GB1272224A - Improvements in or relating to discriminator circuits - Google Patents

Abstract

1,272,224. Road traffic detectors. MARCONI CO. Ltd. 22 Aug., 1969 [15 Oct., 1968], No. 48801/68. Heading G4Q. In a traffic detection arrangement employing a traffic sensor as disclosed in Specification 1,209,506, each axle of a small vehicle may produce an output signal as at A, Fig. 2, whereas a very large slow moving vehicle may produce a large decaying oscillatory output signal as at B for each axle. Thus a detecting arrangement responsive to a positive signal excursion would give an indication every positive half cycle of the decaying oscillation of signal B. The invention relates to a discriminator circuit which processes the output signal so that a single vehicle produces only a single indication. As shown, Fig. 1, the discriminator circuit comprises an input 1 to which signal B is applied. The output of amplifier 2 is connected to a peak signal detector 3 and to a gate 4. The output of detector 3 has the form C. Since energy is lost in detector 3 the amplitudes of the positive excursions of the signal B exceed the detector output. The leading edge of signal C is passed by a gate 6 to trigger a monostable 7 which generates a pulse D. Pulse D is fed back to the inhibit input of gate 6 to prevent retriggering of monostable 7 for the pulse duration. The trailing edge of pulse D triggers monostable 8 which produces the first one of pulses E, this pulse E opening gate 4 and permitting the signal B to pass to a comparison circuit 5 where it is compared with signal C. If during the open period of gate 4 the signal B exceeds the signal C, the comparator output triggers a monostable 9 which produces the first one of pulses F. Pulse F is used to inhibit monostable 7 and its trailing edge is used to retrigger monostable 8 to effect a further comparison in circuit 5. The cycle repeats providing that signal B exceeds signal C. As shown, the signal C is not exceeded in the fourth comparison during the fourth pulse from monostable 8 and the cycle is terminated. The outputs of the monostables 7, 8, 9 are fed to an OR gate 10 whereby this gate produces a single output pulse G, at terminal 11. If the signal A is applied to input 1, this will not exceed the output of detector 3 and hence monostable 9 will not be triggered. The output will thus comprise the pulse from monostable 7 and a single pulse from monostable 8 as indicated by the shorter pulse G defined by the dash line. Thus the length of pulse G gives some indication of the size of the vehicle causing it.