GB642495A - Self checking means for apparatus for comparing the impedance of two electric circuits - Google Patents

Abstract

642,495. Indicating presence of railway vehicles. WESTINGHOUSE BRAKE & SIGNAL CO., Ltd. July 21, 1948, No. 19499. Convention date, Sept. 6, 1947. [Class 105] [Also in Group XXXVI] A relay in the output diagonal of a Wheatstone bridge and released when the bridge is balanced, is arranged so that when released it causes the bridge to become unbalanced so that the relay is then again energized, whereby the unbalancing is removed to drop the relay again, these actions being repeated and the consequent periodic actuation being shown by an indicator. The invention is described with reference to detecting the presence of a vehicle on a stretch of railway track. A loop L, the inductance of which is altered by the presence of a vehicle, is arranged in an A.C. bridge circuit 1, 2, 3, 4 whose output is amplified at V3, V4, V6 and applied to gas-filled valve V5 whose anode circuit is fed with A.C. and includes a direct current relay CF. The bridge is balanced only when arm 4-2 includes a resistor R1 placed in circuit by a front contact b of the relay CF. Accordingly, when relay CF is energized, the bridge becomes balanced to cause the relay CF to drop and thereby to unbalance the bridge to re-operate the relay CF which thus continues in periodic operation. The periodic operation of relay CF controls slow release relays FP, CD to light lamp G. With a vehicle present in the loop L, the bridge remains permanently unbalanced and relay CF thus remains actuated so that while relay FP remains up the relay CD drops, thus lighting lamp R. A fault produced by ageing of valve V5 results in relay CF remaining unenergized and the lighting of lamp R. The resistance R1 is such that the periodic change of potential in the bridge arm is less than that arising on the minimum change of loop impedance required to be detected. The relay CF is made to respond to this smaller change, arising from the periodic action of R1, by adjustment of the cathode voltage on the valve V5. In a modification, the resistance R1 is connected between point 1, 4 instead of 4, 2, Fig. 2 (not shown), but over a back contact of the relay CF and giving an unbalancing effect. The rails 1a, 1b may be connected by low resistances or condensers to reduce leakage of any current induced in the rails through the ballast. The invention is applicable to the detection of movable objects on a road or an airfield..