GB1390606A - Vehilce detector systems - Google Patents

Abstract

1390606 Detecting vehicles; controlling traffic signals PLESSEY CO Ltd 3 Aug 1972 [2 June 1971] 18562/71 Heading G4Q A vehicle detection system, which may be used for controlling traffic signals, comprises a conductor 1 which can be supplied with pulse signals from a pulse generator 2, and a detector which is arranged to sense signals induced in the conductor 1 due to the decay of a magnetic field produced in the vicinity of the conductor 1 and to produce an output signal when a vehicle is in the vicinity of the conductor 1. Conductor 1, which normally is buried in the road, can be laid diagonally across the traffic flow to facilitate the detection of bicycles. The pulses are fed to the loop via an amplifier 3. If conductive metal objects are present in the vicinity of the loop, signals induced in the loop as a result of eddy currents decaying in the objects are sensed by preamplifier 4 whose input is protected from large current pulses by diodes 6, 7 or a Zener diode. The output signals from amplifier 4 are sampled to establish a reference signal which takes account of low frequency loop noise, and a signal which, when compared in comparator 8 with the reference signal, provides an indication of the presence or absence of a vehicle moving through the vicinity of the loop. An averager 16 which provides an output signal which only registers signal increases relative to an ambient signal lever prevents vehicles which are parked in the loop vicinity from being taken into account. A number of loops, each associated with its own detector circuit, may be fed from a single pulse generator and drive amplifier (Fig. 3, not shown).