Magnetic flaw detector for conducting materials - uses phase comparison between exciter and detector coils to locate discontinuities

摘要

The magnetic flaw detector for determining discontinuities in materials, esp. electrically conducting materials, is suitable for continuous measurements on pipelines for example. The detector comprises a frequency generator (53) which feeds an induction coil (57) placed on the upper surface of the material (1). Two identical pick-up coils connected in opposition (73) are connected to an analyser (77) and to a pre-amplifier (61) in a differential mode. The output of the pre-amplifier feeds a phase-sensitive rectifier (63) whose amplified output is fed back to the generator to control its frequency. A phase-shift network (67) is connected between the rectifier and the exciter coil. The phase difference between the excitation and detected fields is maintained constant irrespective of the thickness and electrical conductivity of the material under test.