Observer-based adaptive robust control of friction stir welding axial force

Friction stir welding (FSW) is a relatively new and promising joining process that is the subject of much current research. With constant welding parameters the axial force can vary significantly due to changes in workpiece temperature and other process variations, producing welds with inconsistent microstructure and tensile strength. Control of the axial weld force is desirable to improve the weld quality. In this work an observer-based adaptive robust control (ARC) approach for the axial force of FSW is used to overcome process disturbances and model errors stemming from the simplistic dynamic models suitable for control. Good correlation is shown between spindle power and axial force, allowing readily available power measurements to be used for feedback. A model of the axial force is developed as a combination of a nonlinear static gain and linear dynamics. A force controller is constructed using the ARC approach and estimated state feedback from the adaptive divided difference filter (ADDF). Verification experiments are conducted on a vertical milling machine configured for FSW using an open architecture controller. The combined ARC/ADDF technique is shown to dramatically reduce axial force variations.