Patent GB-1039163-A (GB)
Title: Improvements in control systems
Inventor: GILCHRIST EDGAR SHEPPARD
IPC: G01L1/22
Filed: 1963-05-28
Abstract: 1,039,163. Automatic control. CONSOLIDATED ELECTRODYNAMICS CORPORATION. May 28, 1963,No. 21213/63. Heading G3R. In a control system which operates a recorder, a square wave signal having an amplitude proportional to a variable to be recorded is applied through an amplifier and demodulator to control, electromagnetically, the shaft of the recorder which is coupled to a device which applies a square wave negative positional feedback signal to the amplifier input. Deviations of a variable such as temperature, pressure, liquid level or humidity are detected by a thermocouple or other suitable transducer such as strain gauge D.C. bridge 10 output from terminals 13, 14 of which is converted into a square wave signal by switching transistors 63, 65 controlled by the output of a square wave signal generator 80 comprising transistors 140,142 associated with an output transformer 82. The square wave signal so produced is amplified by an A.C. transistor amplifier 95 and converted into a D.C. signal by a switching transistor 97 also controlled by square wave generator 80. The D.C. signal is then amplified by a further transistor 101 the output from which, at a terminal 104, is opposed to a D.C. reference voltage from a potentiometer 50 which serves to set the desired value of the variable. Any difference, representing the error, is transformed into a square wave signal by a balanceable modulator 218 comprising semi-conductor diodes 214, 216 alternately rendered conductive by an output from square wave generator 80 applied to a transformer input winding 219. The resulting square wave error signal is then amplified by a transistor amplifier 233 and converted into D.C.by switching transistors 245, 246 also controlled from generator 80. Finally the D.C. error signal is amplified by transistors 262, 266 and 271 before application to control means, such as a valve positioner connected to terminals 273, which restore the variable to the desired value. Feedback and bias signals. The D.C. output from transistor 101 at terminal 104 is converted to a square wave signal by transistors 110, 111, controlled by generator 80, and fed back, together with a square wave fixed bias signal from resistor 122, through a transformer 70 to the input of amplifier 95. A constant direct voltage across a Zener diode 41, converted into a square wave voltage by transistors 153, 154 controlled by generator 80, is applied through an adjustable ratio transformer 162 and rectifier 172 to provide D.C. bias for combination with the output of bridge 10. Resistor 166 is temperature responsive to provide temperature compensation. The D.C. error signal applied to transistor 271 is also applied to transistor 280 the output of which is transferred in two stages to condensers 295,296 by pairs of transistors 291, 293 and 290, 292 which are rendered conductive in succession by generator 80. This provides the necessary isolation between transistor 280 and a network including condensers 322, 330 which feeds back a signal to the input of modulator 218 to provide differential and integral components of control. Power supply. The output of a full-wave rectifier 361 which is automatically controlled by a regulator including a series transistor 365, a Zener diode 380 and a current limiting transistor 382 provides the necessary power supply. The output is applied to bridge 10 and reference potentiometer 50 in series so that the error voltage is independent of changes in supply voltage which may occur despite the automatic regulation. The current in this series circuit is further stabilized by a transistor 42. Manual control. The variable may be controlled manually by changing over a switch 270 from the position shown and using a potentiometer 343 to control the input to transistor 271. Smooth changeover is ensured by adjusting the potentiometer until meter 350 indicates equality before changeover. A signal is applied to transistor 280 during manual control, and the network including condensers 322, 330 is modified by a switch 347 so that the signal applied to the input of modulator 218 is modified to keep the output of transistor 266 in readiness for smooth reversion to automatic control. Indicating and recording. The square wave output of transistor amplifier 101 at a terminal 131 is compared with a constant square wave signal derived through a transformer 180 from the transistors 153, 154 and if the difference is excessive an alarm 189 is operated through an amplifier 187 and a demodulator 188. Noise pick-up is minimised by electrostatic and magnetic screening e.g. at 92 and filters 60, 107 and 212 are associated with the D.C. signals. The square wave control signal at terminal 131 is also applied to a transistor amplifier 400 to operate, through a demodulator 401, a recording pen motor 402. Positional follow-up square wave signals are applied to the input of amplifier 400 from an inductive device 403 coupled to and arranged in a unitary structure with the pen motor.