Multivariable geometry control of welding. Part I. Process modeling

While numerous examples of single variable control of in-process welding geometry outputs exist, there are no multivariable geometry cases. In this paper a control model for Gas-Metal Arc Welding is developed for the outputs of bead width, height and depth. Analytical models are shown to be inadequate for this problem, and limited process understanding and inherent complexity precludes the use of numerical analysis. Consequently, a model is derived empirically, fitting measured transient responses with pulse transfer functions. It is found that a second order model with constant damping ratio and velocity dependent natural frequency is most appropriate. This velocity dependance can be eliminated if a distance rather than a time basis is used for the model. More importantly, the static system behavior is found to be significantly non-linear, and results in a bivalued relationship between the available inputs of wirefeed rate and travel speed, and the outputs. It is also shown that the control authority available for decoupled two-output control is very limited. From this model it is clear that single variable control is possible, but that non-linear control methods will be necessary, and that useful multivariable control will necessitate process redesign to enhance the range of controllability.