An axial force controller with delay compensation for the friction stir welding process

Friction stir welding (FSW) is a promising welding technology and is being extensively researched because it can produce high-quality joints. In the welding process, due to improper clamping, deformation of back support and other process variations, the axial force can vary significantly and produce welding defects. To solve this problem, a feedback controller of FSW axial force is desirable. However, time-delay, which widely exists within the FSW systems, may degrade the controller effect and make the system unstable. To regulate the axial force and compensate the time delay, this paper designs an axial force controller for FSW system with delay. At first, by utilizing a time delay compensation scheme, FSW systems with delayed control actions is transformed into standard differential equations without time delay terms. Then, an axial force controller is designed based on the linear-quadratic regulator (LQR) technique. Since the time delay is incorporated in the system model throughout the whole controller design process, the system performance and stability are assured and the control system is also simplified. Utilizing the LQR technique, aspects of system dynamic properties and simplicity are considered simultaneously. Experimental validations were carried out on a FSW platform. The experimental results demonstrate that the time delay is compensated effectively. The controller maintains the constant axial force and shows desirable dynamic behavior, even when the disturbance is encountered during the welding process.