Patent GB-713782-A (GB)
Title: Improvements in methods of, and systems for, maintaining a variable condition at a pre-determined value
IPC: G05B11/44
Filed: 1951-12-31
Abstract: 713,782. Fluid-pressure servomotor-control systems. TAYLOR INSTRUMENT CO'S. Dec. 31, 1951 [Jan. 10, 1951], No. 30463/51. Class 135 A variable condition is maintained at a predetermined value by detecting changes in the condition from said value, developing a primary response proportional to the sum of the detected change and the rate of the detected change, effecting a separate second response proportional to the sum of the primary response plus the integral with respect to time of the primary response, and using the second response to modulate the application of a correcting medium to restore the condition to the predetermined value. The invention is shown applied to controlling the temperature of a process P, Fig. 8, wherein steam is supplied to the process through a pipe 5 in which is interposed a diaphragm actuated flow-controlling valve 6. The temperature of the process is measured by a thermal bulb which transmits a corresponding pressure to a converter 80, the latter supplying air at a pressure inversely proportional to the detected temperature change to a chamber 88 of the first stage of the control apparatus. This first stage comprises three interconnected diaphragms defining chambers 87 ... 89 and carrying a baffle plate 91 the position of which controls the leak from a nozzle 93. A loading pressure is supplied to chamber 89 past a reducing valve V, the value of this loading pressure determining the set point of the system. Air under pressure is supplied to the nozzle past a restriction 94, and the controlling pressure established upstream of the nozzle in accordance with the position taken up by baffle plate 91 is applied to chamber 87 and a capacity 99 past a restriction T. The control pressure produced will thus include components proportional to the change in the temperature plus the rate of such change, and it is applied through a conduit 97 to an isolating relay 100 and past a pressure dividing network to the second stage of the control apparatus. Relay 100 consists of a diaphragm subject on one face 102 to the pressure from line 97 and on its other face 101 to pressure air supplied past a restriction 106. The diaphragm controls a vent 104 to atmosphere so that the pressure delivered to the first restrictor G1 of the dividing network will be substantially the same as that delivered to the relay. A counter flow of pressure air is supplied past a reducing valve 108 and a restrictor G2 and the combined flow is led off through a conduit 112. Restrictors G1, G2 are adjustable so that the sensitivity of the system can be controlled. The second stage of the control apparatus is similar in construction to the first stage. The controlling pressure from conduit 112 is fed into chamber 44 and the output or regulating pressure then produced upstream of the nozzle 51 is applied to a booster relay 60. The final regulating pressure produced by the booster relay is applied to actuate the valve 6 and is also fed back through a conduit 74 to chambers 45 and 46 of the second stage of the controller. The feed-back is unrestricted into chamber 45 but passes through a restrictor R before reaching chamber 46 to which a capacity 54 is connected. The pressure supplied to the booster relay will thus include a component corresponding to the time integral of the output of the first stage of the controller. The booster relay comprises two interconnected diaphragms defining chambers 63 ... 65 of which chamber 63 receives the incoming signal, chamber 64 is connected to atmosphere, and chamber 65 is connected to the output conduit 73. Pressure air is supplied at 71 and the connection of chamber 65 to the pressure supply or to the exhaust chamber 64 is controlled by interconnected inlet and exhaust valves 72, 69 respectively. In a modification of the first stage of the controller the varying pressure obtained from the thermal bulb is applied to a Bourdon tube 9, Fig. 7, which, through a link 10, actuates a hinged baffle plate 12 controlling the leak from a nozzle 13. Air under pressure is supplied to the nozzle past a restriction 18, and the control pressure obtained upstream of the nozzle is applied to a diaphragm 20a which actuates a valve 20 to provide a modulated pressure to conduit 112 leading to the second stage of the controller. This modulated pressure is also applied past a restrictor T to a bellows 22, the latter acting through a rod 23 and levers 25, 26 to re-position the nozzle 13 to its set position relative to the baffle 12. The restrictor. T and bellows 22 are responsible for the component in the modulated pressure in line 112 corresponding to the rate of change of the variable. The sensitivity of the system can be adjusted by varying the position of the common abutment 27 between levers 25, 26. The set point is varied by adjustment of a link 16 to which the baffle 12 is hinged, and the value of this set point is indicated by the pointer 17 connected to link 16. The value of the variable is indicated by a pen 14 connected to link 10. The invention is also applicable to hydraulically and electrically actuated systems.