Inductive loop vehicle detector
First Claim
1. Apparatus useful in conjunction with a roadway for detecting the presence of a vehicle on a specified area of the roadway, said apparatus comprising:
- a loop of wire supported adjacent the surface of said roadway within said specified area;
oscillator circuit means connected to said loop of wire for oscillating at a frequency dependent on the inductance of said loop;
timing means for measuring the time duration of a fixed number of cycles of said oscillator circuit means;
reference means defining a reference duration;
comparison means for determining the difference between said measured time duration and said reference duration; and
threshold means responsive to said difference exceeding a threshold value for generating a signal indicative of the presence of a vehicle in said specified area.
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Accused Products
Abstract
Apparatus for use in combination with an inductive loop for detecting metal objects, e.g. vehicles, in the immediate vicinity of said loop. The loop may, for example, be a coil of wire buried in a roadway in a plane parallel to the roadway surface. Oscillator circuitry is operatively connected to the loop with the frequency of oscillation being determined by the loop inductance, which in turn is dependent on whether or not the vehicle is over the loop. The loop frequency is monitored by digital circuitry including a loop counter which counts loop oscillator cycles and a duration counter which measures the time duration of a fixed number of loop oscillator cycles. The measured time duration is compared with an adaptable reference duration to ascertain whether the loop oscillator frequency has increased or decreased. The presence of a vehicle over the loop decreases loop inductance, increases loop frequency, and thus reduces the measured time duration of a fixed number of loop cycles. A reduction in the measured time duration by an amount greater than a preselected threshold value produces an output signal or "call" to indicate the vehicle'"'"'s presence.
52 Citations
20 Claims
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1. Apparatus useful in conjunction with a roadway for detecting the presence of a vehicle on a specified area of the roadway, said apparatus comprising:
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a loop of wire supported adjacent the surface of said roadway within said specified area; oscillator circuit means connected to said loop of wire for oscillating at a frequency dependent on the inductance of said loop; timing means for measuring the time duration of a fixed number of cycles of said oscillator circuit means; reference means defining a reference duration; comparison means for determining the difference between said measured time duration and said reference duration; and threshold means responsive to said difference exceeding a threshold value for generating a signal indicative of the presence of a vehicle in said specified area. - View Dependent Claims (2, 3, 4, 5)
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6. Detection apparatus useful for detecting the presence of a vehicle on a specified area of a roadway comprising:
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a conductive loop mounted proximate to the surface of said roadway enclosing an area substantially coincident with said specified area; circuit means disposed remote from said loop and operatively connected thereto for producing an oscillator signal whose frequency is related to the inductance of said loop; sequence control means defining successive detect cycles and including means for generating a start signal during each detect cycle; a first counting means for counting cycles of said oscillatory signal occurring after said start signal and for generating a first overflow signal after M counts; a source of clock pulses; a second counting means for counting clock pulses occurring after said start signal and for generating a second overflow signal after N counts; preset means for loading a count number into said second counting means prior to said start signal; and means responsive to said first overflow signal occurring prior to said second overflow signal for generating a call signal indicative of the presence of a vehicle in said specified area. - View Dependent Claims (7, 8, 9, 10, 11, 12)
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13. Apparatus including oscillator circuitry adapted to be operatively connected to a remote inductive loop for monitoring the frequency of oscillation, said apparatus comprising;
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timing means for measuring the time duration of a fixed number of cycles of said oscillator circuitry; reference means defining a reference duration; comparison means for determining the difference between said measured time duration and said reference duration; feedback means responsive to said difference for varying said defined reference duration in a direction tending to reduce said difference toward zero; and call threshold means responsive to said difference exceeding a call threshold value for generating a call signal. - View Dependent Claims (14, 15, 16)
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17. Apparatus including oscillator circuitry adapted to be operatively connected to a remote inductive loop for monitoring the frequency of oscillation, said apparatus comprising:
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loop counter means for counting a fixed number of cycles of said oscillator circuitry; reference means defining a reference duration; a source of clock pulses; error counter means for counting the number of clock pulses representing the difference between the duration of said fixed number of cycles and said reference duration; means responsive to said error counter means for varying said reference duration in a direction to reduce said error counter means count toward zero; and call means responsive to said error counter means counting above a threshold value for generating a call signal. - View Dependent Claims (18, 19, 20)
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Specification