Patent GB-1017999-A (GB)
Title: Traffic density measuring apparatus
IPC: G06G7/184
Filed: 1962-09-11
Abstract: 1,017,999. Road traffic measurements; electric analogue calculating. LABORATORY FOR ELECTRONICS Inc. Sept. 11, 1962 [Sept. 11, 1961], No. 34760/62. Headings G4G and G4Q. [Also in Division H3] Traffic density is calculated by dividing a signal represent ing average traffic volume by a signal representing average traffic speed. A vehicle detector switch 1 operates a multivibrator 2 the output of which is averaged by capacitor 10 and resistance 11, and the charge on capacitor 10 is averaged over a period of one minute by resistances 12 and 13 and capacitor 14. An output from cathode follower 16 is fed back to the charging circuit of capacitor 10 through Zener diode 18 and diode 19 to linearize the charging at high charging rates; the voltage drop across the Zener diode also reduces the gain of the feedback circuit to less than one thereby preventing regeneration. The output at junction 20 is supplied to transistors 27 and 29 which have a feedback path through a tungsten filament lamp 23 forming a variable resistance so that the output on line 46 is proportional to the square root of the input and the heat generated in heater 47 for thermistor 40 is therefore proportional to the traffic volume. Measuring average traffic speed.-Individual vehicle speed pulses at terminal 30 charge a capacitor 33 to a voltage proportional to the speed of the last vehicle. The charge on capacitor 33 is bucketed into capacitor 36, the charging circuit of which includes resistor 35 and thermistor 40 which is connected to a tap 45 on resistor 44 at a voltage representing average traffic speed. Since the resistance of thermistor 40 decreases as traffic volume increases, the effect of this circuit is to decrease the effect of a charge on capacitor 33, if different from average, on the voltage across capacitor 36 as the traffic volume increases. Switch 37 is normally connected to terminal 39 but is connected for an instant to terminal 38 whenever a vehicle passes. A chopper 48 alternately connects the voltage on capacitor 36 and the voltage on tap 45, and any difference is amplified and used to drive motor M which drives the taps 45 and 68 on resistors 44 and 67 until the system is balanced. Dividing traffic volume by traffic speed.- Assuming that the potential on line 90 represents traffic density, this is multiplied by traffic speed, represented by the position of tap 68, to give an assumed traffic volume signal on line 82 which is compared with the measured volume on line 85, and the comparison is used to correct the output voltage on line 90. An A.C. source 72 is coupled through capacitor 83 and oppositely poled diodes 70 and 71 to the lines 85 and 82. The effect of this is that capacitor 83 assumes the mean potential of the two diodes, and the output at the junction of resistances 73 and 74 is a clipped sine wave having a peak to peak level equal to the potential difference between lines 82 and 85. The positive peaks are short-circuited by diode 76 whilst the negative peaks pass through diode 77 to an amplifier 80 and rectifier 86 to line 90. The output voltage may be connected to a visual or graphic recorder, to a traffic controller, or to a classifier circuit to indicate one of a plurality of ranges of traffic density.