Determining the Electromagnetic Polarizability Tensors of Metal Objects During In-Line Scanning

In metal detection systems, the response of a detector to a metal object may be approximated from the electromagnetic polarizability tensor of the object. Conversely, the tensors may be determined from multiposition measurements as the detector and object are moved relative to each other. This paper introduces and sets out the general approach to determining the tensor during in-line scanning. Two common application scenarios are considered, which share a similar consideration in the calculation of the tensor components. The first is the case for detectors of landmines or explosive remnants of war, where the detector is scanned on a surface above the object. The condition of this inverse problem depends on the geometry of the coil(s) and the measurement protocol, which at present is not fully understood. Our results consider two cases, namely, a single line scan over the object as two extreme cases. The results suggest that tensor inversion is possible for the 2-D raster scan, but not for a single line scan. The second application is a conveyor-type metal detector, which is used with a typical detector for industrial process lines. Here, a new rotation measurement method is proposed and examined for the case of simple coaxial sensor coils and in-line scanning. Finally, different inverse methods are analyzed for the new rotation measurement method.