| 摘要 |
An electronic system for detecting the presence of an external object in the field of one of the reactors in the tank circuit of the first one of a pair of oscillators. The oscillators are interconnected for frequency synchronization. The tank circuits of the oscillators are tuned to have different natural resonant frequencies in the absence of an object whereby the oscillator signals differ in phase. The presence of an object affects the tuned resonant frequency of the tank circuit of the first oscillator in a direction or sense to decrease the phase difference between the oscillator output signals. The oscillator output signals are limited, fed to a phase detector, and a signal is developed representing the phase differential. When this signal decreases below a predetermined threshold, an output indicator is triggered to provide an indication of the presence of the external object. The threshold is established in part by a capacitor having a charge representing ambient conditions when no object is present. The charging circuit for the capacitor provides a short time constant to follow gradually changing ambient conditions but provides a long time constant to prevent change in the charge on the capacitor as the result of the relatively more rapid and large change in the impedance of the reactance element as an external object arrives within the detection field. The long time constant circuit is operative regeneratively to change the charge on the capacitor after a long delay following the detection of an arriving object in case the object has not in the interim departed from the field. The departure of the object from the field prior to the time that the charge has leaked from the memory capacitor, returns the capacitor charge to ambient condition. The system is particularly adapted to indicate presence of vehicles within the field of a loop disposed slightly below the surface of a roadway or driveway.
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