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Field sensor

  • US 4,240,528 A
  • Filed: 05/24/1978
  • Issued: 12/23/1980
  • Est. Priority Date: 05/04/1977
  • Status: Expired due to Term
First Claim
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1. A field sensor control mechanism for detecting obstructions in front of the leading edges of linearly moving motor-operated sliding elevator doors, comprising at least two insulated electrodes mounted in a leading edge of a slidable door at different levels;

  • means, including a first capacitor and a first inductor, forming a first oscillating circuit electrically connected to one of said electrodes;

    means, including a second capacitor and a second conductor, forming a second oscillating comparison circuit electrically connected to the other of said electrodes and having a base oscillating frequency at least substantially the same as that of said first circuit; and

    means including a feedback winding and an amplifier connected in circuit with said first and second circuits and operative for so coupling said first and second circuits that the oscillations of said first circuit vary as a function of capacitance changes in said electrodes.

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