Analysis and development of a real-time control methodology in resistance spot welding

The single-parameter, inprocess monitor and automatic control systems for the resistance spot welding process have been studied by many investigators. Some of these have already been commercialized and used by sheet metal fabricators. These control systems operate primarily on one of the three process parameters: maximum voltage or voltage drop, dynamic resistance, or thermal expansion between electrodes during nugget formation. Control systems based on voltage or dynamic resistance have been successfully implemented for industrial appl ications. A great amount of experience on these two control methods has been accumulated through trial-and-error approaches. The expansion-based control system is not commonly utilized due to lack of experience and understanding of the process. Since the expansion displacement between electrodes during welding responds directly to the weld nugget formation, this control parameter provides a better means to produce more precise spot welds. However, the control algorithm of this method is more complex than that for the other two methods. Fruitful development of such a system cannot be obtained by trial-and-error approaches. This paper presents a systematic approach to develop the expansion-based control algorithm for resistance spot welding. The finite element method was used to simulate the welding process and to determine the physical response of C. L. TSAI, W. L. DAI and D. W. DICKINSON are with the Department of Welding Engineering and J. C. PAPRITAN is with the Department of Agricultural Engineering, The Ohio State University, Columbus, Ohio. the joint material to the various welding conditions. Direct correlations between nugget formation and expansion displacement between electrodes were obtained. By systematic computer simulations, the weldability characteristic curves for resistance spot welding were developed. These weldabi l i ty curves show inadequate welding conditions that would cause nugget expulsion and current shunting. A welding duration curve, which shows appropriate time for squeeze, weld and hold cycles, was developed and used as a basis for in-process resistance spot welding control.