Electrode displacement measurement dynamics in monitoring of small scale resistance spot welding

Small-scale resistance spot welding (SSRSW) process is widely used in medical and electronic devices manufacturing, where it is used to join thin foils and fine wires. Although tremendous work has been focused on traditional RSW process monitoring and control, SSRSW has received less attention, one reason being the process variables that are used in monitoring and control are very small in magnitude. A good example of a process variable that presents this difficulty is electrode displacement, which is generally regarded as one of the variables that can provide real-time information useful for monitoring and controlling RSW process quality. However, in SSRSW production, it is difficult to measure the displacement because its magnitude is very small. In this work, a fibre optic sensor was used for displacement monitoring. It was discovered that the implementation of the sensor introduced extraneous transient fluctuations into the measured signature. To develop techniques to improve the accuracy of the fibre optic sensor displacement measurements, more reliable electrode movement data were obtained using a high-speed video set-up. It was found that the displacement obtained by the fibre optic sensor had a delay and oscillatory feature compared to true displacement. Although it is a valuable research tool, the high-speed video system is not practical for online monitoring and control, while the fibre optic sensor has more potential for real-time monitoring and control implementation. Therefore, attempts were made to recover true displacement from the contaminated fibre optic displacement. A number of dynamic models in state-space form were identified to decode and predict the true displacement from fibre optic sensor measurement. Model parameters were estimated from two sets of known displacement data with fibre optic sensor displacement as input and high-speed video displacement as output. A fifth order innovations form model optimized by the subspace method was found to have the best accuracy compared to high-speed video measurement. Compared to uncorrected fibre optic and high-speed video displacement, fibre optic measurements corrected by this model are promising for real-time monitoring and control algorithms that use electrode displacement measurements.