Méthode d'étalonnage d'une machine-outil à cinématique parallèle à cinq axes à grands angles d'inclinaison

A machine-tool is able to manufacture precisely only when it is calibrated. Calibration consists in correcting the model of the machine so that it represents the real kinematics. On no account calibrating a structure consists in changing some parts of the machine. The industrial machining prototype of the Hita-STT (Stiffness Tracking Technology) machine-tool has a four-axis parallel kinematics associated to a turning table as left hand. The spindle is fixed to the parallel kinematics; the workpiece is fixed to the turning table. This new machine-tool was developed by the Laboratoire de Systèmes Robotiques from EPFL and Willemin-Macodel S.A. For its industrialization, it needs an absolute accuracy of 10 microns after calibration considering a repeatability of 5 microns. These specifications should be obtained in the entire workspace between the spindle and the turning device. This doctoral thesis deals with the calibration of the Hita-STT machine-tool. Several methods can be developed. They differ in: the geometrical model that is used. For each kinematics, three geometrical models exist: the implicit model, the forward geometrical model and the inverse geometrical model; the way the corrections are applied. Pose errors are compensated correcting the commands sent to the controller; or the parameters of the structure are identified and updated on the controller. This last solution allows to use a model for the control of the machine that is closer to the real kinematics. These different methods are presented and compared. An optimal calibration method is proposed. Whatever calibration methodology is chosen, it is divided into four steps: modeling the kinematics. An unique distribution of the parameters of the Hita-STT parallel kinematics is proposed; measurement of the pose errors. The choice and the design of the measuring device are presented. A set-up and characterization procedure is explained. It is compatible with the industrial environment. The pose errors measurements are automatic thanks to special ISO functions added to the controller. Data processing is automatic; calculation of the corrections of the model. Several methods are compared in terms of accuracy that can be obtained. An "ideal" method is proposed for the parallel kinematics calibration. It combines the two families of calibration methods, it means an identification method followed by a direct calibration method; implementation. The "ideal" method is tested on the industrial machining prototype Hita-STT. The structure of the programs used to automate the calibration process are given. Tests used to verify the calibration quality are presented. They can also be used to make sure that the parallel kinematics is accurate enough to satisfy the required quality of the manufactured parts.


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