Patent GB-1160078-A (GB)
Title: Electronic Process Control Apparatus
IPC: G05B5/01
Filed: 1966-11-01
Abstract: 1,160,078. Automatic temperature control; voltage and current regulation. FOXBORO CO. Nov. 1, 1966 [Nov. 15, 1965], No. 49017/66. Headings G3R and G3X. [Also in Division H3] A control system for automatically regulating a variable in an industrial process, for example the regulation of temperature by means of a valve, comprises the arrangement shown in Fig. 3 wherein an error signal derived from comparison between a measuring circuit 12b, 14, 18 and a set point circuit 10, 12a, 16 is applied via a resistor 22 and terminal 106 to a first input 24 of a high gain D.C. amplifier 26 see Fig.4 whose output is used to regulate the variable and is also fed back to the control system via the terminals 28, 30. The second input 52 of the amplifier is connected via a terminal 124 to an RC circuit 46, 48 having a large time constant so that rapid changes of potential at the second input are prevented. Upon the occurrence of an error signal, the initial feedback action is negative via members 32, 34, 36 and tends to maintain the potential of the first input 24. Thereafter the potential of the second input 52 gradually changes due to the charge (or discharge) of the capacitor 48, the change tending to increase the potential difference between the amplifier and so produce a positive feedback, reset action which is counteracted by the regulating action to restore the variable to its desired value. The Specification discusses the advantages of the circuit positions in the control system of the capacitor 48 and the potentiometer 32 which determines the "proportioning band" of the controller. An RC circuit 62, 64 provides a rate of change signal of the variable, which signal is in series with the reset signal of capacitor 48 through an on/off switch 134 and an amplifier 66. The rate of change circuit is isolated from the set-point circuit. A resistor 154 ensures that the capacitor 62 is always charged to the measured value, to prevent disturbance when the switch 134 is closed to insert the rate of change circuit in the system. A change-over switch 100 enables the set-point circuit 10, 12a, 16 to be replaced by a set-point circuit 184, 186 to whose terminals 180, 182 is applied a remotely-controlled signal, e. g. one derived from the output of another controller responsive to a second variable condition of the process. Operation on manual or automatic control A switch 72 having contacts 70a, 70b, 70c provides for changeover from one condition to the other, the connections of this switch with the other circuit components and a memory capacitor 74 ensuring smooth changeover even if the variable is changing at the instant of switchover. On automatic operation the capacitor 74 is connected to the output terminal 30 and, with manual switch 78a in its open-circuit position, will follow the output of the amplifier 26. Movement of switch 72 to the manual position disconnects the error signal from input 24; connects capacitor 48 to junction of members 22, 36 so that this capacitor is charged by the difference between the error signal and the amplifier output from 30, 34 thus providing a memory of the relationship between the variable and the amplifier output; and connects the capacitor 74 to the input 52 to fix the amplifier output at the level existing just before changeover to manual, the capacitor voltage being compensated by the simultaneous connection of input 24 to the output terminal 30. During manual operation, adjustment of switch 78a alters the charge on capacitor 74 and hence the output of the amplifier. Upon return to automatic operation the inputs 24, 52 are initially at the same potential due to the connections of the switch 72. Amplifier 26 As shown in Fig. 4, this comprises an oscillator, direct-coupled transistor amplifiers 314, 330, coupling circuits 336, 338 and a power amplifier 346. The capacitance of non-conducting diodes 306, 308 varies according to the D. C. input signal at 24, 52 and adjusts the amplitude of the oscillations to a value corresponding to this signal, this variation in amplitude controlling the value of D. C. in the final control circuit including the transistors 366, 368 which conduct alternately. Further transistors 388, 396 (normally non-conductive) operate to respectively prevent the amplifier output from falling below a minimum or rising above a maximum. Generation of set-point signal As shown in Fig. 2, a circuit 200 has first and second Zener diodes 206, 222, the latter providing a closely regulated voltage for setting the potential of the base of a transistor 212 and being energized through a pair of compensating diodes 224 and a fixed resistor 226 connected to the first diode 206. The method of calibrating the circuit is described.