Patent GB-783169-A (GB)
Title: Pressure fluid operated balanced control system
IPC: G05B6/05
Filed: 1952-09-29
Abstract: 783,169. Fluid-pressure servomotor-control systems. FISHER GOVERNOR CO., Ltd. Sept. 29, 1952 [Sept. 29, 1951], No. 24363/52. Class 135. A system for controlling the magnitude of a condition comprises a primary member positioned in accordance with the existing magnitude of the condition and controlling the pressure of fluid applied to one of a pair of pressureresponsive devices acting in opposition on a secondary control member which is suspended by said devices and by a separate, opposing, biasing device associated with each pressureresponsive device, and a valve controlled by the secondary member to vary an output pressure which is applied to the other pressure-responsive device and to means for affecting the condition to be controlled. As shown, a liquid level controller comprises a float 36 for detecting variations of level in a tank 10, a transmitter 20 in which an air pressure dependent upon the position of the float is set up and transmitted by a conduit 23 to a receiver 25, and a pressureoperated valve 30 controlling flow to the tank 10 and subject to output pressure from the receiver. Pressure air is supplied through a regulator 22.to a relay valve 44 where it passes through a restriction 115 into a diaphragm chamber 116 which has a connection 50 with a nozzle 58 associated with a blade 38 movable by the float. Any variation in liquid level, therefore, causes movement of the diaphragm bounding the chamber 116, resulting in actuation of a movable seat 110 of an outlet valve member 109 by which a chamber 96 may be connected to atmosphere through an outlet 122. Rigidly connected with the valve member 109 is a valve member 107, by which, on downward movement of the diaphragm, the chamber 96 is connected to the air supply. The output from the relay valve 44 is connected to the conduit 23 and also, through a proportional-band control valve 48, to a Bourdon tube 60 by which the nozzle 58 is carried. The valve 48 incorporates a series restriction and a restricted vent to atmosphere both of which are adjustable. An adjustment means incorporating a cam 56 sets the nozzle 58 in relation to the blade 38. A pressure gauge 46 measures the output pressure of the transmitter to indicate the instantaneous value of the liquid level. The output from the transmitter is led through a reversing valve 62 to a bellows 67 between which and a normally vented bellows 69 is mounted one end of a beam 68 which coacts with an adjustably mounted nozzle 75 and whose other end is carried between two further bellows 70, 71. Air under pressure is led to a relay valve 73 similar to the valve 44 by a conduit 28 and the diaphragm chamber 116 is connected to the nozzle 75 so that the output pressure from the valve 73, which is led to the actuator of the valve 30, depends upon the position of the beam 68. A connection from the outlet of the valve 73 is also made through a proportional-band control valve 82 similar to the valve 48 to the bellows 71, and a further connection through an adjustable valve 85 to the bellows 70 removes the offset introduced by the bellows 71. The nozzle 75 is adjustable along an arm 76 by which it is carried for varying the proportional band. The bellows 69 may be connected to a source of fluid pressure for varying the control point of the system. The reversing valve 62 enables an increase in the output pressure of the transmitter to result in either an increased,or decreased output pressure from the receiver. In a modification the valve 85 is omitted and the bellows 70 is vented. In this case, however, a change-over valve is provided whereby the output from the valve 73 may be applied to the bellows 70 instead of to the bellows 71 so that the apparatus functions as a snap-acting, on-off control.