Kinematic Calibration of a Parallel Mechanism Machine Tool

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Although more than ten years have been passed since the first parallel kinematic machine tool was introduced, they are not widely accepted in today's industry. Many parallel kinematic machine tools available in the today's market have issues in their stiffness against external forces and motion accuracy compared with conventional serial kinematic orthogonal-axes machine tools. We conducted the following researches in order to improve the motion accuracy of a parallel mechanism machine tool mainly from the viewpoint of its motion control.1)Compensation of motion errors caused by the gravity.On a parallel mechanism machine tool, since its spindle unit is supported only by six struts, the deformation of its mechanical structures caused by the gravity has a large effect on its positioning accuracy. In order to accurately predict and compensate such a gravity-induced motion error, a model of the parallel kinematics was proposed and improved.2)Improvement of a calibration method of kinematic parametersFor high-accuracy motion control of parallel kinematic feed drives, it is the most critical issue to calibrate various kinematic parameters such as the reference length of struts or the location of base joints. A calibration method based on circular tests to measure the machine's contouring accuracy in a circular operation has been proposed. We proposed the improved calibration methodology under the cancellation of gravity-induced errors, such that only the calibration error of kinematic parameters can be evaluated in the calibration process. By applying 1) and 2), it was validated by using a commercial parallel mechanism machine tool that the motion error was reduced to less than 20 μm over the entire workspace, which the motion error was as large as 200 μm without the proposed schemes when the spindle is near an edge of the workspace and/or the tilt angle of a spindle is large.