Patent GB-823149-A (GB)
Title: Improvements in or relating to railway signalling systems
IPC: B61L3/12
Filed: 1955-05-24
Abstract: 823,149. Electric tests. SIEMENS & GENERAL ELECTRIC RAILWAY SIGNAL CO. Ltd., HILBOURNE, R. A., and JONES, D. D. May 23, 1956 [May 24, 1955], No. 14982/55. Class 37. [Also in Group XXX] In a system for signalling between the railway track and a vehicle on the track (see Group XXX), a signalling receiver is responsive to the electric signal supplied by a " Hall effect " device arranged so that its output is modified when the vehicle passes a signalling position along the track. As described, the Hall effect device is carried on the vehicle and the signal output is modified by the passage of the device over magnetic fields arranged at the signalling positions on the track, and the signals received are employed to give an indication corresponding to the track signals or to effect a braking of the vehicle. As shown, an oscillation generator 3 supplies the Hall effect device 2, e.g. using a strip of germanium, through terminals 33, 34 with oscillatory current through a transformer 4, the output signal current being taken from the terminals 5, 6 through a resistance 8, from a tapping 9 on which the signal is applied to an amplifier transistor 12. Partial compensation for the residual voltage between terminals 5, 6 is provided by the compensating network 7 so as to provide a compensating voltage, from the tapping on resistance 31, in series with output voltage on terminals 5, 6, the amplitude and phase of this compensating voltage being adjusted by the tappings on resistances 31 and 32. The output from the amplifier 12 is fed through a transformer 13 having two secondary windings 15, 16 to provide two outputs in phase opposition, each feeding into respective phase detectors 17, 18 to determine whether the output signal passed by the amplifier 11 is in-phase or in anti-phase with the output signal from the generator 3. To this end a voltage derived from generator 3 in the secondary winding 37 of transformer 4 is applied to the collector electrodes of the junction transistors 19, 26. Due to the rectifier action between the base and emitter electrodes of the transistor, current flows through the base electrode during alternate half cycles, so that for the portion of the half cycle during which negative going pulses are applied to the collector electrodes, concurrently with the current due to the rectifier action, a negative current pulse is fed to a capacitor 21 serving to smooth out the output from the phase-sensitive detector. The voltage across the capacitor 21 is amplified in a D.C. junction transistor amplifier 22 and fed to the operating winding of an electromagnetic relay 24. When the device 1 passes between the poles of a permanent magnet at a signalling position on the track, the operating winding 24 is energized and contacts 25 closed in a control circuit to apply the train brakes or otherwise give a visual or audible signal if the contacts 29 are not closed within a predetermined time, as when the device subsequently passes through a magnetic field of opposite sense produced, e.g. by an electromagnet to effect energization of the relay 28, thereby indicating that the railway signal is in the clear aspect. To eliminate the small output of the device 1 in the absence of a magnetic field, due to rectifier action at the terminals 33, 34, a resistor 35 of approximately the same value as the A.C. resistance of the strip 2 is inserted in the feed circuit. To render the phase detectors 17, 18 insensitive to small changes in phase, a direct current source 36 is connected in series with the secondary winding 37 and the resultant voltage is limited by two rectifier elements 38, 39 to limit the voltage pulses supplied to the collector electrodes of transistors 19, 26 so that they each occur for a period of time appreciably less than a half cycle.