Online position correction approach of an industrial robot by using a new photogrammetric measurement system

Abstract

Six-axis industrial robots are widely used, mainly because of their versatility. However, they have one drawback: they are not as accurate as conventional machine tools. There are currently a number of approaches to increase the accuracy of these robots. This paper reports on the online correction of an ABB IRB 120 industrial robot using a novel photogrammetric measurement device, the Multi Aperture Positioning System (MAPS). As described by Luhmann, multi-camera solutions were previously required for the online measurement of the six degrees of freedom (6DOF = position and orientation in 3D space) of the robot TCP. MAPS offers the advantages of a single-camera system and at the same time the features and accuracy of a multi-camera system. To increase the accuracy of the robot, a correction algorithm is developed. It uses the measured values from MAPS to calculate the deviation of the robot Tool Center Point (TCP) position from the planned position and corrects it online if necessary. In this paper, three experiments are presented and their results discussed. Two to determine the position and trajectory accuracy of the robot and one to determine its z-stability on a trajectory in the xy plane. In each of these experiments, the robot end effector is tracked with the MAPS measurement device. After the actual state of the robot was recorded, all experiments were performed again, this time with the online correction. In all experiments, a significant increase in the accuracy of the robot can be observed. For example, the mean absolute error (MAE) of the position accuracy improved by a factor of 7 from 0.0207 to 0.0029 mm and the path accuracy (MAE) from 0.1140 to 0.0151 mm.