GB577405A - Improvements relating to electric signalling systems for railways - Google Patents

Abstract

577,405. Apparatus for detecting passage of rail wheels along a track. SIEMENS & GENERAL ELECTRIC RAILWAY SIGNAL CO., Ltd., and RIDDLE, H. J. N. May 12, 1944, No. 9154. [Class 105] The passage of a railway wheel along a track rail is detected by an arrangement of two inductances at least one of which has its characteristics modified by the presence of a wheel whereby either such inductance is brought into balance with the other inductance so that an associated relay energized when the inductances are out of balance (as by an inductance-bridge arrangement) becomes de-energized, or the modification of the flux from one inductance (energized by oscillating current) which in its normal distribution causes an e.m.f. in the other inductance, which other inductance in this case is situated at the same side of the track rail, is such that an e.m.f. is no longer induced in the said other inductance whereby a relay controlled from said other inductance becomes de-energized. As shown an inductance A is energized by oscillating current of audio frequency from an oscillator OSC and acts on an inductance B which has its axis arranged at right angles with its mid point offset from the axis of inductance A. The flux from inductance A being asymmetric relative to inductance B causes an e.m.f. in the latter which is applied to the control grid of a valve V1. The grid has a negative bias but the positive half-waves of applied voltage give a pulsating anode current which, amplified if necessary by a valve V<2>, energizes a transformer OT feeding an axle-counting relay TRA through a rectifier bridge MP. The anode circuits may include tuning condensers. Two such arrangements may be provided immediately adjacent to give a directional counting, for example a second inductance C and associated relay TRB may be provided for the other end of the energized inductance A. The two arrangements may be associated with the opposite rails of the track to avoid operation by stray iron such as a tool. One arrangement for the balancing inductance form of apparatus comprises a split transformer primary having the two inductances inserted in the split primary half-circuits and acting in opposition. Normally the inductances are unbalanced and an e.m.f. is induced in the secondary winding but the presence of a wheel brings the inductances into balance and thereby prevents an e.m.f. being induced in the secondary winding. Energizing current may be switched on only when a train is approaching.