Improved Temperature and Depth Control During FSW of Copper Canisters Using Feedforward Compensation

The welding procedure to seal copper canisters requires variable power input throughout the 45 minute long weld cycle to keep the probe temperature within the process window. By using a cascaded loop that determines the power input requirement, the controller will not be dependent on repeatability in the necessary power input between weld cycles, and the lag time in the probe temperature measurement will not be critical. Due to fast-changing thermal boundary conditions during the downward sequence, a feedforward to the power input was designed to further improve controller performance.

In addition to the cascade controller adjusting the tool rotation rate, a depth controller is adjusting the axial force to control the shoulder depth. The purpose is to eliminate flash due to excessive shoulder depth and to control the position of the probe tip, which influeces the size and shape of the hook defect produced. Controlling depth is challenging for several reasons, including deflection in the welding machine and thermal expansion of the weld material, and also results in cross-coupling between axial force and spindle torque. The cross-coupling was handled by another feedforward compensator that adjusts the tool rotation rate based on the commanded axial force.

The implemented controllers and feedforward compensators have been evaluated over several welds with good results, where the depth is kept within ±0.1mm when the tool reaches the joint line sequence and the probe temperature is held within ±5°C during full circumferential welds compared to a process window of ±60°C.