Process controller having electrochemical cell integrating means

1,045,051. Automatic control. GENERAL ELECTRIC CO. Jan. 28, 1965 [Feb. 24, 1964], No.3833/65. Heading G3R. [Also in Division G4] In a process control system, a current corresponding to error in the magnitude of a measured variable of the process is applied through a differentiator and an electrochemical integrator to maintain the variable at a desired value. Current from a transducer 11, dependent on the magnitude of a process variable, is compared with current, corresponding to a desired value of the variable set at a source 12, and any difference is differentiated by a circuit including a capacitor 25. With switches 30 in the upper position the output of the differentiator is applied between input and common electrodes 19, 16 of an integrating, electrochemical, cell 14 having iodine-iodiode electrolyte, an output electrode 17, and a guard electrode 18. The cell electrodes are of platinum. Zener diode limiters 23, 24, 39 and a resistor 31 are provided to prevent overloading of the cell. Output electrode 17 is biased with current to permit larger input currents to be used and this current is maintained despite temperature variations by a temperature variable resistor 33. The voltage bias of the output and guard electrodes are made equal to prevent gassing. The current from integrator output electrode 17 is amplified by transistors 40, 42, 49 to provide two output current supplies 43, 45 varying in opposite senses, the appropriate one of which is used to correct deviations of the variable automatically. The amplifier is provided with temperature compensating negative feedback through resistors 47, 48. With switches 30 in the central position, automatic control is disabled and manual control may be effected by operating a switch 50 in either direction from an off position to apply current in a corresponding sense through cell 14. Earthing by switch 30b ensures smooth reversion to automatic control. A circuit 60 including biased diodes is effective to maintain the output of amplifier 13 between limits by controlling the input to cell 14 if the limits are exceeded. By moving switches 30 to the lower position, manual control may be effected by varying the input to cell 14 by means of a potentiometer 80 in circuit 60, this arrangement having the advantage that, due to permanent feedback from potentiometer 80 to the cell, drift does not occur when manual control is used for long periods. Smooth changeover with this form of manual control is effected by first sett