Study On Real-time Sensing Of Welding Pool Oscillation Frequency Based On Laser Photoelectric Conversion

There exists a physical relationship between weld pool oscillation frequency and states of weld beam penetration. But the detection of weld pool oscillation frequency is very hard. Especially, it is very difficult for the traditional sensing methods through arc voltage and arc light emission in continuous welding to detecting weld pool oscillation.According to this problem, an innovative sensing method named laser photoelectrical conversion was proposed to real-time measure weld pool oscillation frequency in continuous welding. The theoretical basis and the feasibility of the proposed method were also studied and analyzed. The principle of the method is as follows. A five-line laser pattern is projected onto the whole weld specular pool surface. The projected laser lines will be reflected through the welding pool surface because of the welding pool surface with the property of mirror. The variation of welding pool surface can cause the synchronization variation of reflected laser brightness, and the variation of brightness of reflected laser ray is very obvious. The reflected laser brightness will be converted into electrical signal for sensing pool oscillation frequency after the reflected laser lines being into the special enclosure including photoelectric converter.Meanwhile, the variation of reflected laser brightness can also cause the electrical signal synchronization variation. Thus, the welding pool oscillation can be characterized by the variation of electrical signal. Through processing of the collecting electrical signal the welding pool oscillation frequency can be obtained. The proposed method successfully solves the problem that the real-time detection of weld pool oscillation frequency is very hard, especially in continuous welding. The method to further realize the real-time continual pulsed GTAW penetration sensing and control laid a foundation. The proposed method was studied by following four parts.(1) According to the measure principle of laser photoelectric conversion, the experiment system of sensing pool oscillation frequency for pulsed GTAW was designed. Comparing with sensing system of laser vision method, the sensing system real-time is significantly improved because of bringing into photoelectric conversion principle. The sensing system is simple and can be easily realized with low cost.(2) To verify the feasibility of the proposed method used to real-time detect welding pool oscillation frequency, the experiments sensing the welding pool oscillation frequency in stationary and sequential welding for pulsed GTAW were carried out by the established experimental platform. The result showed that the proposed method can successfully and stably obtain the welding oscillation frequencyin stationary welding for pulsed GTAW when the states of weld beam are full penetration or partial penetration, and the welding oscillation frequency will be decreased with welding time increasing. The proposed method can also successfully and stably obtain the welding oscillation frequency in sequential welding for pulsed GTAW when the states of weld beam are full penetration or partial penetration, and the travelling speed has a few influence on the quality of welding oscillation signal. The proposed method can obtain the welding oscillation signal under different thick work-pieces. The welding pool oscillation frequency obtained by the proposed method was consistent with laser vision method. At last, To study the effects of arc light and laser on sensing weld pool oscillation signals for pulsed GTAW, the comparative analysis about the sensing effects under two different conditions was carried out. The results show that the periodic variation of the basic electrical signal was caused by the reflected laser stripes; however, the effect of arc light on the signal variation was very faint.(3) The influence of the parameters of optical path of laser on the welding pool oscillation signal was analyzed qualitatively. Finally, the variation range of the parameters of optical path of laser was obtained. In addition, the phenomenon that the location of the enclosure has a great influence on the quality of welding pool oscillation information was found through experiments. The corresponding relation among the position of laser, welding pool and enclosure was reversely simulated by establishing the standard model of weld pool, incident, reflected ray and enclosure position’s mathematical model. The appropriate position of enclosure can be obtained before the sensing experiments by established model. Meanwhile, the effect of welding parameters such as arc length, pulsed frequency and welding speed on measurement was also studied. As a result, the variation range of the welding parameters in which the high quality of the welding pool oscillation signals can be obtained was found.(4) The relationship between welding oscillation frequency and weld geometry size was analyzed. And the variation of welding oscillation frequency with welding parameters was studied. The results show that the states of weld beam can be sensed by the welding oscillation frequency extracted by the proposed method; the welding oscillation welding frequency of states of weld beam partial penetration is higher than states of weld beam full penetration; there is an abrupt transition in the frequency from the partial penetration to complete joint penetration.