GB919420A - Improvements in or relating to apparatus for detecting the presence of a railway train - Google Patents

Abstract

919,420. Train detectors. PHILIPS ELECTRICAL INDUSTRIES Ltd. May 10, 1960 [May 11, 1959], No. 16501/60. Class 105. The presence or absence of a train on an insulated rail section R 1 , R 2 is detected by apparatus comprising an A.C. supply source WB connected through the rails to a transformer T whose output is rectified, smoothed and controls the frequency of a generator G 1 which forms part of an indicator circuit. A mixing stage M mixes the output of G 1 with that of a stable generator G 3 , the output then being fed via a low-pass filter F 1 and a rectifier K 2 to the gate circuit P 1 which is included with the gate circuit P 2 in the energizing circuit of the relay VR. The relay H is energized by the output voltage of the filter F 1 so that the contact h is closed. The output voltage of the mixing stage M and the voltage of the generator G 2 are compared by the discriminator D 2 which supplies via the delay device V1 a control voltage to stabilize the frequency of the generator G 1 . The indicator is thus not sensitive to slow variations of P.D. between the track rails, but abrupt short-circuiting of the rails by a train entering the section causes the frequency output of the mixing stage M to rise rapidly to a value higher than the cut-off frequency of the low-pass filter F 1 de-energizing the relay H, thus increasing the frequency of the generator G 2 and blocking the gate circuit P 1 , de-energizing the indicating relay R 1 . Similarly abrupt deshorting of the tracks when a train leaves the section causes the relay h 1 to energize. For a D.C. supply source between the rails a pulse from a discriminator (Fig. 1, not shown) is fed to a bi-stable triggger circuit which controls a gate circuit to energize the indicating relay. In a further modification (Fig. 4, not shown) two absolute frequency discriminators tuned to two different frequencies are used alternately, the change over being effected by blocking or unblocking of gate circuits when a train enters or leaves the section.