KNN classification of metallic targets using the magnetic polarizability tensor

Abstract

Walk-through metal detectors are used at check points for preventing personnel and passengers from carrying threatening metallic objects, such as knives and guns into a secure area. These systems are capable of detecting small metallic items, such as handcuff keys and blades, but are unable to distinguish accurately between threatening objects and innocuous items. This paper studies the extent to which a K-Nearest Neighbour-classifier can distinguish various kinds of metallic objects, such as knives, shoe shanks, belts and containers. The classifier uses features extracted from the magnetic polarisability tensor, which represents the electromagnetic properties of the object. The tests include distinguishing threatening objects from innocuous ones, classifying a set of objects into 13 classes, and distinguishing between several similar objects within an object class. A walk-through metal detection system is used as source for the test data, which consists of 835 scans and 67 objects. The results presented show a typical success rate of over 95% for recognizing threats, and over 85% for correct classification. In addition, we have shown that the system is capable of distinguishing between similar objects reliably. Overall, the method shows promise for the field of security screening and suggests the need for further research.

Electromagnetic induction tomography

“…This has the advantage of helping in the estimation of the target's tensor and location. Examples of the use of the dipole approximation for WTMDs have been reported by Peyton (2013, 2014) for single and multiple objects, respectively, and by Makkonen et al (2013) using classification techniques. Several workers have investigated the dipole model for identification of buried ERW notably (Norton & Won, 2001).…”

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]).…”

“…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]). Thus, by making measurements with metal detectors, the PT is commonly used by electrical engineers to identify and chararacterize metallic targets.…”

“…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.…”