GB1495759A - Digital loop detector system - Google Patents

Abstract

1495759 Digital detecting apparatus GULF & WESTERN INDUSTRIES Inc 24 Dec 1974 55782/74 Heading G1N [Also in Division G4] Digital detecting apparatus for creating an output signal when an electrically conductive object comes within the field of effect of a loop comprises means for creating a pulse train having a frequency controlled primarily by the inductance of the loop; means for counting the pulses of said pulse train during a selected time interval to produce a count generally representative of the frequency of said pulse train during said time interval; means for creating a representative count; means for comparing the representative count with the reference count; and means for creating an output signal when the representative count differs from the reference count by at least a pre-selected amount in a selected numerical direction. The apparatus may be used in detecting vehicles for traffic control purposes, or in a metal detector for security checks. As shown, a digital loop detector system A includes a loop B generally positioned in or adjacent to a road, capacitor C and oscillator D. The latter produces a pulse train the frequency of which is determined by the inductance of loop B and the capacitance of C. The output of oscillator D is amplified by squaring amplifier E to produce a pulse train tL which is fed into a processing circuit F. The pulse train tL is gated to counter 12 via a gate 18 which is controlled by a crystal controlled interval generator 20 through line 22. The counting interval may be selected to be 50, 100; or 200 ms. The sequence control and decision logic circuits 30 are controlled by counter 10, reference counter 12 and comparator 14. Line 32 represents the function of incrementing counter 10 by a unit change. Line 34 represents the function of gating a completely new count into counter 10. Output line 36 controls the output of the detector, which may be a relay, transistor switching circuit, or optical coupled output. The circuit 30 may be controlled externally by a forced drift oscillator 50, the function of which is to enable the detector A to forget a vehicle remaining in the field of loop B for a prolonged time interval. The output of oscillator 50 may be adjusted via line 52. The interval generated by generator 20 may be selected by logic on lines 54. A line 56, controlled by a manually adjusted switch, may be provided to change the system "A" from a pulse mode to a presence mode of operation. In the pulse mode, a uniform pulse is created in output line 36 for each detection, whereas in the presence mode, the output line is activated for the time that a vehicle is being detected and before it is forgotten by the forced drift feature using oscillator 50. Reference has been directed by the Comptroller to Specification 1398937.