Direct Sampling of the Pulsed Eddy Current Response From Non-Ferromagnetic Metals

Abstract: The non-destructive testing method Pulsed Eddy Current, or PEC, can be used to measure the thickness and resistivity of conducting and non-ferromagnetic materials. In this project I investigated if a gauge at ABB, which uses PEC technology to measure the thickness of aluminium strips in rolling mills, can extract more information about the strip by sampling the direct signal with a higher resolution than before. Using the gauge with this new sampling method, I first compared the noise level of the integrated signal obtained by using analog and digital integration. Then I used an upper noise level limit to determine how thick the strips could be without the noise level being too high. Finally, I used simulations and measurements of clad strips to determine if more information, such as the resistivity distribution, can be extracted from the measured signal and if the sampling frequency is high enough to detect the information. I found that the noise level can be reduced by replacing an analog integrator with a low-pass filter and integrating the signal digitally. I also found that the thickness can be between 1.3 mm and 12 mm whereas it can be between 0.50 mm and 8.0 mm with the previous sampling method. Finally, I found that the sampling frequency is high enough and that it is possible to determine if a strip has a clad layer or not. The results in this project suggest that it is beneficial to change the way the signal is sampled to this new method.