Feedforward compensation of contour errors in robotic machining system using compliance model

Abstract

Industrial robots are receiving attention as alternatives for large machine tools owing to their high flexibility, large workspace, and cost-effectiveness. However, their low machining accuracy hinders their adoption for machining processes. The major error source is its compliance error caused by the cutting force resulting from the low stiffness of the robot manipulator. To increase the machining accuracy, an external measurement system and a compliance model were used to measure or estimate the compliance errors, and a control algorithm was subsequently applied to compensate for those errors. However, the control algorithm in terms of improving the dimensional accuracy of the workpiece has not been presented thus far. Unlike previous methods that deal with each axis compliance error separately, in this study, a contour control algorithm is proposed to effectively improve dimensional accuracy of the machined workpiece. The compliance model and a force torque sensor were used to predict the actual position of the cutting tool, and the contour errors were calculated using the interpolated reference positions and the predicted actual positions. A feedforward controller was designed to compensate for the estimate contour errors by modifying the reference positions. Experiments were conducted to evaluate the effectiveness of the proposed algorithm.