification 670,419 is referred to.