Real Time Path Correction with Direct End-Effector Feedback

Abstract

In contemporary industries the demand for very accurate robots is continuously growing. Yet, robot vendors are limited in the achievable accuracy of their robots, as they have no means to provide a direct end-effector feedback. Therefore most approaches aim to evaluate an accurate model of the system, thus providing compensation factors to correct the paths. Models, however, are unable to represent the real physical system in a sufficient manner for path correction. The non-linearities in robotic systems are difficult to model and the dynamics are very fast. A better approach is therefore to use direct end-effector position and orientation feedback from an external sensor. The measured data can directly be compared to the nominal data from the path interpolator. Hence, they are not dependent on the kinematic robot model. The residual errors can be used to calculate correction values for the individual robot joints, thus providing a fast path correction algorithm. In this paper such a method is discussed, implemented and tested. The feedback data (absolute end-effector position and orientation) for the control-algorithm is provided by a Leica Absolute Laser Tracker and a 6 DoF T-Mac, mounted on the end-effector. It provides data at a rate of 1 kHz, which are transmitted over EtherCat. The data are processed on an external embedded PC from Beckhoff, which is part of the EtherCat network and then transmitted to the robot.