MULTIPLE FREQUENCY MAGNETIC FIELD TECHNIQUE FOR DIFFERENTIATING BETWEEN CLASSES OF METAL OBJECTS

摘要

The invention relates to an A.C. metal detection apparatus employing a means for subjecting metal objects to oscillating magnetic fields of more than one frequency, and measuring circuits for detecting at each frequency the magnetic field due to the presence of the object and analyzing the magnetic fields to classify the objects on the basis of parameters such as size, shape, thickness, and surface to volume ratio.