Microprocessor-based arc voltage control for gas tungsten arc welding using gain scheduling

Limitations of the traditional automatic voltage controller (AVC) for gas tungsten arc welding are explained, and the characteristics of the welding arc that cause these limitations are presented. The design of an adaptive AVC using a microprocessor-based controller is described. The enhanced AVC uses gain scheduling to maintain optimum gain in the feedforward loop of the system and offers a constant arc length control mode that is extremely useful under conditions where the welding current is varied over a wide range. Both of these features are implemented with the use of look-up tables that define the correct parameters for the current operating point. The construction of these look-up tables must be offline in the design phase. A prototype system has been constructed and tested. It has proved to provide a full control of the arc length during weld pass termination, and experiments have shown that tail-out cracking in alloys prone to these kinds of defects, like Inconel 718, can be reduced by controlling the heat input during this process