Patent GB-1223442-A (GB)
Title: Apparatus for producing a signal representative of an average speed
IPC: G01P7/00
Filed: 1968-02-21
Abstract: 1,223,442. Road traffic measurements. LABORATORY FOR ELECTRONICS Inc. 21 Feb., 1968 [21 Feb., 1967], No. 8417/68. Heading G4Q. Road traffic is controlled by a signal representative of the " average " speed of a number of vehicles passing a detector in succession. The detector is of the kind which generates a pulse having a duration equal to the time a vehicle is within the detection zone and thus the pulse length is inversely proportional to the vehicle speed. Due to the different shapes and lengths of vehicles the pulse lengths derived from vehicles travelling at the same speed vary and thus the control signal is derived from a processing operation performed upon a number of the pulses to obtain an " average " value which more accurately represents the speed of a succession of vehicles. A pole-mounted sonic detector is briefly described in conjunction with Fig. 1 (not shown), but an inductive loop detector may be used. This detector supplies for each detected vehicle, a pulse on a line 9 having a duration inversely proportional to the vehicle speed. Fig. 2 shows in block diagram form a circuit for processing the pulses on line 9 to obtain a traffic control signal at output 40. The output at 40 is a voltage having a level which represents the " average " speed of a number of vehicles. The pulse on line 9 passes through a noise-eliminating filter 10 and inverting amplifier 12 to a ramp generator 14 which generates a signal having a level proportional to the pulse length. At the end of the pulse on line 9, pulse generator 20 is triggered via diode 18, the output of the generator 20 acting through OR gate 24 to clamp the output voltage level of the generator 14. The output of generator 20 also passes via a capacitor 22 to one input of a variable length pulse generator 16 to activate it. The other input of generator 16 is connected to the generator 14, whereby the generator 16 produces a pulse having a duration inversely proportional to the clamped voltage level of the generator 14. Thus the pulse length at the output of generator 16 is proportional to the speed of the detected vehicle. The output of generator 16 passes through a diode 26 to a " last input storage " 28 and also passes to the OR gate 24 to continue the clamping action on the generator 14 until the cessation of the output pulse from generator 16. The store 28 generates a voltage signal having a level directly proportional to the length of the pulse from generator 16 and thus directly proportional to the vehicle speed. Any previous signal in the store 28 is cancelled by the pulse from generator 20 before the entry of the next pulse from generator 16. It is now assumed that the signal which has entered the store 28 is derived from the first vehicle to have passed the detector since the apparatus was switched on. Thus there is no earlier signal to be cancelled in store 28, no signal in stores 32, 36 and no output on line 40. The signal in store 28 passes to averaging circuit 30 which generates an output signal having a level proportional to the signal in the store, the output signal of circuit 30 passing into a buffer store 32 having its entry controlled by pulse generator 34 triggered upon the termination of the output pulse from generator 16. After a time delay produced by delay 38 the pulse from generator 34 causes transfer of the signal in the store 32 into the average speed store 36, the output of store 36 constituting the traffic control signal on line 40. The output of store 36 also is supplied as a second input to the averaging circuit 30. When the next signal enters the store 32, due to the second vehicle passing the detector, this signal is transferred to the store 36 and this process repeats for each succeeding vehicle whereby the signal stored in store 36 becomes an accumulated signal consisting of a contribution made up from all previous signals. However, each contributtion from the successive signals stored in store 28 is reduced by a mixing process performed in averaging circuit 30. This mixing relates to the mixing of the two input voltages supplied to the circuit 30 and the ratio of this mixing may be varied by a multiple-position switch described with reference to Fig. 3B (not shown). This switch has positions numbered 3, 5, 7, 10, 15, 20, 30 corresponding to the different mixing ratios.- Thus, in position 5 the circuit 30 functions to mix the two inputs so that its output consists of the sum of <SP>4</SP>/ 5  of the voltage level on line 40 and 1/5 of the voltage level in store 28. Hence the output on line 40 is continually updated by each succeeding vehicle. Fig. 5 shows the pulse forms in different parts of the circuit and their times of generation in relation to a common timing datum. In Fig. 5, " a" is generated by the vehicle detector, " b " by the amplifier 12, " c " by the generator 14, " d " by generator 20, " e " by generator 16, " f " by generator 16, " g " by generator 34, and " h " by delay 38. The electronic circuitry used is described with reference to Figs. 3A, 3B (not shown).