Inductive loop vehicle presence detector

Claim 2

Apparatus for sensiNg the presence of a vehicle, said apparatus comprising a first oscillator including an inductive loop, said first oscillator being operable to generate a signal which will increase in frequency when a vehicle enters a field established by the loop, a second oscillator for generating a reference signal of a frequency differing from the frequency of the loop oscillator, a beat frequency detector coupled to both oscillators and operable to generate a signal having a frequency equal to the difference between the frequencies of the oscillator signals, a low-pass filter coupled to the beat frequency detector, means coupled to the low-pass filter for generating a direct voltage corresponding with the amplitude of waves passed by the filter, and means controllably coupled to the reference oscillator and coupled to receive the direct voltage whereby the reference frequency of the second oscillator is shifted when the direct potential level varies.

Claim 3

Apparatus for sensing the presence of a vehicle, said apparatus comprising a first oscillator including an inductive loop, said first oscillator being operable to generate a signal which will increase in frequency when a vehicle enters a field established by the loop, a second oscillator for generating a reference signal, the reference signal having a normal frequency less than the frequency of the first oscillator, a beat frequency detector coupled to both oscillators and operable to generate a signal having a frequency equal to the difference between the frequencies of the oscillator signals, a low-pass filter coupled to the beat frequency detector and operable to pass the difference frequency signal when both oscillators are generating normal signals, said low-pass filter being further operable to block the difference frequency signal when a vehicle enters the loop and the first oscillator increases in frequency, and a switching means coupled to the low-pass filter and controllably coupled to the second oscillator for causing the second oscillator to decrease in frequency when the low-pass filter blocks the flow of the difference frequency signal.

Claim 4

Apparatus for sensing the presence of a vehicle, said apparatus comprising a first oscillator including an inductive loop, said first oscillator being operable to generate a signal which will increase in frequency when a vehicle enters a field established by the loop, a second oscillator for generating a reference signal, the reference signal having a normal frequency less than the frequency of the first oscillator, a beat frequency detector coupled to both oscillators and operable to generate a signal having a frequency equal to the difference between the frequencies of the oscillator signals, a low-pass filter coupled to the beat frequency detector and operable to pass the difference frequency signal when both oscillators are generating normal signals, said low-pass filter being further operable to block the difference frequency signal when a vehicle enters the loop and the first oscillator increases in frequency, and a switching means coupled to the low-pass filter and controllably coupled to the second oscillator for causing the second oscillator to decrease in frequency when the low-pass filter blocks the flow of the difference frequency signal.

Claim 5

Apparatus for sensing the presence of a vehicle, said apparatus comprising a first oscillator including an inductive loop, said first oscillator being operable to generate a signal having a normal frequency and being further operable to generate a signal of increased frequency when a vehicle enters a field established by the loop, a second oscillator for generating a reference signal having a normal frequency less than the frequency of the first oscillator, a beat frequency detector coupled to both oscillators and operable to generate a signal having a frequency equal to the difference between the frequencies of the oscillator signals, a low-pass filter coupled to the beat frequency detector and operable to pass the difference frequency signal when both oscillators generate normal signals, said low-pass filter being further operable to block the difference frequency signal when a vehicle enters the loop and the first oscillator increases in frequency, said second oscillator including a tuned circuit having a capacitor therein for establishing the frequency of oscillation, a switching means coupled to the low-pass filter, and a further capacitor coupled between the resonant circuit of the second oscillator and the switching means, said switching means being operable to couple the second capacitor in shunt with the first capacitor when the low-pass filter blocks the flow of the difference frequency signal whereby the frequency of the second oscillator is decreased below the normal frequency thereof.

