GB1148876A - Improvements in or relating to automatic process control systems - Google Patents

Abstract

1,148,876. Automatic control. GEORGE KENT Ltd. April 14, 1966 [April 14, 1965], No. 16045/65. Heading G3R. In an electrical system in which a process variable is automatically corrected by a controller producing one or other of two outputs according to the sense of error detected by comparison of actual and desired values of the variable, the controller outputs are controlled by an interrupter having an on/off time ratio which depends on the magnitude of the actual or desired value of the variable. As described, an electric signal dependent on the magnitude of a variable to be controlled, e.g. the CO 2 content of gas generated by a carburizing or carbo-nitriding furnace, is compared with a reference representing the desired magnitude and any error produces a signal of constant magnitude, corresponding to the sense of the error, which energizes one or other of two relays to operate a servomotor in the appropriate direction to correct the variable, e.g. by varying the fuel supply. An electronic delay unit operated periodically by pulses and having a time of operation which is made dependent on the magnitude of the variable or the magnitude of the reference interrupts the energizing circuit of the relays so that the rate of correction is dependent on the magnitude of the quantity chosen. The rate of correction may, in addition, be made dependent on the magnitude of the error. Alternative methods of producing interruption having a variable mark-space ratio are referred to. Continuously operating selector switches may apply several variables and their reference values in succession to the system the output of which is applied in succession to appropriate correction devices.