Mathematical description of AC resistance spot welding control problem and limits of its controller design

Resistance spot welding (RSW) is widely used in sheet metal fabrication industries even though the medium frequency type has appeared. Since there are lots of demands to significantly improve its performances depending on better control schemes, it is valuable to answer the question: how much of the performance it might improve if more advanced control strategies could be used. In addition, various modeling work has been done in the past twenty years. However, there are few new papers showing controller design based on these models and improving its performances effectively. This paper presented the complete mathematical description of the control problem firstly and revealed the critical points in the model of the whole process which significantly affect the control strategy design. Then it explained why most feedback strategies ordinarily used lose their theoretical foundations and few existing control algorithms can be directly used for this control problem with RMS valued current as the output and input constraints. Moreover, a practical control strategy with solid theoretical support was proposed, which seems to be simple but actually with lots of complicated theoretical issues concerned and is one of the few reasonable control algorithms considering the unique properties of RSW's 'single phase AC model, input constraints and nonlinearity of the output'. In the end, this paper verified the proposed scheme against the experimental results, and introduced a new interesting general control problem for later researches.