GB816834A - Road traffic detection and control system - Google Patents

Abstract

816,834. Radar. EASTERN INDUSTRIES Inc. May 18, 1956 [May 31,1955], No. 15658/56. Class 40 (7). [Also in Group XXXVIII] A road-traffic detector system for controlling traffic signals comprises an ultra-high frequency directional antenna 15 disposed above the road and directed at a small angle to the vertical in the direction of approaching traffic 10, the antenna being associated with a Doppler radar system 30-37, the pass band of which for the detected beat frequency due to the Doppler shift is restricted to a low-frequency range and such that an output is obtained only in response to a vehicle passing through a zone substantially under the antenna where its velocity relative the antenna, and with it the Doppler shift, falls momentarily to zero. The particular embodiment described utilizes an antenna beam width of 30 degrees, the beam being directed at 15 degrees to the vertical. The frequency of operation is 2455 Mc/sec., and the pass band, which is determined by the combination of filters 31 and 33, is approximately 20 c/s wide centred on 30 c/s. The output signal, comprising a pulse for each vehicle, is detected at 35 and applied to actuate a relay 37 via an amplifier 36. The duration of each pulse is inversely proportional to the speed of the vehicle producing it and may provide information of speed to a suitable utilization circuit. A gain control following filter 33 controls the sensitivity of the system and hence the effective lateral coverage of the detector. By inclining the antenna so that one edge of the beam is substantially vertical, no pulse is produced in response to a detector leaving the detection zone. Reference is made to the use of a beam inclined away from the approaching traffic. Fig. 4 (not shown) illus. trates a detailed circuit for the system; the antenna comprises an in-phase array of eight dipoles arranged in two columns side-by-side and backed by a reflector plate, and the oscillator-detector 30 comprises a lighthouse-type triode. The remainder of the circuits of the system are conventional. Fig. 5 (not shown) illustrates a housing arranged for attachment to a support arm and having a lower part in the form of a radome to accommodate the antenna and an upper part to accommodate the associated circuits. The antenna and circuits, which are arranged to be removable from the housing as a unit, may be placed therein in either of two positions 180 degrees apart to allow for use with either left- or right-hand mountings. Locating means are provided to ensure correct positioning.