Comparative study on resistance and displacement based adaptive output tracking control strategies for resistance spot welding

Abstract

Abstract To achieve high quality resistance spot welds (RSW) through intelligent manufacturing of automotive body structures, adaptive control for RSW processes becomes imperative and has gained global attention. Tracking ideal process signal profiles predetermined offline is a commonly-used strategy to realize the adaptive control of RSW and is found effective in compensating for the adverse impacts caused by shunting and electrode wear. However, the validity of this control strategy under other types of disturbance is still unproven. In this article, typical abnormal welding conditions, i.e. initial sheet gaps and edge proximity, were designed to test the performance of output tracking control strategies for steel RSW. Two types of process signals including dynamic resistance and electrode displacement were tracked to approach the referenced signal profiles through the adjustment of weld parameters. Comparative studies were conducted based on metallographic analysis and mechanical tests to validate the two tracking control strategies. Results demonstrated that gap and edge proximity conditions decreased the nugget diameter and the tensile-shear strength of the welds. When satisfactory tracking of the reference resistance signal was realized under these two types of abnormal conditions, weld properties are further degraded, owing to a reduction in welding current. On the contrary, tracking the electrode displacement signal mitigated the negative influences from the disturbance through increasing welding current properly. When the disturbance was too severe, the displacement-based strategy raised the risk of expulsion. The findings suggest that tracking the dynamic resistance signal is not optimal for adaptive control of steel RSW, while tracking the electrode displacement signal is more effective. However, to achieve a more satisfactory adaptive control result, the output tracking control should be used in combination with expulsion suppression techniques.