Online Precision Measurement of Weld Indentation in Resistance Spot Welding Using Servo Gun

Resistance spot welding (RSW) is the predominate joining method used for steel auto body assembly. Significant effort has been made to control and inspect the RSW quality in real time to guarantee the security and the reliability of body structure joints. Among the various methods, monitoring of the surface indentation is proven to be a promising method to actualize online weld quality assessment. In this article, a new approach to online measurement of indentation depth for the RSW process using a servo gun is proposed. The approach entails the development of a modified measurement system to eliminate the transmission error of the toothed belt drive and compensate the deflection of the fixed electrode arm, which enables precision measurement of the electrode tip displacement (ETD). Furthermore, consideration is given to explaining the downtrend of the displacement curve during the hold time within the RSW process, and the indentation depth at ambient temperature is finally estimated through curve fitting and experimental correction. The experimental results show that the error of indentation estimation could be limited to ±30 μm (±2σ) at 95% confidence level, which far surpasses the performance of the previous method. Based upon sensitivity analysis, the precision is proven to be robust to different workpieces ranging from low-carbon steel to high-strength steel.