Patent GB-980343-A (GB)
Title: Improvements in or relating to road traffic signalling systems
IPC: G08G1/01
Filed: 1960-12-06
Abstract: 980, 343. Road signals for controlling traffic. LABORATORY FOR ELECTRONICS Inc. Dec. 6, 1960 [Dec. 7, 1959], No. 42039/60. Heading G4Q. [Also in Divisions H3 and H4] In a vehicle detection system utilizing the Doppler principle the Doppler signal is employed to control a multivibrator such that output pulses of number proportional to the duration and/or amplitude of the Doppler signal are produced. The pulses so produced operate a relay which is employed to control traffic signals &c. in accordance with traffic flow, the latter being deduced by proportioning the time-constant circuits of the multivibrator such that a single vehicle of average length (e.g. a passenger motor car) travelling at average speed produces two pulsings of the relay whilst a number of closely spaced such vehicles or a vehicle of greater length (e.g. a truck with trailer &c.) or a number of average length vehicles side-by-side produce a larger number of pulses which may be arranged to occur at the rate of two pulses for each equivalent vehicle of average length. In the case of a number of vehicles sideby-side, which produce an increased amplitude Doppler signal, the pulse repetition rate is increased so that, generally, the number of pulses in a given time is proportional to the traffic density. The system may employ either an ultra-sonic or a c.w. radio Doppler detection arrangement the embodiment described being an example of the latter. As shown in Figs. 4 and 5, the Doppler frequency is generated by an oscillator detector system 41 operating on e.g. 2455 mc/s and directing a beam of width sufficient to cover the desired number of traffic lanes from a di-pole radiator 21 Figs. 1, 2 and 4, (which may comprise an array such as 60 Fig. 5) angled at about 30 degrees to the horizontal. A H.P. filter 42 and a L.P. filter 44 select a frequency band of about 30 c.p.s. centered on about 25 c.p.s. from the Doppler beat frequency which, after amplification in 45 and rectification in 48 is supplied as a positive D.C. control signal via resistor 146 (Fig. 5) to control the multivibrator 49 comprising valve section 165 (normally conducting in the absence of a signal on resistor 146) and valve section 155 (normally cut-off). When a vehicle is first detected the increasing D.C. signal through resistor 146 is effective, via capacitor 179 and diode 182, on the grid of section 155 to cause the latter to conduct, section 165 to become non- conducting and relay 200 to operate to close contacts 199. When the D.C. signal becomes stablized (or capacitor 179 fully charged) but before capacitor 147 has charged significantly, the potential on grid 157 of section 155 falls to approximately ground level thus reversing the condition of the two sections and causing relay 200 to release contacts 199. With continued application of the D.C. signal however capacitor 147 becomes charged to an amount sufficient to cause section 155 to again conduct and section 165 to cut-off so that a second operation of relay 200 results. The multivibrator continues to operate in this manner until the D.C. control signal terminates. If such termination occurs when section 165 is conducting the reduction of the charge on capacitor 178 is effective as a negative pulse via capacitor 177 and diode 171 to cause section 165 to become non-conducting which results in a further operation of relay 200 (as section 155 conducts) after which, as capacitor 147 discharges, the grid of section 155 returns to ground potential and relay 200 is de-energized. Thus, detection of a single vehicle (e.g. a motor car) resulting in a short duration D.C. signal, of average amplitude, produces at least two pulse operations of relay 200. Larger vehicles, e.g. trucks or buses, in addition to producing a greater number of pulses due to their greater length also cause such pulses to occur at a more rapid rate since such vehicles return a Doppler signal of greater amplitude and thus capacitor 147 charges in a shorter period so that the rate of oscillation of the multivibrator is increased. Specification 816,834 is referred to.