In-process monitoring of micro series spot welding using dual accelerometer system

Micro series resistance spot welding (MSRSW) is a common practice to form two neighboring nuggets at the same time in industrial fabrication of precision parts, especially in EV battery pack manufacturing. However, because of the Peltier effect, unbalanced Joule heat generation results in different nugget sizes. Therefore, weld quality monitoring systems are necessary to improve weld quality. Unfortunately, no work has been reported. It is still a challenge to develop a monitoring system. A series of monitoring systems have been developed for conventional resistance spot welding (RSW). Among them, electrode movement caused by heat expansion and contraction during the welding process is an important information. GAP sensor and LVDT are major sensors used in the measurement of electrode movement. However, the physical dimension of these sensors is too huge to be applied in MSRSW. This study proposes an in-process electrode movement monitoring system for MSRSW using dual accelerometers. Two-minute accelerometers were mounted on top of each electrode holders. Thanks to its good sensitivity and frequency response characteristics, an accelerometer signal was successfully picked up and integrated twice to reflect an electrode expansion/contraction movement. The correlation between two signals was analyzed by applying the Lissajous figure technique. Experimental verification of this monitoring system has been carried out. A major finding is that dual accelerometer signal and its double-integrated signal, electrode movement, reflect thermo-physical phenomena during MSRSW process.