Patent GB-700385-A (GB)
Title: Improvements in or relating to vehicle-carried equipment for cab signalling systems for railroads
IPC: B61L3/22
Filed: 1951-06-12
Abstract: 700,385. Railway cab-signalling systems. GENERAL RAILWAY SIGNAL CO. June 12, 1951 [June 30, 1950], No. 13917/51. Class 105. Cab signalling apparatus is operated by interruption-coded unidirectional current in the rails through an inductive oscillatory collector circuit having a damping resistance included such that there arises only a transient pulse at the beginning and end of a code pulse in the rails, these two transients being opposite in polarity ; the transients control an amplifying apparatus to operate a relay at the code rate and this in turn through de-coding means gives the appropriate signal in the cab. False signalling by stray steady current of commercial frequency is prevented by the amplifier including a shunt tuned to such frequency. The oscillatory collector circuit is moreover tuned to a frequency different from such commercial frequency. The rail current may be direct current or half or full-wave rectified alternating current. The receiver coils 60, 61 carried by the locomotive and disposed above the rails and inductively affected by the coded rail current pulses so that the effects in the two coils are additive are joined to a condenser 62 and parallel resistance (provided by two equal resistors 63, 64) to provide a critically damped oscillatory circuit. The effect of one pulse of the rail-current code is to give in the damped oscillatory circuit one transient pulse at the start of the rail code-pulse and a transient pulse of opposite polarity at the termination of the rail code-pulse. These two time-spaced opposite transients actuate through an amplifier arrangement the two windings of a relay CR having an armature magnetically retained in its lastoperated position. Thus a single pulse of the coded rail current closes first the front and then the back contacts of the relay CR. Accordingly the repeated pulses of the coded rail current cause the contacts of the relay CR to vibrate at code rate and thus its contacts 90, 91 energize the slow-dropping front and back repeater relays CRFP, CRBP while its contact 94 alternately energizes the two halves of a midtapped transformer winding the secondary of which feeds a circuit 96, 97 tuned to the " clear signal " rail code rate (180 pulses a minute). A decoding relay 180 R is fed from the circuit 96, 97 through a transformer coupling and fullwave rectifier 98. If 180 code is being received a " clear signal" lamp G is lit over a front contact 100 of the relay CRBP and a front contact 101 of the decoding relay 180 R. If a caution code is being received (75 pulses per minute) the contact 101 is on its back point and a " caution signal " lamp Y is lit. If no proper code is being received (alternate opposite transients per rail pulse) then the relay CR ceases to vibrate and the relay CRBP drops whereupon its back contact and back contact 104 of decoding relay 180 R light a " danger signal " lamp R. Any stray commercial frequency current picked up is shunted by an inductance-capacity line 83,82 or 85,84 before reaching the final tube 77 or 79 of the amplifying apparatus. The amplifying apparatus comprises tubes 69, 71 the grids of which are respectively connected to the resistances 63, 64 while their anode circuits respectively include resistances 74, 75. A transient which flows in the downward direction through the resistances 63, 64 increases the anode current of tube 69 and decreases the anode current of tube 71. The reduced current through resistance 75 raises the voltage applied (through a blocking condenser 78) to the grid of tube 79 normally biased to cut-off point which thereupon becomes conducting and briefly energizes the lower winding of the relay CR. The increased anode current of tube 69 lowers the voltage at the top end of the resistance 74 applied (through a blocking condenser 76) to the grid of tube 77 so that this is biased still further beyond cut-off and so no anode current arises in tube 77. A corresponding action occurs with the opposite transient but the upper winding of the relay CR is then the one briefly energized. Another amplifying arrangement, Fig. 4, uses a single tube 110 with its anode circuit having transformer couplings with two thyratron tubes 114, 115 the anode circuits of which are joined by a condenser 126 and include resistances 124, 125 respectively in the lines to the two windings of the relay CR. The two time-spaced opposite transients are amplified by the tube 110, the first transient igniting the thyratron 114 and the second transient igniting the thyratron 115 the brief current flow resulting from the change of charge on the condenser 126 causing a temporary voltage drop at the anode sufficient to extinguish the first thyratron. Each thyratron remains in action until the other one is ignited and thus one or other of the windings of the relay CR is always energized. A somewhat similar arrangement but using, instead of the thyratrons and condenser, ordinary vacuum tubes and with resistance connections to the grid of one tube from the anode of the other tube is described, Fig. 5 (not shown). For supplying the coded unidirectional rail current and for approach-lighting the wayside signals the system shown in Fig. 2 may be used. The trackcircuit current in a track section such as 5T is coded by a vibrating contact 40 of a relay 5CT supplied with interrupted current at a rate of 75 or 180 interruptions a minute according as the ahead section 6T is occupied or free as reflected by a relay 6H which is energized, if coded current is present in the track circuit of section 6T, by the track relay 6TRA vibrating its contact 42 to pulse the two halves of a midtapped transformer winding the secondary halves of which feed the relay 6H through a rectifying contact provided by the track relay 6TRA. If section 6T is occupied no current reaches the track relay 6TRA and the relay 6H is not energized ; a back contact 43 of the relay 6H then applies 75 rate code (given by the continuously operating coder 5-75CT) to the relay 5CT the vibrating contact 40 of which applies a 75-code to track section 5T from track battery 5TB. At the entrance end of section 5T this code vibrates the track relay 5TRA to energize the relay 5H steadily and at its vibrating contact 17 to energize a slow relay 5ATP and a relay 5CTA which during each offperiod of the rail code (as applied at the exit end of section 5T) applies a return-code pulse whereby at the exit end a slow relay 5AP is energized from an exit end track relay 5TRB. The duration of the return code pulse is determined by the drop away time of the relay 5ATP. Approach lighting of the wayside signal 6 for entrance to section 6T does not occur until the train has entered section 5T and cut off the return code to drop the relay 5AP a back contact of which then lights the appropriate lamp G, Y, R according to the ahead traffic conditions as reflected by the relay 6H and a relay 6D the latter being energized through a decoder 180 DU only if 180 code is applied in section 6T. In the case of a system in which the relays H are polarized relays controlled over line wires, Fig. 1 (not shown), the coders may be approach energized over a line-wire circuit.