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Method and apparatus for magnetically detecting defects in an object with compensation for magnetic field shift by means of a compensating coil

  • US 5,512,821 A
  • Filed: 02/01/1993
  • Issued: 04/30/1996
  • Est. Priority Date: 06/04/1991
  • Status: Expired due to Fees
First Claim
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1. A magnetic detector comprising:

  • means for generating a magnetic field;

    a magnetic sensor for detecting a leakage flux resulting from a magnetically defective portion of an object moving in and relative to said magnetic field and for producing a signal corresponding thereto;

    a low-pass filter for extracting a low-frequency signal component contained in said signal that is output by the magnetic sensor, said low-frequency signal component being indicative of a floating flux which crosses said magnetic sensor and which is a result of the object moving through said magnetic field;

    an integrator for integrating the low-frequency signal component extracted by the low-pass filter;

    an amplifier for amplifying the low-frequency signal component integrated by the integrator; and

    a compensating coil excited by the amplified low-frequency signal component output by the amplifier, for generating a magnetic flux which cancels out said floating flux crossing said magnetic sensor.

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