Patent GB-949382-A (GB)
Title: Traffic density computer
IPC: G08G1/01
Filed: 1960-05-25
Abstract: 949,382. Analogue computers for road traffic. LABORATORY FOR ELECTRONIC Inc. May 25, 1960 [May 29, 1959], No. 18567/60. Headings G4G and G4Q. Traffic density is computed by feeding the Doppler beat output of a sensing element into a Speed and Volume impulse translator, the volume output signal being processed in computer 13-20 and the speed output fed into a speed averaging computer 21, a dividing device 75 yielding an output representing traffic density. Volume computer. The sharp translator pulses are fed through tube 26, differentiator 14 and the isolation diode 27 rendering multivibrator part 28b conductive for the pulse, the duration being determined by potentiometer 31-34. The tube 37 gives a square output to the counter block 17 in which the capacitor 41 is charged accumulatively by a constant amount for each vehicle detected and is subject to constant bleeding so that the voltage on the capacitor is a measure of traffic volume. The voltage fed back from cathode follower tube 43, a resistor 46 and diode 47 makes the charging voltage independent of the voltage in the storage capacitor. The instantaneous volume output is smoothed by capacitor 48 and amplified by the cathode follower in block 20 for work circuits after lead 78<SP>11</SP>. Speed computer. Capacitor TC1 begins to bleed through TR2 and relay 54b closes. Voltage on capacitor 54 will correspond to the vehicle's speed and is applied at 61 to a circuit comparing it with the old average speed represented by the voltage at 62. The two are compared in a chopper 66, and servo amplifier 67 and any difference produces an A.C. voltage driving servomotor 68 and potentiometer arm 64 to give at 62 a new average. Time base compensation is given by resistancevarying thermistors 58, 59, the latter being heated by a current controlled by valve 70, the grid potential of which is influenced by the volume computer output 78<SP>11</SP>. In measuring density a value is assumed and multiplied by speed and the result compared with the measured value of volume. The difference adjusts the assumed value of density until the error is a minimum. The voltage on potentiometer 74 represents the assumed density and arm 76 is mechanically coupled to arm 64 in block 21 indicating average speed. Arm 76 is connected through diodes 77, 78 in series to the volume output, and the junction 79 of the diodes is coupled through capacitor 80 and resistor 81 to an A.C. source. If the trial volume is lower than the computed volume no current will flow in the diodes, and the output of amplifier 82 is high and after rectification will raise the assumed value of the density across potentiometer 74 towards its true value. If the trial volume is high, current flows in the diodes and reduces the amplifier output correcting the trial value of density. Closure of switch 120 connects the density value to potentiometer 124 indicating a predetermined value which if exceeded operates a sensitive relay 122 and a warning device. Several such relay circuits may be combined to indicate different levels of traffic density. Block 85 provides a second computation of traffic density at very low speeds where the inverse speed is a first approximation of density. The arm of potentiometer 87<SP>1</SP> is set to a speed value below which the normal computation is not correct. If the speed computer output voltage at 62 is lower than this setting, the junction of diodes 86, 87 will be a high impedance point and an A.C. voltage will be present on resistor 92 at the input of amplifier 82, the latter selecting and amplifying only the signal having the larger value (Fig. 1a not shown). A servo method of dividing density by speed and comparing the resultant with the known value of the volume is described (Fig. 3 not shown), a chopper comparing the trial and actual values of the volume and driving servo motor to bring the density output to its true value. Different density output responses for increasing or decreasing density are also described. Specification 929,631 is referred to.