Machining, Measurement, and Control Laboratory

A Study on the Improvement of Motion Accuracy of Hexapod-type Parallel Mechanism Machine Tools (2nd Report)
---A Calibration Method to Evaluate Positioning Errors on the Global Coordinate System---
Soichi Ibaraki (Kyoto University)
Takeshi Yokawa (Kyoto University)
Yoshiaki Kakino (Kyoto University)
Masao Nakagawa (Okuma Corp.)
Tetsuya Matsushita (Okuma Corp.)

This paper presents a methodology to calibrate kinematic parameters in a Hexapod-type parallel mechanism machine tool. In a conventional procedure of the DBB (Double Ball Bar) test to measure a machine's contouring accuracy in a circular operation, the center deviation in an error profile is ignored to cancel setup errors. In such a case, the machine's positioning error can be evaluated only locally. We propose to use a specialized jig plate to fix a ball of a DBB device in order to evaluate the machine's positioning error in the global coordinate system. Kinematic parameters in a Hexapod-type parallel mechanism can be calibrated based on contouring error profiles. Experimental comparison shows the machine's global positioning error is significantly improved by using the proposed calibration method. In particular, the parallelity error and the straightness error are reduced by 89% and 76%, respectively, compared to the conventional calibration.
Key Words:   parallel mechanism machine tools, kinematic parameters, calibration, circular test, Double Ball Bar.