Modeling and Simulation for Welding Automation

Doktorsavhandling, 2010

Fusion welding of metals is an important manufacturing process. This thesis presents a modeling and simulation strategy for welding automation. It deals with the problem of mitigating discontinuities caused by this process, and focus is on a high joint performance and structural integrity. The strategy emphasizes on generic solutions for flexible fixturing and feedback control. The meaning of flexible fixturing is to design constraints and clamping for the structure being welded in such way that residual stress and deformation can be mitigated compared to conventional fixtures designed only for stiff fastening. Here is suggested to use simulations to predict these phenomena and then use this information in the design. The use of feedback control is here based on finite element simulations and analyses of welding processes. By using a calibrated and validated model it is possible to perform model based controller design. It is also possible to evaluate and decide on what physical configuration is needed for sensors to measure and feed back important process quantities. By this evaluation, trials and visualization can be made already in the design phase of the production chain aside or prior to continuous production and thereby reduce the need for physical experiments. Welding of selected materials and two types of welding heat sources have been investigated. Suggestions for systematic model calibration methods have been proposed including global and local optimization. Model order reduction has been evaluated and controller design has been performed based on convex optimization methods involving linear matrix inequalities. Experimental work has also been carried out to verify the simulation results and the usefulness of the methods. The suggested approach is not independent of physical experiments, since it is based on models that have to be calibrated, but the experiments needed are assumed to be carried out in a simple and cost efficient way. The thesis suggests how these experiments can be performed. The simulation-based strategy has been evaluated and proven to work successfully in different types of applications. The proposals given in this thesis are assumed to form an efficient tool for the welding engineer in order to obtain high weld quality in automated welding. The research presented indicates that the methods work well in real situations and that further work for more robust industrialization will be beneficial for the welding community.