Use of robotic friction stir welding (FSW) has gained in popularity as robotic systems can accommodate more complex part geometries while providing high applied tool forces required for proper weld formation. However, even the largest robotic FSW systems suffer from high compliance as compared to most custom engineered FSW machines or modified computer numerical control (CNC) mills. The increased compliance of robotic FSW systems can significantly alter the process dynamics such that control of traditional weld parameters, including plunge depth, is more difficult. To address this, closed-loop control of plunge force has been proposed and implemented on a number of systems. However, due to process parameter and condition variations commonly found in a production environment, force control can lead to oscillatory or unstable conditions and can, in extreme cases, cause the tool to plunge through the workpiece. To address the issues associated with robotic force control, the use of simultaneous tool interface temperature control has been proposed. In this paper, we describe the development and evaluation of a closed-loop control system for robotic friction stir welding that simultaneously controls plunge force and tool interface temperature by varying spindle speed and commanded vertical tool position. The controller was implemented on an industrial robotic FSW system. The system is equipped with a custom real-time wireless temperature measurement system and a force dynamometer. In support of controller development, a linear process model has been developed that captures the dynamic relations between the process inputs and outputs. Process validation identification experiments were performed and it was found that the interface temperature is affected by both spindle speed and commanded vertical tool position while axial force is affected primarily by commanded vertical tool position. The combined control system was shown to possess good command tracking and disturbance rejection characteristics. Axial force and interface temperature was successfully maintained during both thermal and geometric disturbances, and thus weld quality can be maintained for a variety of conditions in which each control strategy applied independently could fail. Finally, it was shown through the use of the control process model, that the attainable closed-loop bandwidth is primarily limited by the inherent compliance of the robotic system, as compared to most custom engineered FSW machines, where instrumentation delay is the primary limiting factor. These limitations did not prevent the implementation of the control system, but are merely observations that we were able to work around.
Morphology and strength of acrylonitrile butadiene styrene welds performed by robotic friction stir welding
