Robotic machining with higher volumetric accuracy

Soichi Ibaraki

Graduate School of Advanced Science and Engineering, Hiroshima University

Kagamiyama 1-4-1, Higashi-Hiroshima 739-8527, Japan

Serial articulated manipulators, or industrial robots, are typically programmed by the teach method. Whereas machine tools are fully programmed by the “offline programming” based on a virtual model, its application to industrial robots is still limited. It is partly due to significantly larger volumetric errors of an industrial robot. The "volumetric error" represents the three-dimensional positioning error for arbitrarily given command poses over the entire workspace. Ensuring higher volumetric accuracy for industrial robots is a key to further extend the flexibility and adaptability of their implementation in manufacturing. Potential new applications of higher-accuracy industrial robots include robotic machining. A robot, as a "portable machine tool," can be a cost-, space-, and energy-efficient alternative to a machine tool, particularly for large-sized workpieces.

Our group has proposed a new kinematic model of a six-axis industrial robot containing the angular positioning deviation of all the rotary axes as error sources. Based on this model, our numerical compensation has significantly improved the robot's volumetric accuracy. The "direct" measurement of the angular positioning deviation of each individual rotary axis by using a laser tracker will be first presented. Our recent works on the "indirect" schemes, using less expensive measuring instruments, to identify rotary axis angular positioning deviations will be also presented. The proposed schemes have been tested on several different robots.

Our recent study on the machining accuracy by a six-axis large-sized robot will be presented. The improvement in the geometric accuracy of the machined test pieces, by applying off-line compensation based on our model, has been studied. Technical issues with robotic machining will be also reviewed and discussed.