Patent GB-1104506-A (GB)
Title: Improvements in or relating to process control apparatus
Inventor: ELDRED ROBERT JAMES
IPC: G05B13/02
Filed: 1965-07-09
Abstract: 1,104,506. Fluid-pressure servomotor systems. FOXBORO-YOXALL Ltd. June 17, 1966 [July 9, 1965], No.29268/65. Heading G3P. In a process control system in which a controlled variable depends on one or more manipulated variables and in which the controlled variable has an optimum value, such as maximum or minimum, which it is desired to maintain, the manipulated variable is caused to creep slowly forwards and backwards through a region corresponding to the optimum value of the controlled variable and a memory element stores a previous value of the controlled variable. When the controlled variable is closer to the optimum value than the value in the memory, the memory is updated to the new value of the controlled variable. The direction of creep is reversed when the controlled variable departs from the optimum. If the controlled variable continues to depart after reversal, the creep is halted and the value in the memory is allowed to equalize slowly with the controlled variable. When equality is achieved, the creep of the manipulated variable is resumed. The control apparatus may be pneumatic, electric or hydraulic. In the pneumatic embodiment of Fig. 2, the controlled variable in line 6 is representative of a parameter of a process 1. The manipulated variable in line 2 controls the process and is itself controlled by a unit 5 via a valve 34. The various sources of compressed air are indicated at 13 and it is desired that the value of the controlled variable should be a maximum. The controlled variable A and the previous maximum value B of this variable act on diaphragm 23 and control a relay R 1  via flapper-nozzle unit 24, 25. The output of R 1  acts on spring-loaded bellows 26 to position a rod 29 carrying flappers which cooperate respectively with nozzles 30-33. If the output of R 1  drops below say 5¢ p. s. i., due to A exceeding B, then rod 29 moves to the left to uncover nozzle 30 to open valve X to up-date the value B. This resets flapper 24 and rod 29 returns to the right to close nozzle 30 and valve X. The cyclically varying input to unit 5 derives from relay R 2  via valve S and restriction W. Relay R 2  is controlled by flapper-nozzle units 35, 36, valves Z, V, relay R 3  and nozzles 31, 32. Nozzles 31, 32 are closed when the output of R 1  reaches 6¢ and 7 p. s. i. So long as the output of relay R 1  remains above 5¢ p.s.i. but below 7¢ p.s.i., the value B remains constant and the variable A rises and falls whilst the output of R 2  alternately increases and decreases. The arrangement is such that when the output of R 2  is increasing the rod 29 moves to the left while A rises to a maximum and then to the right while A falls to a minimum. This rightward movement culminates in the output of R 2  being changed from increasing to decreasing, whereupon the rod 29 moves to the left and then to the right again, at the end of which reciprocation the output of R 2  is changed to increasing, and so on. If the process does not respond promptly when the output of R 2  changes, the rod 29 continues its leftward movement and closes nozzle 33 to cause valve Y to open. The signal in the memory then leaks away through restriction 48 until A and B are equal. At the same time, valve S is closed to stop the creep of the input signal 2. Leftward movement of the rod 29 then takes place to open valves Y and S by uncovering nozzle 33. The normal cycle of operations as described above then continues. The process 1 may comprise a furnace in which it is required that the temperature of the products of combustion be maximized by manipulating the air and/or fuel flow. There may be more than one input signal 2, in which case the respective controls 5 are connected to the output of R 2  in sequence by selector 50.