Patent GB-684251-A (GB)
Title: Improvements relating to train or vehicle describers
IPC: B61L11/08
Filed: 1950-03-21
Abstract: 684,251. Train describer and points setting apparatus controlled by interacting train and track apparatus. UNION SWITCH & SIGNAL CO. March 21, 1950. [April 30, 1949; July 16, 1949] No. 7015/50. Classes 104 (i) and 105. A train describer to indicate at a station the route of an approaching train is controlled by the interaction of a train-carried coil, tuned to an appropriate frequency denoting the particular route. with a trackway coil which forms one arm of a Wheatstone bridge arrangement supplied with currents of a plurality of frequencies each frequency corresponding to a particular one of the possible routes. The Wheatstone bridge is normally balanced but when the train carried coil interacts with the one arm thereof there arises across the detecting points of the bridge a potential difference which is of the frequency to which the train-carried coil is tuned. The multiple frequencies may be obtained from the harmonics of a square waveform. The arrangement may also be used to control the setting up of the route. The apparatus may be arranged so as to guard against false operation by the passage of the metallic mass of the vehicle. As shown in Fig. 1 a train-carried coil VC is tuned to an appropriate one of a plurality of possible frequencies by switching in the appropriate condenser such as 46, each frequency being assigned to a particular route. As the train passes a trackway coil TC forming one arm AI of a Wheatstone bridge A1 ... A4 fed through filters 17, 19 and transformers T1 T2 from a plurality of oscillators OSC1, OSC2 each supplying one of the different frequencies assigned to the various routes there arises across the detector points of the Wheatstone bridge a potential difference of the frequency to which the train-carried coil has been set, this being due to the variation in the load drawn from the oscillators by the trackway coil TC when the train-carried coil VC passes thereover. The detected voltage is applied through tuned transformers and amplifiers T3, AM1, T5 or T4, AM2, T6 to the grid of a gaseous discharge tube GT1 or GT2 which is thus fired and the anode current actuates a relay R1 or R2 whereby a lamp D1 or D2 is lit to illuminate the appropriate sign on the train describer panel. The relays R1, R2 may be used to set the points for the appropriate route. The plurality of oscillators may be replaced by a single oscillator giving a square wave-form the harmonics of which are used to supply the multiple frequencies, Fig. 2, (not shown). To discriminate against the metallic mass of the vehicle causing a change of impedance of the trackway coil and so giving an incorrect train description the arrangement shown in Fig. 3 is used. The voltage across the detection points of the Wheatstone bridge differs in phase according to whether it arises from the reaction of the tuned train-carried coil or from the eddy current reaction of the metallic mass of the vehicle and this phase difference is checked against a reference voltage applied from the oscillators OSC1, OSC2 to the grids of the gaseous discharge tubes GT1, GT2. Thus a reference voltage from oscillator OSC1 is applied by transformer T7 to a circuit of rectifiers and resistances K1, 70, 72, K2, the mid point 74 of the resistance section being applied to the grid of gaseous discharge tube GT1 and to earth through a condenser 76 and the mid point 64 of the transformer being applied to earth through a resistance 68. As the points 64 and 74 are at the same potential no current flows in the line from 64 to resistance 68, earth, condenser 76 to point 71 until the potential arising at the detection points of the Wheatstone bridge is applied to the resistance 68 whereupon if the relative phase is correct the current through the resistance 68 will raise the potential of point 64 to cause positive pulses to flow to condenser 76 and so fire the gaseous discharge tube. Instead of the train description being given by a single frequency response the train description may be given by a code group of frequency responses. The train description is cancelled by an operator at the station ST depressing a push 31 or by the departing train causing a contact 29 of a track relay TR to open, either action opening a point in the anode circuit of the gaseous discharge tube.