RResearch and development of algorithm for determination of absolute conductivity value without calibration by eddy current techniques

无损检测方法通过确定材料的载荷极限，对技术部件的安全可靠运行至关重要。广泛使用的程序是导电材料的涡流检测。除了检测材料缺陷外，还可以无损地进行复杂的材料表征。材料性能由其对电性能的影响决定。电磁场在材料中感生涡电流，其值受材料参数（电导率、磁导率、介电常数、几何形状）影响。这些影响可以通过确定线圈的阻抗来测量。这些阻抗值取决于频率和提升，例如线圈和待测材料之间的距离。到目前为止，通过涡流测量电导率是基于经验的方法。使用具有不同电导率的一些参考样品确定校准曲线。该校准仅对一个设备、一个频率有效，并且通常仅对非常有限的提升范围有效。与此同时，存在的问题是，通常没有足够的精确定义和良好分级的参考样品来确定这些曲线。通过仿真结果发现了一种新的理论方法。这是另一种可以直接测定电导率的方法。在这种情况下，它是直接从物理定律推导出来的，而不再是从经验校准曲线推导出来的。因此，它不限于特定的测量频率。此外，提离值可以直接分配给测量的阻抗值。该方法使得能够将电导率层析成像执行为切片深度剖面。在材料内部的场分布上的积分应导致电导率值。必须找到积分值和电导率之间的关系，这在数学上要求很高，因为它会导致一个逆问题。传播体积的电特性必须根据表面上的点上的测量值来确定。假设电导率仅在深度（z方向，波传播方向）上变化，而在平面上具有相同的值。通过频率改变穿透深度，改变一定深度的材料对计算积分有不同的影响。以这种方式，必须开发用于确定作为深度的函数的电导率的算法。在薄层的情况下，似乎可以同时确定层的厚度和电导率，并且另外确定衬底的近似电导率。这一新方法必须加以详细研究，并为实际应用做好准备。使用修改的硬件不仅测量接收线圈中的电压，而且还测量激励线圈中的电流，从而可以计算测量装置的阻抗。

Non-destructive testing methods are essential for safe and reliable operation of technical components by determination of load limits of the material. A widely-used procedure is eddy current testing of conductive materials. Besides detection of material defects it is possible to perform complex material characterization non-destructively. Material properties are determined by their influence on electrical properties. The electromagnetic field induces eddy currents in the material whose values are influenced by the material parameters (conductivity, permeability, permittivity, geometry). These influences can be measured by determining the impedance of the coil. These values of the impedance depend on the frequency and the lift off, e.g. the distance between the coil and the material to be tested. Until now the measurement of conductivity by eddy current is based on a empirical approach. Using some reference samples with different conductivity a calibration curve is determined. This calibration is valid only for one device, one frequency and mostly only for a very limited range of lift off. At the same time there is the problem there are often not enough exactly defined and well-graduated reference samples for determining those curves. A new theoretical approach was found by simulation results. It is another way that makes possible direct determining of conductivity. In this case it is derived directly from the physical laws and no longer from the empirical calibration curve. Thereby it is not limited to a specific measurement frequency. Additionally the lift off value can be assigned directly to the measured impedance value. This method enables carrying out a conductivity tomography as sliced depth profiling. Integration over the field distribution inside the material should result the conductivity value. The relationship between the value of the integral and the conductivity has to been found. This is mathematically very demanding because it leads to an inverse problem. The electrical properties of the propagation volume have to be determined from the measured values on a point of the surface. It is assumed that the conductivity changes only in the depth (z direction, direction of wave propagation), whereas it has in plane the same value. Changing the penetration depth by frequency change the material in a certain depth has a different influence on the calculated integral. In this way an algorithm for determining conductivity as a function of depth has to be developed. In case of thin layers it seems to be possible to determine the thickness and the conductivity of the layer at the same time and additionally the approximate conductivity of the substrate. This new approach has to be examined in detail and prepared for the practical use. A modified hardware is used measuring not only the voltage in the receiving coil but additionally the current in the exciting coil making possible the calculation of the impedance of the measurement set.