Abstract
The electrical signals of laser plasma were measured by a passive probe based on the principle of plasma sheath effect. A high-speed CCD camera was synchronously employed to observe the dynamic behavior of the plasma. To eliminate the effect of zero drift and equipment noise on electrical signals, wavelet packet transformation (WPT) was adopted to process the original signal. The approximate coefficient of frequency band ranging from 0 to 195 Hz was reconstructed to form the denoised signal, which was found to have a good correlation with the welding quality. In order to obtain the characteristic information of the denoised signal, ensemble empirical mode decomposition (EEMD) was performed and the residual term representing the variation trend of an electrical signal was obtained. The results showed a strong connection between the value of the residual term and the weld defect. Based on the detection results of electrical signals, the formation mechanism of the surface defect was proposed according to the morphology of the plasma plume acquired by the high-speed CCD camera.
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