Claim 6

Apparatus for sensing the presence of a vehicle, said apparatus comprising a first oscillator including an inductive loop, said first oscillator being operable to generate a signal having a normal frequency and being further operable to generate a signal of increased frequency when a vehicle enters a field established by the loop, a second oscillator for generating a reference signal having a normal frequency less than the frequency of the first oscillator, a beat frequency detector coupled to both oscillators and operable to generate a signal having a frequency equal to the difference between the frequencies of the oscillator signals, a low-pass filter coupled to the beat frequency detector and operable to pass the difference frequency signal when both oscillators generate normal signals, said low-pass filter being further operable to block the difference frequency signal when a vehicle enters the loop and the first oscillator increases in frequency, said second oscillator including a tuned circuit having a capacitor therein for establishing the frequency of oscillation, a switching means coupled to the low-pass filter, and a further capacitor coupled between the resonant circuit of the second oscillator and the switching means, said switching means being operable to couple the second capacitor in shunt with the first capacitor when the low-pass filter blocks the flow of the difference frequency signal whereby the frequency of the second oscillator is decreased below the normal frequency thereof.

Claim 7

Apparatus in accordance with claim 3 wherein the switching means comprises two transistors each having an emitter electrode, a base electrode and a collector electrode, the emitter electrode of a first of the transistors being connected to the collector electrode of the second transistor and being further connected to the second capacitor, the collector electrode of the first transistor anD the emitter electrode of the second transistor being connected together and being connected to a point of reference potential, the base electrode of both transistors being connected together and a biasing means responsively coupled to the difference frequency signal passed by the low-pass filter, and coupled to the base electrode of both transistors.

Claim 8

Apparatus in accordance with claim 3 wherein the low-pass filter comprises a constant-K section and an M-derived section and wherein the filter provides a characteristic including a low-pass band, a point of minimum response and a high frequency rejection band, and wherein the second oscillator includes a capacitive means for shifting the signal frequency thereof whereby the first oscillator may be tuned to a minimum response point of the filter characteristic.

Claim 9

Apparatus in accordance with claim 3 wherein the first oscillator comprises a resonant circuit including the loop and a tuning capacitor connected across the loop, a coupling transformer having a winding coupled across a loop and the tuning capacitor and having another winding coupled to a reference potential whereby the resonant circuit of the loop is floating and ungrounded to minimize the effect of stray capacitance.

Claim 10

Apparatus for sensing the presence of a vehicle, said apparatus comprising two oscillators tuned to different frequencies, a beat frequency circuit coupled to receive signals from both oscillators and operable to generate a signal of a frequency equal to the difference between the oscillator frequencies, a low-pass filter coupled to receive the signal from the beat frequency circuit, an output circuit coupled to the low-pass filter, each of the oscillators including a tuned circuit for determining the oscillator frequency, a first of the oscillators having an inductive loop coupled thereto as a part of the tuned circuit and as a frequency determining element whereby a vehicle may cause a variation in the inductive value of the loop and cause a corresponding variation in the frequency of the oscillator, a transistor switch responsively coupled to the output circuit, said transistor switch being coupled to the tuned circuit of one of the oscillators and being operable to vary the frequency thereof in response to signal variation from the output circuit whereby the presence of a vehicle will cause a variation in the signal of the output circuit to partially self-sustain the output signal.

Claim 11

Apparatus for sensing the presence of a vehicle, said apparatus comprising two oscillators tuned to different frequencies, a beat frequency circuit coupled to receive signals from both oscillators and operable to generate a signal of a frequency equal to the difference between the oscillator frequencies, a low-pass filter coupled to receive the signal from the beat frequency circuit, an output circuit coupled to the low-pass filter, each of the oscillators including a tuned circuit for determining the oscillator frequency, a first of the oscillators having an inductive loop coupled thereto as a part of the tuned circuit and as a frequency determining element whereby a vehicle may cause a variation in the inductive value of the loop and cause a corresponding variation in the frequency of the oscillator, a transistor switch responsively coupled to the output circuit, said transistor switch being coupled to the tuned circuit of one of the oscillators and being operable to vary the frequency thereof in response to signal variation from the output circuit whereby the presence of a vehicle will cause a variation in the signal of the output circuit to partially self-sustain the output signal.

Claim 12

The apparatus in accordance with claim 7 comprising a capacitor coupled to the tuned circuit of said one oscillator and to the transistor switch, whereby the capacitor will become a part of the tuned circuit and will shift the frequency of said one oscillator when a vehicle moves over the field of the loop.