Machining, Measurement, and Control Laboratory

Compensation of Gravity-induced Errors on a Parallel Kinematic Machine Tool

Toshihiro Okuda (Kyoto University)
Soichi Ibaraki (Kyoto University)
Yoshiaki Kakino (Kyoto University)
Masao Nakagawa (Okuma Corp.)
Tetsuya Matsushita (Okuma Corp.)

This paper presents a disturbance estimation methodology on a Hexapod-type parallel kinematic machine tool of the Stewart platform. The disturbance estimation by monitoring an armature current of servo motors has been commonly done on conventional serial mechanism feed drives. On a parallel mechanism feed drive, a disturbance observer must have a more complex structure with a precise model of the gravity effect on servo motor loads, which significantly varies depending on the position and the orientation of a spindle unit. The estimation performance of the proposed disturbance observer is validated through static load tests and cutting experiments. As an application example, the proposed disturbance estimation scheme is applied to the compensation of the tool deflection in end milling processes by tilting the spindle.
Key Words:   parallel kinematics machine tools, Stewart platform, disturbance observer, servo motors, stiffness.