Although there have been many investigations into monitoring and control of resistance spot welding (RSW) of sheet metal having a thickness greater than 0.5 mm, that of thinner components has rarely been investigated. Monitoring of expulsion in a small scale RSW process was carried out via measurement of voltage, electrode displacement, and force change during the welding current pulse. It was found that electrode displacement increased steadily during the current pulse for an expulsion free weld. For welds with visible expulsion, the electrode voltage had a small but readily observed spike; the electrode displacement showed a dip or a decrease in total amplitude; the force change during a welding cycle was of greater magnitude. Since the system uses a constant current power supply, the voltage increase corresponds to an increase in dynamic resistance associated with the expulsion event. It was also observed that the magnitude of the electrode displacement dip was directly related to the volume of...

Monitoring of expulsion in small scale resistance spot welding

To develop an effective nondestructive evaluation method for the welding quality of the resistance spot welding, the electrode displacement signal during the resistance spot welding process is monitored, and the acquisition data of the signal are innovatively presented as the radar chart format. Some geometric features of the radar charts are extracted to reflect the welding quality. The decision tree classification technique is adopted to build a classifier and to provide a visible and intuitive diagnostic procedure for welding quality assessment. Test results of the decision tree classifier show that it is feasible and reliable to evaluate weld quality based on the graphics features of the radar chart. The features and the weld quality are closely related, and their extraction avoids complex algorithm. Meanwhile, when there are small samples, the decision tree classifier can identify good or bad weld accurately and rapidly, even though the weld is from an abnormal welding process, such as expulsion, current shunting, greasy surface, and small edge distance condition.

A new method for nondestructive quality evaluation of the resistance spot welding based on the radar chart method and the decision tree classifier

A novel quality evaluation method for resistance spot welding based on the electrode displacement signal and the Chernoff faces technique

The paper presents a new image processing-based algorithm for electrode tip displacement measurement. Its accuracy is evaluated by a comparison with a conventional optical sensor. The results show a clear advantage of the proposed image processing algorithm. A special program was written in order to analyze the algorithm. The electrode tips are detected by the program. It also determines the size of the reference areas on the electrode tips and tracks these areas during the welding process. Due to a multi-thread implementation of the algorithm the execution time was reduced significantly as compared to previously presented work. Consequently this approach is now applicable for control-related resistance spot welding purposes.

https://iopscience.iop.org/article/10.1088/0957-0233/23/6/065401/pdf

Image-based electrode tip displacement in resistance spot welding

Automatic welding quality classification for the spot welding based on the Hopfield associative memory neural network and Chernoff face description of the electrode displacement signal features

A high-speed video imaging system was employed to directly measure the electrode displacement of the small scale resistance spot welding (SSRSW) process. This measurement technique was chosen because it eliminates a number of potential error sources inherent in other electrode displacement measuring techniques. Careful observation of the heating and cooling portions of the electrode displacement curves revealed that each is comprised of two identifiable segments. Distinct high-velocity segments of the displacement curves were thought to correspond to solid-liquid phase transitions in the weld nugget, while lower-velocity portions corresponded to thermal expansion or contraction of solid material. It was found that the magnitudes of the high speed portions of the electrode displacement were more closely correlated with the weld nugget thickness than was the overall magnitude of the electrode displacement. Furthermore, all measures of electrode displacement were more closely correlated to weld nugget thickness than to nugget diameter.

K. Ely

Papers

Monitoring resistance spot nugget size by electrode displacement