Primary Circuit Dynamic Resistance Monitoring and its Application to Quality Estimation during Resistance Spot Welding

Resistance spot welding is one of the most widely used processes in sheet metal fabrication. Improvements in quality assurance are important to increase welding productivity. Until now, quality estimation was performed using dynamic resistance at the secondary circuit, which is easily measured. However, this method has many problems when applied to in-process real-time systems. In this study, weld quality was ensured through the use of a new dynamic resistance monitoring method. This method used instantaneous current and voltage values measured at the primary circuit. A weld quality estimation system is suggested using these variables. For quality estimation, ten factors relating to quality were extracted from the primary circuit dynamic resistance, measured in the timer. The relationship between these factors and weld quality was determined through a regression analysis. Four regression models were developed using the results of the analysis in order to estimate weld strength and nugget diameter. Neural network models were also suggested using these factors. The results of the neural network were compared with those of the regression models. This was designed so weld quality estimation could be obtained upon completion of welding. Moreover, the proposed method using the primary circuit dynamic resistance has an advantage over the conventional one, such as obtaining information on quality without the use of extra devices. Y CHO is a Research Associate, Department of Mechanical Engineering, The University o f Michigan, Ann Arbor, Mich. S. RHEE is an Associate Professor, Dept. of Mechanical Engineering, Hanyang University, Seoul, Korea. This paper was presented at the A WS 81st Annual Convention, April 25-27, 2000, Chicago, Ill. Introduction For several decades, resistance spot welding has been an important process in sheet metal fabrication. The automotive industry, for example, prefers spot welding for its simple and cheap operation. Recently, however, attempts are being made to reduce the number of spot welds to increase productivity. In order to minimize the number of spot welds and satisfy essential factors such as strength, weld quality must be obtained. Traditionally, to check weld quality, destructive and nondestructive tests were used on randomly sampled workpieces at the production site. These processes, however, can only be examined off-line, making it impossible to receive pertinent information regarding the weld quality during the process. Weld quality estimation must be done in real time to monitor and repair weld defects as they occur. As resistance spot welding has electromechanical elements, various weld quality control techniques have been proposed based upon the following process parameters: dynamic resistance, welding current, voltage, and electrode displacement (Refs. 1, 2). In general, electricitybased systems, which maintain the volt-