Patent GB-1164909-A (GB)
Title: Industrial Process Control Apparatus.
IPC: G05D7/06
Filed: 1967-10-19
Abstract: 1,164,909. Amplifiers. FOXBORO CO. 19 Oct., 1967 [24 Oct., 1966], No. 47724/67. Heading H3T. [Also in Division G3] A control system employs a multi-stage transistor amplifier having in the first stage a differential amplifier to one input of which the control signal is applied, this is followed by an intermediate stage and then an output stage from which negative feedback is taken to the second input of the differential amplifier. The invention is described in connection with a system for controlling the rate of flow of fluid in a pipe either manually or from measurement of the actual flow compared with a (variable) datum value. Means are provided for limiting the rate of change of the output signal when changing from manual to automatic operation or vice versa. Automatic operation.-In Fig. 1 the rate of flow of fluid in a pipe 14 is measured by a pressure-drop transducer 16 of the force-balance type (see Specification 875,785) to give a corresponding voltage signal across resistor 20. With switch 26 in the top position this signal is passed to a twin-Darlington differential amplifier 28, 30, 32 whose negative power line is returned via switch 40 to a potentiometer 42 which sets the zero level from which deviations of the transducer signal are measured. The differential amplifier output, limited by diode 52, is applied to a three-stage amplifier 50, 54, 56 which has negative feedback to the second stage via resistor 60. The output of transistor 56 is passed to a Darlington output stage 64,68 and thence to a transducer 72 which generates a corresponding pneumatic pressure in line 74 to actuate a valve 76 to control flow in the line 14. The output transistors 64, 68 have a separate power supply 86 to prevent load variations from affecting the earlier stages. The signal at the emitter of transistor 68 is fed back via line 104 to a capacitor 106, and any change in the signal causes this capacitor to charge or discharge through a network 107, 108, 110 which is coupled through switch 24D to the second input of the differential amplifier by line 112. The time-constant of this circuit is about 30 sec. and the output of the final amplifier will adjust itself to maintain the capacitor charge or discharge current nearly constant. Since 106 is an electrolytic capacitor a polarizing voltage is supplied through resistor 114 even when transistor 68 is cut off. Transistor 130 and Zener diode 134 limit the amplifier output by back-biasing transistor 64. Manual operation.-Switch 26 is put into its second position when a meter 116 is connected between the top of a potentiometer 118 and a feedback line 120 taken from the emitter of the output transistor 68. The potentiometer 118 is adjusted to correspond with the existing level and the switch 26 is then put into its fourth (bottom) position. The valve 76 is now controlled by the potentiometer 118 and the transducer 16 is disconnected. A smooth changeover is effected by this preliminary balancing and by a time-constant circuit 122 which prevents a rapid change in the input to the differential amplifier. To change back to automatic operation potentiometer 42 is adjusted for zero reading on meter 116, thereby ensuring that the voltage across resistor 20 corresponds to the setting of potentiometer 42. Capacitor 106 is connected to the junction of resistors 108 and 108A to charge it to a value producing zero voltage on line 112. Switch 26 has a third position (electrically identical to the fourth) to provide the same sequence of operations as for changing from automatic to manual.