GB673505A - Vehicle detector - Google Patents

Abstract

673,505. Traffic indicating apparatus; bridge circuits. TELEFONAKTIEBOLAGET L. M. ERICSSON. May 23, 1950 [May 23, 1949], No. 12924/50. Class 37. [Also in Group XXXVIII] In highway traffic indicating and control apparatus of the kind comprising A.C. fed detectors which co-operate to produce a differential voltage in the presence of a vehicle to actuate an indicator a compensating bridge circuit is included which functions to produce a delayed counter voltage nullifying the differential voltage produced by a stationary vehicle, e.g. a vehicle that cannot proceed due to breakdown. In Fig. 2 a detector D1 comprising a pair of inductively coupled coils D11, D12 is spaced from a detector D2 comprising similarly a pair of inductively coupled coils D21, D22 and both are laid in the highway so that vehicles in a line pass across them. The primary coils D11, D21 are in series and energized from a supply transformer T1. The secondary coils D12, D22 are in series opposing so that with no traffic there is zero voltage across output lines a1, b. If for example a vehicle encounters firstly D1 and secondly D2 voltages opposite in phase appear successively across a1 b and operate a polarized triple-throw relay R1 via a transformer T3 and phase-sensitive amplifier F1 to close firstly for a short interval relay contacts 11/12 and secondly for a short interval relay contacts 11/13 and actuate an indicator via suitable relay arrangements to show both the passing and the direction of passing of the vehicle. A normally balanced bridge comprising equal transformer secondaries S1, S2 within a transformer T2 and equal temperature variable resistors N11, N21 is interposed in series between output line al and transformer T3. A transformer T4 has as its load two heating resistances arranged so that N12 heats N11 and N22 heats N21. In parallel with F1 there is a further amplifier F2 controlling a double-throw relay R2 whose contacts 21/23 when closed short out heating resistor N22 and whose contacts 21/22 when closed short out heating resistor N12. If, for example, a vehicle breaks down over detector D1 there will be applied to T3 a voltage which operates R1 to close its contacts 11/12 and operates R2 to close its contacts 21/23 so that N11 is heated and N22 cools. After a short duration of the latter the opposing voltage thereby produced across T3 will effect release of R1. After a further short interval the opposing voltage effects release of R2 so that N22 is then heated and N11 cools with closure of R2 contacts 21/22. In the operation of the equipment R2 will reverse regularly to maintain as far as possible N11 and N21 at equal temperatures and prevent any prolonged operation of R1 other than for passing vehicles. A resistor M in series with the primary of the bridge transformer T2 produces the phase opposition character of the compensating voltage.