Standard Practice for Electromagnetic (Eddy Current/Magnetic Induction) Sorting of Ferrous Metals

Abstract

5.1 Absolute and comparative methods provide a means for sorting large quantities of ferrous parts of stock with regard to composition, condition, structure, or processing, or a combination thereof. 5.2 The comparative or two-coil method is used when high-sensitivity testing is required. The advantage of this method is that it almost completely suppresses all internal or external disturbances such as temperature variations or stray magnetic fields, provided both the coils and both the reference parts are exposed to the same conditions which are not of relevance. 5.3 The ability to accomplish satisfactorily these types of separations is dependent upon the relation of the magnetic characteristics of the ferromagnetic parts to their physical condition. 5.4 These methods may be used for high-speed sorting in a fully automated setup where the speed of testing may approach ten specimens per second depending on their size and shape. 5.5 The success of sorting ferromagnetic material depends mainly on the proper selection of magnetic field strength and frequency of signal in the test coil, fill factor, and variables present in the sample. 5.6 The degree of accuracy of a sort will be affected greatly by the coupling between the test coil field and the test specimen and the accuracy with which the specimen is held in the test coil field during the measuring period. Testing with harmonics can, to a large extent, reduce the sensitivity to accuracy of location. 5.7 When high currents are used in the test coil, a means should be provided to maintain a constant temperature of the reference standard in order to minimize measurement drift.