GB603169A - Vehicle detector - Google Patents

Abstract

603,169. Radiolocation. AUTOMATIC SIGNAL CORPORATION. June 21, 1945, No. 15863. Convention date, May 17, 1944. [Class 40 (vii)] [Also in Groups XXX and XXXVIII] In a traffic detection system frequencymodulated ultra - high - frequency waves are directed along a traffic lane and waves reflected from a vehicle in the lane are combined with the transmitted waves to produce a beat frequency characteristic of the distance of the vehicle from the transmitter, the beat frequency being passed to a frequency selective device responsive to the frequency or frequency range characteristic of a vehicle at a predetermined distance or within a predetermined zone. As described, ultra-high-frequency waves are generated by a klystron or like tube 23, Fig. 2, comprising a heater 27, cathode 28, control electrode 29, focusing electrodes 31, 32, accelerating electrodes 38, 39 and a collecting electrode 50, the tube being jacketed in a cavity resonator 24. The ultra - high - frequency waves are velocity modulated by a 60-cycle A.C. power supply 49 applied to the accelerating voltage supply 42. The frequency-modulated waves are collected from the resonator 24 by a lead 22 and transmitted along a traffic lane by an antenna 21 via a wave-director 17. Reflected waves from a vehicle are received on the antenna 21 and combined with the transmitted waves in the resonator 24. The beat frequency, characteristic of the vehicle distance, is demodulated by a detector 68, amplified by an automatic volume control type amplifier 75, passed through tuned circuits 92, 87 and 96, 88 and the rectified output used to operate a relay or indicator-meter 99. The resonant frequency of the cavity resonator 24 is varied synchronously with the velocity modulation of the electron stream by means of a cam-operated plunger 57. A second synchronously driven cam 62 prevents beat frequency currents from reaching the indicator or like during the periods when the modulating wave is changing direction. By thus suppressing the peaks of the modulation, an approximately linear relation between vehicle distance and beat frequency is obtained and false beat frequencies which may occur around the positive and negative modulation peaks are suppressed. Switches 82, 85 in their upper position disconnect the filter circuits 92, 87 and 88, 96 and allow the indicator 99 to respond to frequencies within the pass-band of the amplifier 75 and hence to respond to the presence of a vehicle within a wide zone. To allow vehicles at several points or in several zones to be detected, the circuit subsequent to the amplifier 75 may be duplicated in parallel (Fig. 3, not shown). The speed and direction of travel of a vehicle travelling in a detection zone may be shown on a meter (Fig. 4, not shown) by measuring the rate of change of beat frequency and the direction of change respectively.