Patent GB-736084-A (GB)
Title: Improvements in or relating to railway signalling systems
IPC: B61L3/22
Filed: 1952-07-22
Abstract: 736,084. Railway cabsignalling systems. GENERAL RAILWAY SIGNAL CO. July 22, 1952 [Aug. 13, 1951], No. 18493/ 52. Class 105. Cab-signalling apparatus is operated by interruption coded currents of different kinds of direct or alternating current applied to the rails through inductive receiver circuits designed to be shock excited by the commencement and termination of a code pulse to produce transient voltages at the resonant frequency of the receiver which are applied to an amplifier designed to have a maximum gain for a limited band of frequencies including only the principal frequency components of the transient voltages but substantially attenuating components of lower and higher frequencies, the output voltages of the amplifier being employed for establishing different code circuits in accordance with the repetition rate of said output voltages. As shown, the cab apparatus comprises a pair of receiver circuits each comprising coils 15, 16, Fig. 1A, arranged ahead of the leading wheels and axles of a train so as to be influenced inductively by the rail current flowing down one track rail through the axles and wheels of the train and back through the other track rail. Each receiver comprises a laminated core, Fig. 2, of inverted U-shape, with a coil 15 or 16 of insulated wire on each leg and so wound and connected in series that the voltages induced therein are cumulative for changes in magnetic flux passing in opposite directions through the coils, but are opposing for changes in flux in the same direction through the coils. These receiver circuits are tuned by condensers 17, Fig. 1A, connected across the coils. A resistor 18 provides damping of the oscillatory circuit to a degree that the transient voltage is of sufficient duration to utilize the frequency characteristics of the circuit, but subsides before the next transient voltage is produced. The voltages in the two receiver circuits are applied separately to respective amplifiers the outputs of which are combined so that when in time phase a pulse is produced which is used to operate a decoding relay. Thus the outputs from the two amplifiers on wires 45, 48 are applied to respective grids of a thyratron tube COT, Fig. 1B, which ignites when the two outputs are in time-phase and is self-extinguishing to produce for each transient voltage a series of unidirectional pulses in the output which are applied to a timing device comprising a diode D and-triode T arranged to give a single pulse output for each series of input pulses. The output pulses from the timing element are applied to operate a flip-flop circuit, e.g. of the Eccles- Jordan type, so that it takes up alternate stable conditions for successive input pulses. The output circuits of the triodes T1, T2 of this flip-flop circuit are applied to respective windings of a decoding master relay MR of the polar type to bring its armature and contact finger 60 to opposite positions to control the energization of the centre-tapped primary winding of a master decoding transformer TR generating alternating voltages in the secondary winding which have a periodicity of the code rate at which relay MR is operated. The secondary winding of the transformer TR feeds a plurality of resonant transformer units 75DU, 120DU and 180DU, each relating to one of the different code rates used, e.g. 75, 120, 180 interruption per minute, and feeding unidirectional energizing current to the corresponding code-responsive relay 75R, 120R or 180R. These relays control circuits for lighting suitable cab signal-indicating lamps R, G, Y/G and Y. The amplifiers in the two channels for the outputs of the receivers are similar. The shock-excited oscillatory circuit feeds the grid of the first valve V1 by a capacitor 19 and a grid leak resistor 20 forming a high-pass filter to attenuate grid potentials for the lower range of frequencies. The grid of valve V1 is biased by a resonant circuit 21, 22 in the cathode circuit and tuned to some frequency for which the. maximum degree of degeneration is desired, e.g. for stray foreign currents in the track rails at, say, 60 c/s.; (commercial power frequency) and is directly connected to its plate by a capacitor 23 to provide negative feedback for higher frequencies so as to exclude unwanted amplifier outputs for certain forms of extraneous influences and to obtain a more uniform amplifier output for different locations of the vehicle in the track circuit so as to compensate for intensity difference and wave-form variation of code pulses as the train passes from the entrance and to exit end of a track'section. An auxiliary resistor 24 supplements the grid leak resistor 20. The output variations in plate potential of valve V1 are supplied through capacitor 25 to the grid circuit of second stage valve V2, which circuit is the same as that of valve VI, with the high-pass filter 25, 26 and negative feedback capacitor 27 and auxiliary resistor 28. The grid of valve V2 is biased for " Class A " operation by resistor 29 preferably not by passed by a capacitor in order to retain the degenerative effect characteristic of the cathode resistor and make the gain of the amplifier more uniform for variations such as in replacement tubes or other practical operating, conditions. The variations in plate potential are supplied through capacitor 31 to a parallel connected inductor 32 and capacitor 33 providing plate circuit tuning at a prescribed frequency to provide the desired band pass. The output voltage across circuit 32, 33 is applied to grid circuit of valve V3 through a potentiometer 34 for regulating the gain of the amplifier, which together with a similar resistor 35 in the other amplifier adjust the respective gains to obtain absolute and comparative levels for the control of the thyratron COT. Stage V3 is similar to stage V2 and the output generated across tuned circuit 40-41 is applied to the control grid of amplifying valve V4 having grid biasing cathode resistor 44. The A.C. output of valve V4 on line 45 is applied to one of the control grids of the thyratron COT through a grid-current limiting resistor 50, Fig. 1B, and the corresponding output from the last stage 47 of the other amplifier on line 48 is applied through limiting resistor 52 to the other control grid, the negative grid biasing voltage being chosen so that the grid will assume its critical potential for firing the tube subject to the existing control by the other grid when both transient A.C. voltage outputs from the amplifiers are in time phase. Resistance load 54 in the plate circuit and the capacitor 55 render the tube COT self-extinguishing. When this tube is conductive, the capacity 58 coupling it to the cathode of the diode D of the timing device, discharges through the thyratron COT and a voltage is developed across the cathode resistor 59 of the diode, rendering the diode conductive thereby swinging the grid of the triode T abruptly negative beyond cut-off while the capacitor 57 is charging and providing an impulse in the form of a sharp rise in the potential of the plate of triode T as it is rendered non-conductive. When thyratron COT extinguishes, further charging of capacitor 58 takes place and conduction through the diode D ceases. The capacitor 57 discharges slowly through the plate resistor 56 of the diode so that the diode is maintained non-conductive for a predetermined time after the initial spring of the thyratron thereby providing a single effective output during a transient period. The receiver windings are so mounted that they vibrate mechanically only at frequencies higher than the electrical resonant frequency of the windings so that the signals induced in the windings by the relative motion between the windings and extraneous magnetic fields are of frequencies that are not transmitted to the decoding means. Specification 700,385 is referred to.