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Resistance spot welding (RSW) is one of the most widely used processes in sheet metal fabrication. Although used in mass production for several decades, RSW has a major problem of inconsistent quality from weld to weld, which results from the complexity of basic process as well as from various process conditions, noise and errors. So far there is no satisfactory on-line non-destructive quality evaluation. In this research, a data acquisition system was developed to on-line monitor the dynamic signals in resistance spot welding, such as electrode voltage, welding current, dynamic resistance and dynamic electrode force. Of these signals, dynamic resistance and electrode force signals were found to provide plenty of quality information. When an expulsion occurred, the dynamic resistance signal experienced a sudden drop and dynamic electrode force signal has a vibration signature.