Three-dimensional object location and inversion of the magnetic polarizability tensor at a single frequency using a walk-through metal detector

Abstract

A tomographic metal detection and characterisation system has been designed and built for recovering information about magnetic and/or conductive objects within the detector space. This information is gathered as a result of a "walk-through" scan of a candidate in the same manner as for a typical security metal detector archway. Following the passage of the candidate, the system uses measurements from an array of coils to calculate the polarisability tensor, which describes the low frequency electromagnetic characteristic of a small metallic object when it interacts with an AC magnetic field. In addition to the magnetic polarisability dyadic tensor the position of the perturbation is also determined as a product of the inversion algorithm. The system has been tested and is capable of inverting object tensors with <20% typical parameter variation, and determines three-dimensional object location with a typical error of less than ±3 cm. In this paper results are shown from a set of four different test object examples, each with a different magnetic polarisability tensor. This object set consists of a ferrite sphere, a ferrite rod, and phantom aluminium and steel handgun shapes.

Classification of materials for conducting spheroids based on the first order polarization tensor

“…Previously, it has been used to improve electrical imaging [1] (for biomedical or industrial purposes) as well as investigating electrosensing fish [2,3,4,5,6,7] and also metal detection for security screening [8,9,10] or landmine clearance [11,12,13]. It arises from the studies about the perturbation of electric or electromagnetic field in a free space (such as 2D or 3D) due to the conductivity contrast between the space itself and some small conducting objects.…”

“…Some examples regarding computation of the PT based on the explicit formula and according to the geometry as well as the material of a conducting object can be found in [17,20,21,22,23]. On the other hand, the PT is sometimes referred as the polarizibility tensor in engineering literatures and can be obtained from field measurements either in laboratory or fieldwork [8,9,10,12,13]. Since the measurements can be made as long as there is a conducting object perturbing the electric or electromagnetic field, any other information about the object is not needed to find the PT (see Makkonen et al [10]).…”

“…Besides, given a PT for some objects, Ahmad Khairuddin and Lionheart [6] and Khairuddin and Lionheart [25] have suggested that it is sometimes necessary to find an ellipsoid that has the similar PT with that objects. Similarly, in metal detection, it is necessary to use the PT obtained from metal detectors to identify and describe detected metallic targets [8,9,10,11,12,13,24]. In these two situations, it is important to understand what is described by the PT before applying it.…”

Evaluation of the thin-skin approximation boundary element method for electromagnetic induction scattering problems

“…If we know the tensor of the object beforehand, we can use it to distinguish different metallic objects from one another purely from their magnetic response. This approach has significant application in areas such as walk-through metal detectors [3], [4] and in landmine detection and clearance [5].…”

Design of electromagnetic sensor arrays optimised for inversion of the magnetic polarisability tensor

“…The design of a system which is capable of identifying buried or visually obscured metallic items has the potential to significantly reduce the time required to clear a given area of threats. Our previous research has shown the possibility of inverting the magnetic polarisability tensor, ↔ M, and its use in classifying targets [7] [8].…”