Machining, Measurement, and Control Laboratory

Compensation of Gravity-induced Errors on a Hexapod-type Parallel Kinematic Machine Tool
Soichi Ibaraki (Kyoto University)
Toshihiro Okuda (Kyoto University)
Yoshiaki Kakino (Kyoto University)
Masao Nakagawa (Okuma Corp.)
Tetsuya Matsushita (Okuma Corp.)
Tomoharu Ando (Okuma Corp.)

This paper presents a methodology to compensate contouring errors introduced by the gravity on a Hexapod-type parallel kinematic machine tool with the Stewart platform. Unlike conventional serial kinematic feed drives, the gravity imposes a critical effect on the positioning accuracy of a parallel kinematic feed drive, and its effect significantly varies depending on the position and the orientation of the spindle. We first present a kinematic model to predict the elastic deformation of struts caused by the gravity. The positioning error at the tool tip is given as the superposition of the deformation of each strut. It is experimentally verified for a commercial parallel kinematic machine tool that the machine's contouring error is significantly reduced by compensating gravity-induced errors on a reference trajectory.
Key Words:   Machine Tool, Motion Control, Parallel Kinematic Machine Tool, Gravity-induced Errors, Contouring Accuracy.