Precise Detection of Pose and Assembly Error Based on Partial Scan Data for Fabrication of Tubular Joints

Automated and accurate determination of the pose of a roughly placed workpiece is essential for fabrication tasks such as robotic welding. In this paper, an algorithm based on cylinder fitting and Iterative Closest Point (ICP) alignment is proposed for detecting both the pose and assembly error of tubular joints in a T-, K-, or Y-shaped configuration using partial scan data. A typical TKY joint consists of a brace and a chord component. In order to also compute the assembly error, the poses of the individual joint components are determined separately. The pose information is obtained by aligning scan data points with ideal CAD models using cylinder fitting and ICP matching techniques. Due to uniformity of the chord surface, assembly error due to a translation along, or a rotation about the chord longitudinal axis are not determined. The assembly error obtained can be used to determine whether realignment is required, or combined with the pose information to re-plan paths for subsequent tasks. The effectiveness of the proposed algorithm is verified with simulation studies and application to an actual robotic welding system to locate a tubular workpiece.