Regulation of GMA welding thermal characteristics via a hierarchical MIMO predictive control scheme assuring stability

Abstract

This paper deals with the regulation of the thermal characteristics of gas metal arc welding (GMAW). A complete treatment of the welding control problem requires the regulation of both the geometrical and thermal characteristics of the weld. Both classes of characteristics are of critical importance, but the thermal ones have received less attention in the majority of previous work. The present paper proposes a hierarchical predictive control scheme for the metallurgical characteristics of GMAW. A previously developed model for the regulation of the heat-affected zone, the cooling rate and the nugget cross-sectional area is used for the open-loop predictions. At the first level of the hierarchy, a parameterized generalized predictive control (GPC) algorithm is selected, among other control techniques, due to the inherited difficulty of the welding thermal process and the robustness of this algorithm against modeling errors and parameter variations. The main drawback of GPC is that it does not guarantee stability. The second level of the control hierarchy, where a coordinator specifies a set of reliable values for the parameters of GPC, so that stability is assured, and transmits them to the controller of the first level overcomes this difficulty. This paper provides a representative set of simulation results obtained by the present hierarchical GPC scheme, including a comparison with the classical linear-quadratic optimal control scheme. These results show the superiority of the GPC scheme.