Characterising small objects in the regime between the eddy current model and wave propagation
Characterising small objects in the regime between the eddy current model and wave propagation
复制标题
DOI：
10.1017/s0956792523000207
复制
发表时间：
2022-09
影响因子：
1.9
通讯作者：
P. Ledger;W. Lionheart
中科院分区：
文献类型：
--
作者：
P. Ledger;W. Lionheart
能够在低频下但在麦克斯韦系统的涡流近似中的建模误差变大的情况下对物体进行建模，对于改进当前的金属检测技术是重要的。重要的是，建模误差随着频率的增加而变大，但是涡流模型的精度还取决于对象拓扑结构及其材料，与相同尺寸和材料的其他几何形状相比，某些几何形状的误差要大得多。此外，对于高磁传导材料，涡电流模型与非导磁物体（具有相似的导电率、尺寸和形状）相比在小得多的频率下发生故障，因此，在金属检测器的典型操作频率下使用涡电流来表征由导磁材料制成的小磁性物体并不总是可能的。为了解决这个问题，我们推导出一个新的渐近展开的可渗透的高传导性的对象，是有效的小对象，并持有不仅为频率的涡流模型是有效的，但也为涡流建模误差变大的情况下，应用涡流近似将是无效的。领导阶项，我们推导出导致新形式的对象表征的极化率张量对象的描述，其中的系数可以从解决矢量传输问题。我们希望这些新的特性是重要的，当考虑对象在更大的距离线圈，这是重要的安全关键应用，如识别地雷，未爆弹药和隐藏的武器。我们还希望我们的研究结果是重要的，当表征文物的考古和法医学意义在更大的深度比涡流模型允许，并有进一步的应用停车传感器和提高检测隐藏的，看不见的，金属物体。
Being able to characterise objects at low frequencies, but in situations where the modelling error in the eddy current approximation of the Maxwell system becomes large, is important for improving current metal detection technologies. Importantly, the modelling error becomes large as the frequency increases, but the accuracy of the eddy current model also depends on the object topology and on its materials, with the error being much larger for certain geometries compared to others of the same size and materials. Additionally, the eddy current model breaks down at much smaller frequencies for highly magnetic conducting materials compared to non-permeable objects (with similar conductivities, sizes and shapes) and, hence, characterising small magnetic objects made of permeable materials using the eddy current at typical frequencies of operation for a metal detector is not always possible. To address this, we derive a new asymptotic expansion for permeable highly conducting objects that is valid for small objects and holds not only for frequencies where the eddy current model is valid but also for situations where the eddy current modelling error becomes large and applying the eddy approximation would be invalid. The leading-order term we derive leads to new forms of object characterisations in terms of polarizability tensor object descriptions where the coefficients can be obtained from solving vectorial transmission problems. We expect these new characterisations to be important when considering objects at greater stand-off distance from the coils, which is important for safety critical applications, such as the identification of landmines, unexploded ordnance and concealed weapons. We also expect our results to be important when characterising artefacts of archaeological and forensic significance at greater depths than the eddy current model allows and to have further applications parking sensors and improving the detection of hidden, out-of-sight, metallic objects.