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Eddy current metal surface flaw detector

  • US 3,931,571 A
  • Filed: 06/14/1974
  • Issued: 01/06/1976
  • Est. Priority Date: 08/21/1973
  • Status: Expired due to Term
First Claim
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1. In a device for detecting flaws in a surface of the kind having a detector coil for responding to variations in eddy currents induced in said surface;

  • an oscillator circuit for establishing an oscillatory electromagnetic field at said coil and generating an output signal varying with the impedance of said coil; and

    means actuated by said output signal to provide an indicator signal which is substantially in linear proportion to a dimension of the flaw for the purpose of actuating indicating or measuring means to identify the flaw, the improvement consisting in that the oscillator circuit includes;

    a tuned detector circuit comprising said detector coil,an inductor in series with said tuned circuit,a two-stage amplifier with adjustable gain, connected to said inductor,a feedback circuit maintaining zero phase shift between the said amplifier stages, anda capacitor providing capacitive reactance between the output of the amplifier and earth, means to adjust the oscillator so that the amplitude of the output signal decreases successively when the detector coil is moved from free air to adjacent a flawless part of the surface and then adjacent a flaw in the surface, said actuated means including a discriminator circuit whose discrimination level is set to pass only those oscillator signals whose amplitudes are less than or about equal to the amplitude of the output signal when the detector is adjacent a flawless part of the surface.

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