Design and Implementation of Nonlinear Force Controllers for Friction Stir Welding Processes


In friction stir welding (FSW) processes, force control can be used to achieve good welding quality. This paper presents the systematic design and implementation of FSW force controllers. The axial and path forces are modeled as nonlinear functions of the FSW process parameters (i.e., plunge depth, tool traverse rate, and tool rotation speed). Equipment models, which include communication delays, are constructed to relate the commanded and measured actuator signals. Based on the dynamic process and equipment models, nonlinear feedback controllers for the axial and path forces are designed using the polynomial pole placement technique. The controllers are implemented in a Smith predictor-corrector structure to compensate for the inherent equipment communication delays, and the controller parameters are tuned to achieve the best closed loop response possible given equipment limitations. In the axial force controller implementation, a constant axial force is maintained, even when gaps are encountered during the welding process. In the path force controller implementation, a constant path force is maintained, even in the presence of gaps, and wormhole generation during the welding process is eliminated by regulating the path force.


Mechanical and Aerospace Engineering


Air Force Research Laboratory (Wright-Patterson Air Force Base, Ohio)
Missouri University of Science and Technology. Center for Aerospace Manufacturing Technologies

Keywords and Phrases

Actuator signals; Axial forces; Closed loop response; Communication delays; Controller parameters; Dynamic process; Equipment models; Force controllers; Non-linear forces; Non-linear functions; Nonlinear feedback controllers; Polynomial pole placement; Process parameters; Smith predictor-corrector; Systematic designs; Tool rotations; Traverse rates; Welding process; Welding qualities; Axial flow; Controllers; Electric welding; Equipment; Feedback; Feedback control; Force control; Friction; Friction welding; Gas welding; Nonlinear control systems; Poles; Poles and zeros; Polynomial approximation; Polynomials; Process control; Tribology; Welding; Friction stir welding; Friction welding; Polynomials; Predictive control

International Standard Serial Number (ISSN)


Document Type

Article - Journal

Document Version


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© 2008 American Society of Mechanical Engineers (ASME), All rights reserved.

Publication Date

01 Dec 2008