Recent terrorist attacks and the subsequently perceived threats to national security have meant that the demands of security networks are greater than ever before. The walkthrough metal detector (WTMD) is on the frontline of the personal security screening network and is used to check individuals for metallic threat objects such as knives or guns. In many operational conditions each positive identification of a metallic object is first considered to be a threat, however in many instances the source of the trigger does not necessarily constitute the detection of a threat object as many innocuous metallic items are allowed in certain secured zones, e.g. a mobile phone in an airport. Operators are often required to perform physical searches to confirm that the trigger was not due to a threat object. Often little or no positional information is provided, and as a result the time taken to conduct a physical search is often significant and causes delays to people waiting to be screened. A tomographic magnetic imaging system was designed and built with the intention of recovering detailed information about magnetic and/or conductive objects within the detector space, in addition to identifying the position of the detected object to a greater degree of accuracy than is possible with the current generation of WTMDs. The object-specific information that is recovered by the system is in the form of the magnetic polarisability dyadic tensor, which is considered to completely describe the behaviour of an object when it interacts with a magnetic field. The prototype system that has been produced is capable of inverting object tensors with a typical error of < 20%, and determines three-dimensional object location with an error of ±2 to ±3 cm. Given that both the inversion of an object tensor, and object location to this degree of accuracy have not been observed in the academic or commercial literature it is claimed that both of these factors represent original work. In order to achieve this level of performance a special 8-transmit, 8-receive coil array has been produced which is capable of minimising mutual coupling between all channels during background operation. This allows for greater detection of changes occurring as a result of the presence of the object, rather than from the excitation source. A procedure for modelling the response of this prototype coil array to a specified magnetic polarisability dyadic tensor is derived and tested by examining the response of the system to a ferrite sphere. This testing indicates that the measurements from the WTMD and simulated object response differ by less than 10% in almost all cases. Testing of the inversion algorithm has been conducted with six known objects, and the system has been shown to perform consistently, and to an acceptable degree of accuracy. The procedure for the inversion of an object tensor is based upon the response of the model, and consequently this error contains a portion of the error contained from the model. The detector system was tested in an airport environment where sensitivity was shown to be beyond the levels required for typical operation.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:686712 |
| Date | January 2011 |
| Creators | Marsh, Liam Aubrey |
| Contributors | Peyton, Anthony |
| Publisher | University of Manchester |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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