Importance

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Abstract

A safety arrangement for an automatic door operator wherein the door edge has mounted thereon a stretched electrically conductive cable which, when deflected upon encountering an obstruction, will change the inductance of a loop of which the cable forms a part. A capacitor is in parallel with this loop so that the capacitor together with the inductance of the loop form a resonant tank circuit. The tank circuit is electrically driven by a voltage controlled oscillator. The phase of the loop current is compared with the phase of the voltage across the capacitor and the phase difference is utilized to control the voltage controlled oscillator to maintain the tank circuit in resonance (i.e, zero phase difference). The detection of a sudden change in phase difference, which occurs when the loop inductance is suddenly changed due to the encountering of an obstruction by the door edge, results in the generation of an obstruction signal for activating the safety feature of the door operator.

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First Claim