Title: Inductive loop detector system

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Abstract: Apparatus for use in combination with multiple inductive loops for detecting metal objects, e.g. vehicles, in the immediate vicinity of each of said loops. Each loop may, for example, be a coil of wire buried in a roadway in a plane parallel to the roadway surface. A common oscillator circuit is operatively connected to each loop on a time shared basis with the frequency of oscillation at any time being determined by the inductance of the connected loop. The inductance in turn is dependent on whether or not a vehicle is over the loop. Successive scan cycles are defined with multiple loop phases occurring during each scan cycle. During each loop phase, a different one of the multiple loops is connected to the oscillator circuitry and during a "period measurement" portion of each loop phase, 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 or period of a certain number of such loop oscillator cycles. The measured time duration (period) is then compared with an historical reference duration (accumulated over prior scan cycles) during a "calculation and storage" portion of each loop phase to ascertain whether the loop oscillator frequency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 period. A reduction in the measured period by an amount greater than a preselected threshold produces an output signal or "call" to indicate the vehicle's presence. In order to accommodate loop inductance variations attributable to slowly varying environmental conditions, the historical reference duration is caused to vary or "adapt" over a plurality of scan cycles in accordance with certain defined criteria. The duration counter counts cycles of a crystal oscillator and is used during each "period measurement" portion to measure a time duration and during each "calculation and storage" portion to define sequential control states for the performance of various arithmetic and logic operations.


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