Interval Model Control of Consumable Double-Electrode Gas Metal Arc Welding Process

This application paper concerns the modeling and control of an innovative welding process, namely, the Consumable Double-Electrode Gas Metal Arc Welding. This innovative process can dramatically increase welding productivity and reduce weld distortion. It has demonstrated the feasibility to double the travel speed for automatic welding but requires controls to realize its unique advantages. To reach this goal, the bypass voltage and base metal current were selected as process outputs to be controlled. The bypass current and main wire feed speed were selected as the inputs and the control system was reduced to two single-input-single-output (SISO) subsystems for convenient implementation and design. Physical analysis and derivation show that these subsystems can be approximated as first-order model systems but their parameters depend on manufacturing conditions. Hence, they were described using first-order interval models whose parameters are unknown but bounded by known intervals. Step response experiments were conducted with selected range of manufacturing conditions to identify a few models for each of the subsystems. These models were then used to derive two interval models. To increase the stability margin, the intervals identified were artificially enlarged. Finally, a prediction-based interval model control algorithm was used to control the resultant interval models and closed-loop control experiments verified the effectiveness of the developed control system. Note to Practitioners-Gas metal arc welding (GMAW) is the most widely used arc welding process. However, its current melting the wire is the same as the current heating the base metal. Increasing the melting speed for higher productivity proportionally increases the base metal heat input resulting in increased distortion. To resolve this problem, the authors proposed a simple way to modify the GMAW process such that the base metal current is independent from the melting current. The melting speed can thus be increased, while the base metal heat input and resultant distortion are still controlled at given desirable levels. This modified process, referred to as the double-electrode GMAW or DE-GMAW, has been proved to be capable of doubling the travel speed for automobile welding. However, it is more complex than GMAW and requires appropriate controls to achieve its claimed advantages. This paper is devoted to the modeling and control of this innovative process.