Patent GB-850182-A (GB)
Title: Improvements relating to automatic process control
IPC: G05B13/02
Filed: 1957-04-29
Abstract: 850,182. Automatic temperature control systems. WHITE, R. B. April 29, 1957 [April 30, 1956], No. 13503/57. Class 38 (4). A process is automatically adjusted to and maintained at a point of desired slope on a curve relating input and output quantities of the process by (a) making an incremental change in the input quantity; (b) predicting the resulting output quantity from multiplication of the input increment by the desired slope; (c) measuring the actual resulting output quantity and (d) changing the input quantity in the appropriate sense by an amount proportional to the difference between the predicted and measured output quantities. As applied to steam-heating a liquid-filled tank in chocolate manufacture (Fig. 2, not shown), the curve of Fig. 1 represents tank tempeiature plotted to a base of steam flow. It is required to maintain the process at the optimum point represented by the knee of the curve, where the slope has a predetermined value. The steam supply valve is regulated by the controller, Fig. 5, to which is supplied the tank temperature as a setting on a potentiometer 502, the desired slope as a setting on a potentiometer 520 for amplitude and a switch 522 for sign, and information as to whether the curve is upwardly or downwardly convex as a setting of a switch 505. The value of the incremental change is determined by a motor-operated timer 514. A unit 524 controls the sequence of operations of the controller; the right-hand motor determining the control sequence of a switch 525 and the left-hand motor determining the delay period in which the output settles to a new value following an input change. Considering the conditions obtaining at a point 103 on the curve : the actual tank temperature 108 appears as a setting of potentiometer 502 and the previous predicted temperature 106 as a setting of a potentiometer 504. The difference voltage from the two potentiometers energizes a relay 526 and if the sense of the difference is the same as for the previous change, a comparator circuit 506 remains open so that no reversing command is fed to a memory relay 508 when power is applied to terminal (1) at the first position of controller switch 525. In the second position of switch 525 power is applied via terminal (2) to the motor in a reversing servo 510 which operates through a differential 542 to balance potentiometer 504 with potentiometer 502 so that the next prediction starts at point 108 on the curve. At the same time power is applied through a reversing switch 512 to an output motor 516 regulating the steam supply valve. Timer 514 is also started to switch off motor 516 after a predetermined time to set a maximum permissible adjustment of the steam valve. Power applied to a predictor 518 through a phase-sensitive rectifier 532 runs a motor 534 which rotates a shaft 536 by an amount representative of the product of the desired slope and the incremental change determined by motor 514. In the third position of switch 525 power as applied to a terminal (3) so that a motor 538 operates to return shaft 536 to zero and simultaneously operates a motor 540 to adjust potentiometer 504, through differential 542, to correspond with the next predicted tank temperature. In the fourth position of switch 525 power is applied to terminal (4) to energize a relay 528 releasing relay 508 and changing over reversing switch 512 provided a reversing signal was stored by relay 508 in the first position of switch 525. After a delay period the controller again compares the predicted and actual values and makes an appropriate incremental adjustment of the steam supply. The controller may be completely mechanical and the desired slope may be set to zero for a maximum or minimum. Applications to maintaining an optimum rate of mud supply to oil well drilling apparatus, maintaining a throttle setting of a locomotive to give maximum miles per gallon, and maintaining optimum conditions in a catalytic reaction, are referred to.