Patent GB-620028-A (GB)
Title: Improvements relating to electric control systems
IPC: G05D3/12
Filed: 1944-12-14
Abstract: 620,028. Electric control systems. BRITISH THOMSON - HOUSTON CO., Ltd., and WHITELEY, A. L. Dec. 14, 1944, No. 25140. [Class 40 (i)] An automatic or correspondence electric control system in which.an initial error voltage is changed into an error voltage which decreases with time exponentially to a value which is proportional to the initial error, comprises in cases where the error voltage is an alternating one, means for connecting, alternately in opposite directions and synchronously with the frequency. of the error voltage, a condenser in shunt with an impedance. The voltage of the condenser produces across a series-connected resistance a modified error voltage which is used to restore positional correspondence between the controlling and controlled devices without a tendency to part. In the known system, shown in Fig. 1, motor-driven shaft 2 follows the movements of shaft 1 and toroidal rheostats 3 and 4 are driven by shafts 1 and 2 respectively and are fed from D.C. supply lines 5 and 6. With both shafts initially in the rest position, a sudden turn of shaft 1 will produce the voltage impulse A, Fig. 2, between the lines 9 and 10 to cause an exponential rise of voltage across condenser 12. The potential difference across resistor 13 will then be an exponential decay, curve B, Fig. 2, falling to a steady minimum dependent on the ohmic values of resistors 11 and 13 and the relative displacement of shafts 1 and 2, and is applied to the grid of valve 14 through leads 9a and 10. An increase in the potential of 9a in response to a movement of rheostat 3 causes a fall in potential at the anode of 14 and a rise at the anode of 15, these changes being transferred by leads 14a and 15a respectively to the grids of valves 16 and 17 to produce a change in the direction and strength of the magnetization in coil 20a, the shunt field of the motor controlling shaft 2. The resultant rotation of 2 is directed to follow the motion of 1 and to neutralize the relative displacement of the shafts. In the system according to the invention, Fig. 3, the method is shown adapted to an A.C. system in which 1 and 2 are the controlling and controlled shafts respectively, and 3a, 4a are the rotors of selsyns 3 and 4. Shaft 2 is controlled by two-phase induction motor 11, one phase of which, 13, is fed from a source of the same frequency as the error signal and the second phase 12 is fed by transformer 6 in the anode circuit of valve 5. Lack of positional correspondence between the rotors produces an alternating potential in 4a, substantially proportional to the relation displacement, which is applied to the circuit in dotted lines in series with the resistor 16 and the voltage developed across the latter is applied to the grid of triode 5. The condenser 21, 22 is connected alternately in opposite directions and synchronously with the frequency of the error voltage across resistor 15 by means of rectifiers 17, 18, 19, 20 and transformer 23 which supplies an alternating potential of the same frequency as the error signal. The voltage across the condenser 21, 22 is shown in Fig. 5, curve B, and the resultant voltage across resistor 16, as applied to valve 5 is indicated by curve C. Current flow in phase 12 of induction motor 11 will be of same form as curve C and will produce a corresponding motion of shaft 12 until relative displacement of the rotors 3a and 4a is reduced to zero, when the current in phase 12 vanishes. At this point only phase 13 is excited and hence induction motor 11 produces no torque until shaft 1 is again rotated. Specifications 395,743, [Group XXXV], 576,117 and 581,135 are referred to.