Signal processing and high speed imaging as monitoring tools for pulsed laser welding

Sammanfattning: In Laser Materials Processing there has always been a need for suitable methods to supervise and monitor the processes on line, to ensure correct production quality or to trigger alarms when failures are detected. Numerous investigations have been made in this field, including experimental and theoretical work. It is common practice in this field to monitor surface temperature, plasma radiation and back-reflected laser light, coaxially with the laser beam. Traditionally, the monitoring systems involved carry out no statistical analysis of the signals received - they merely involve thresholds. This thesis looks at the feedback collected during laser welding using such a co-axial setup from a Digital Signal Processing point of view and also uses high speed video photography to correlate signal perturbations with process anomalies.Modern Digital Signal Processing techniques such as Kalman filtering, Principal Component Analysis and Cluster Analysis have been applied to the measurement data and have generated new ways to describe the weld behaviour using parameters such as reflected pulse shape. The limitations of commercially available welding supervision systems have been studied and design suggestions for the next generation of on line weld monitoring equipment have been formulated.