SAE International Improved Accuracy of Unguided Articulated Robots 2009-01-3108

The effectiveness of serial link articulated robots in aerospace drilling and fastening is largely limited by positional accuracy. Unguided production robotic systems are practically limited to +/-0.5mm, whereas the majority of aerospace applications call for tolerances in the +/-0.25mm range. The precision with which holes are placed on an aircraft structure is affected by two main criteria; the volumetric accuracy of the positioner, and how the system is affected when an external load is applied. Production use and testing of off-the-shelf robots has highlighted the major contributor to reduced stiffness and accuracy as being error ahead of the joint position feedback such as backlash and belt stretch. These factors affect the omni-directional repeatability, thus limiting accuracy, and also contribute to deflection of the tool point when process forces are applied. Drawing from common axis configuration in machine tool design, an industrial robot integrated with secondary encoders yields tighter control on axis position and increases system rigidity, thus creating a more repeatable system and, in turn, a system than can be compensated to high accuracies.