應用於非導磁金屬導電率量測之磁電渦流感測器開發

Abstract

本論文提出一種基於磁阻抗感測器之磁電渦流感測系統開發，利用控制頻率的方法，同時預測非導磁金屬板之導電率及厚度，估測金屬板厚度為資訊做補償之導電率預測，增加準確度。在系統開發中，使用分散式電流源模型計算線圈於金屬導體產生的電渦流場及其產生之二次磁場做頻率響應的分析。分散式電流源法可以直接計算由非導磁金屬導體中電渦流產生之磁通量密度，效果比有限元素軟體模擬的結果更佳， 因為有限元素軟體測量非導磁金屬導體之電渦流產生磁場的方法，是透過計算線圈加上非導磁金屬導體產生的磁場，減去僅有線圈時的磁場，會造成較大的雜訊。 分散式電流源模型能有效地進行金屬板不同幾何與物理特性之模擬，應用於設計磁電渦流感測器。利用分散式電流源模型計算求得磁電渦流感測器之頻率響應，建立模擬的模板網格，經過校正後映射到實驗建立的網格。之後再以二維映射的方式將頻率響應的大小與相位，映射至導電率和厚度的座標，使用線性內插的方法估測導電率和厚度。相較於商用金屬導電率測試儀，此磁電渦流感測系統透過激勵線圈、閉迴路電流放大器以控制電路、異向磁阻感測器及相關功能放大電路組成，不僅成本較低，並可同時估測被測非導磁金屬物體之幾何及材料特性。為了準確控制線圈電流，此研究亦開發具有閉迴路功能之電流放大器。 本論文以分散式電流源法開發利用控制電流頻率的方法，同時估測非導磁金屬導電率及厚度之磁電渦流感測器，於測量較薄金屬片時，其導電率估測比商業用導電率量測儀更佳精準。

This paper proposes the development of a magnetic eddy current measurement system based on a magnetic sensor. The accuracy of conductivity estimation is increased by the compensation of metal thickness. Distributed Current Source (DCS) method is used to analyze the eddy current field and its secondary magnetic field. In addition, Distributed Current Source method calculates magnetic flux density generated by eddy current in non-magnetic metal conductor directly, and the result is better than the outcome of finite element method to measure the magnetic field generated by the eddy current of the non-magnetic metal conductor through finite element software, is by calculating the magnetic field generated by the excited coil and the non-magnetic metal conductor minus the magnetic field only contributed by the excited coil. The difference between the magnitude of these two methods is wide apart, so the finite element method will cause a larger error. Distributed Current Source model can simulate sample with different geometric and physical characteristics, which not only provides the basis for the characteristic estimation, but also used to calibrate the experimental data and build the plate for estimation. Use the sweep data simulated by Distributed Current Source model and experimental data to generate the plate, then use two –dimensional mapping method to project the frequency response to characteristic coordinate system. Finally, the properties of the metal are estimated by linearly interpolate. Compared with the commercial metal conductivity tester, the cost of this magnetic eddy current test system consisted of an excited coil, a close-loop current amplifier, an anisotropic magnetoresistive sensor, and related functional circuit is lower. Moreover, this system can estimate the thickness and conductivity of the sample at the same time. In order to control the current which inputs to the coil, the closed-loop control circuit is discussed and developed in this paper. Based on the simulation and experimental results, the proposed system is deemed effective in estimation of conductivity and thick of non-ferrous metal plate.