Design and validation of a universal 6D seam tracking system in robotic welding based on laser scanning

Robot automation increases productivity, the product quality and frees man from involuntary, unhealthy and dangerous work. While computational power has increased exponentially during the last decades, the limitations in flexibility constitute today a bottleneck in the further evolution of robotic systems. One application for intelligent systems is seam tracking in robotic welding. Seam tracking is among others used for the manufacturing of large ships such as cruisers and oil tankers, where relatively high tolerances in the sheet material are allowed to minimize the manufacturing costs (Bolmsjö, 1997; Fridenfalk & Bolmsjö, 2002a). Seam tracking is today typically only performed in 2D, by a constant motion in x and compensation of the errors in y and z directions, see Fig. 1. There exist solutions for higher autonomy that include seam tracking in 3D where compensation is also performed in o direction or around an orientation axis. These limitations make however seam tracking only possible for work pieces with relatively simple geometrical shapes. The next step in the evolution of sensor systems in robotic welding is the introduction of a full 6D sensor guided control system for seam tracking which is able to correct the TCP in x, y, z and around roll, pitch and yaw. Such an ultimate system is per definition able to follow any continuous 3D seam with moderate curvature. This system has many similarities with an airplane control system, where a change in either position or orientation affects all other degrees of freedom. Though seam tracking has been performed in 2D for at least 40 years, the hypothetical design of 6D seam tracking systems was probably first addressed in the beginning of the last decade by suggestions of models based on force sensors (Bruyninckx et al.,1991a; Bruyninckx et al., 1991b) and laser scanners (Nayak & Ray, 1990a; Nayak & Ray, 1990b). It is however difficult to evaluate these systems, since no experimental results are presented, neither any explicit control systems for creating balance between the subsystems. The contribution of this paper is the invention of a universal 6D seam tracking system for robotic welding, validated by simulation experiments based on physical experiments, which proved that 6D seam tracking is possible and even very robust for laser scanning (Fridenfalk & Bolmsjö, 2002b). On a more detailed level, the contributions of this paper are considered to be the introduction of: (1) differential control in laser scanning, using the same techniques