Patent GB-581135-A (GB)
Title: Improvements in and relating to electric control systems
IPC: G05D3/12
Filed: 1942-06-19
Abstract: 581,135. Automatic control systems. BRITISH THOMSON-HOUSTON CO., Ltd., and WHITELEY, A. L. June 19, 1942, No. 8479. [Class 38 (iv)] [Also in Group XXXVIII] In an automatic control system or a, positional correspondence united system in which the predetermined value or correspondence is restored under the control of an A.C. voltage proportional to the error and changing in phase as the sign of the error changes, a component proportional to the first time derivative or the time integral of the error is introduced by demodulating a portion of the error voltage, differentiating or integrating the resultant D.C. which is then used to modulate A.C. for addition to the error A.C. The second time derivative may also be used. The demodulation and modulation is effected by ring bridges of known type fed with voltage of the same frequency as the error voltage. The invention is described in detail with reference to correspondence control systems only. In the resultant field system shown in Fig. 1, comprising transmitter 1 and receiver 2, the deviation voltage at the receiver is applied to amplifier 3 controlling rectifiers 6, 7 supplying the field winding of generation 11 of a Ward- Leonard system which supplies the driving motor, these arrangements being similar to those described in Specification 576,117, [Group XXXVIII]. A portion of the deviation voltage is applied to rectifier bridge 15 fed also by the source of supply and the resultant D.C. depending in intensity and polarity on the deviation, after rough smoothing at 16, 17, is applied to valve 18, preferably a pentode, so that the voltage across inductance 19 in the anode circuit which is proportional to the rate of change of anode current, is proportional to the rate of change of the input voltage. This voltage is applied over a secondary winding 20 to a modulator bridge 21 to provide A.C. of corresponding value which is injected in the input circuit of control valve 3. The second differential is obtained by using a second valve, similar to 18 in cascade. In order to compensate for the loading of inductance 19 by the modulator bridge, the anode transformer may have a third winding feeding back preferably to a series condenser-shunt resistance combination in the input circuit. The integral of the deviation voltage may be obtained as shown in Fig. 4 in which the main control path is the same as that shown in Fig. 1. The deviation voltage is again applied to a demodulating bridge 26 and the resultant D.C. is applied to a series resistance-shunt condenser integrating combination 27, 28 in the input to valve 29. The output of valve 29 taken from resistance network 30-32 modulates the A.C. supply frequency on bridge 33 and the resultant A.C. is injected into the input circuit control of valve 3. The circuit so far described provides a rough approximation to integration, but an improved result may be obtained by feeding back the A.C. output of bridge 33 over transformer 36 to the input side of the demodulating bridge 26, or, alternatively, as in Fig. 5 (not shown), by feeding back into a separate demodulating bridge which provides the voltage across the integrating condenser.