Offline circular path error measurement and compensation for robotic machining applications

Robotic machining provides a flexible and cost-effective alternative to conventional machine tools. However, robots have relatively poor rigidity and accuracy. Performance enhancement in robotic machining typically relies on machining parameter optimization, robot calibration, offline or online path error compensation, and process refinement. This study focuses on offline measurement and compensation of circular path errors using a telescoping ballbar system. To address the inherent limitations of traditional ballbar setups, particularly their restriction to a few fixed measurement radii, a novel out-of-plane ballbar measurement method is introduced along with a custom data processing framework. This configuration enabled error measurements across general circular trajectories with varying radii. A geometric projection model was developed to quantify the measurement distortion induced by the out-of-plane angle and a small-radius adaptor was designed to extend the applicability of the ballbar system. The proposed method was experimentally validated on a robotic machining platform using a laser tracker. The results show that at out-of-plane angles below 30°, the system achieves over 61% compensation accuracy, which is comparable to the 75% achieved using the laser tracker, while requiring less than 20% hardware cost. These findings demonstrate that the proposed approach offers a practical, scalable, and economical solution for circular-path error compensation in robotic machining.