Circumferentially compensating eddy current probe with alternately polarized receiver coil

Claim 1

A circumferentially compensating eddy current probe for detecting localized flaws in a tube having a central axis and made of an electrically conductive material, while compensating for circumferential structural variations therein, comprising: a first coil assembly and a second coil assembly to be located inside the tube under inspection; the said first coil assembly having at least one first coil arranged in a plane perpendicular to the central axis and to be located adjacent to the tube under inspection, the at least one first coil having an axis parallel to the central axis to generate at least one magnetic field of a substantially toroidal shape in and through the tube in the direction of the central axis; the said second coil assembly having an even number of substantially identical second coils to be located between the said first coil assembly and the tube under inspection and symmetrically thereabout and in the said plane perpendicular to the central axis in the said magnetic field of the toroidal shape where radial and axial components of the said magnetic field exist, the said second coils having diameters larger than the width of the first coil and having axes perpendicular to the said central axis and being electromagnetically polarized alternately along their axes so that the second coil assembly senses distortions in the magnetic fields and produces a circumferentially compensated output indicating essentially the presence of localized flaws in the tube.