Apparatus for surface flaw detection on electrically conductive

Abstract

A surface defect detecting apparatus having at least one sensor block for movement in respect of rotating cylindrically shaped electrically conductive metallic material to be inspected by scanning an outer portion of the surface of the material. The sensor block has spaced guide wheels at front and rear parts thereof in the spiral scanning direction which rotate in response to the rotation of the material; and a coil holder for supporting a plurality of eddy-current flaw detecting coils arranged longitudinally in a row in the direction of movement, intermediate between pairs of guide wheels at the front and rear parts opposite to the outer surface of said material with a predetermined gap therebetween. A turning wheel at each of the front and rear parts of the coil holder is rotatable with respect to the outer surface of the material so as to maintain the predetermined gap. An AC signal of a predetermined frequency applied from an oscillator to the coils produces corresponding eddy currents in material to be inspected. The coils are divided into a first group and a second group with one group of coils being defined by coils other than those coils in an adjoining row to that group, phase shifters feed the AC signal to coils of the first group resulting in a shift in phase by a predetermined amount, as reference signal to detectors formed as phase detectors. The phase detectors are associated with coils of the latter group, with a phase different from that of the AC reference signal respectively applied to a second group of coils and to the phase detectors of the first group of coils.