GB1492841A - Analogue control devices - Google Patents

Abstract

1492841 Automatic control GEC-ELLIOTT AUTOMATION SA 2 Jan 1975 [4 Jan 1974] 54/75 Heading G3N An analog control device comprises an analog control circuit operable either in a first mode when controlled by a measured value-desired value difference signal or in a second mode when controlled by the analog output of a reversible digital to analog servo converter which has the output signal of the control circuit as its input in the first mode and a further signal as its input in the second mode. The output signal of the control circuit always provides the control signal to a controlled device. e.g. a valve. The mode of operation of control circuit CR is determined by switches 9, 10 which when put into position c provide an input to amplifier 11 from line 3 and also close switch 16 to provide a proportional plus integral characteristic. In the first mode, with switches 9, 10 as shown, the circuit responds to an error signal derived as the difference between a measured signal on line 2 and a desired signal on line 1 and the circuit provide a PID characteristic. The differential term may be applied to the error signal or alternatively only to the measured signal if the connection 17 is used. The output of control circuit CR is used to control a process regulating device, e.g. valve V, and also is the input to reversible servo-converter SC1 which consists of a closed loop comprising comparator 20, switch 21, up/down counter 22 and D/A converter 23. In the second mode the switches 9, 10 are changed to contact c and the switch 24 in the converter changed to terminal b so that the input to the circuit CR is the signal on line 3 which is the output signal of converter SC1. The converter has an alternative analog input Ea2 and alternative overriding digital inputs selected by switch 25 from an operator console or computer. A digital output may be displayed or used in the controlling computer. The value of the output signal from the control circuit can be stored as a digital number in converter SC1 and used in the second mode to maintain the output signal on line S. In the embodiment shown a similar converter SC2 is used to provide the desired signal either from an external source Ve or from the measured signal itself. The external source signal inputs are used when the control circuits are connected in series with inter-related inputs and outputs. Modifications are described where both the construction of the control circuit is changed and also the way in which the output signal from the converter is applied. Further modifications show further converters used in the feedback path to provide the differential term for the control arrangement.