Combination of geometric and parametric approaches for kinematic identification of an industrial robot

Abstract

Combination of geometric and parametric approaches of kinematic identification is proposed in this article. The experimental strategy is similar to that used in geometric approach wherein each axis of the robot is actuated one after the other. This adds clarity to experimental strategy, which becomes ambiguous while solely using a conventional parametric approach. Therefore it is easier to conduct experiments even if there are restrictions in workspace. The estimation was done using a parametric technique, but in a stage wise manner using a divide and conquer strategy. This allowed measurement of the robot accuracy after removing the errors arising due to the definition of base and end-effector frames. Additionally it is possible to visualize the reduction in errors during the estimation process. In addition to this, the Jacobian matrix that relates the pose errors to the correction in parameters is adapted during estimation using a damped least squares method depending upon the convergence of the parameters. Results were obtained after extensive experiments on industrial robots using three different measurement instruments namely laser tracker, monocular camera and multi-camera system. The proposed method performs better than the conventional approach which uses only geometric approach. Finally thanks to the new approach, it was possible to conduct experiments after dividing the workspace region into those with high and low levels of observability ; which is not easy while using conventional approaches. It was also possible to perform identification in regions closer and farther away from the robot where there is deterioration of observability. The results show that the proposed method could reduce the errors in pose in a consistent manner, even when different measurement instruments were used, or when there was a deterioration in observability due to the choice of poses during identification. • Combination of geometric and parametric approaches of identification makes it easy and less ambiguous to conduct the experiments even in the presence of restrictions in workspace. • Divide and conquer strategy combined with a damped least squares estimation process with checks on convergence of individual parameters to annul the effect of ill-conditioning of the identification Jacobian. • Possibility of measuring errors in pose at different stages, thus permitting assessment of the positioning accuracy of the robot, and its improvement during the estimation process. Estimation of nominal values defining pose of the base and end-effector coordinate system was used to measure the robot accuracy before identification and after refinement at first stage, such that the accuracy solely due to the robot kinematics could also be measured. • Experimental validation on Fanuc robot was through multiple experiments involving different measurement instruments like monocular camera, multi-camera and laser tracker and results show improvement over geometric approach proposed earlier, irrespective of the measurement region or instrument. Monocular camera could reduce the positioning errors though by a smaller scale compared to other expensive instruments suitable for off-line use. • Possibility of partitioning the robot workspace into different regions on the basis of observability. Experiments in these regions validate that improvement of the positioning accuracy of industrial robot is possible in a consistent manner in all the regions; and while using different measurement instruments.