When controlling the Friction Stir Welding (FSW) tool's motion along the joint line, the tool's altitude may be controlled using either force or position control. In the first case the force that arises on contact between the tool and the object is measured and fed back to the control system which provides positional offsets to adjust according to a desired value of the force. In the latter the machine's traditional position control is applied to ensure a proper contact conditions by the use of offsets. One issue that often arises with the latter case involves machine compliance. The lack of rigidity when the applied forces arise on contact suppresses the mechanical structure is a manner for which the positional feedback from the encoders respond with erroneous values. The force control solutions that have been made available during latter years for industrial robots, working in applications such as assembly and grinding, does override the issue with compliance but introduces new undesired behaviours. One issue faced for a typical FSW robot application, welding a considerable length on a small area, as the object collapses due to an overheated joint area. This paper investigates the two approaches for the robotic case in order to pin-point typical scenarios for each solution to be used beneficially.
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