Calibration of the 5-dof parallel kinematics HITA-STT

The parallel kinematics machine tool HITA-STT (Stiffness Tracking Technology) developed by Willemin–Machines S.A. and EPFL is now able to machine. This machine can reach accelerations of 5g. It is characterized by a tilt of ± 60° around the B-axis. This characteristics combined with the disposition of the kinematics allows to machine five faces of a cube without changing its position in the rotary table, what means in one set-up. The machine is patented. The first machining tests showed the necessity to develop a good calibration method in order to reach similar precision as the most precise machine tools, without high manufacturing costs. The particularity of this machine is its parallel mechanism. That means that a movement in a single direction needs a combination of all the actuators’ displacements. The calibration of this structure is then not obvious due to the combination of the actuators’ movements. The goal of this paper is to present a new method to calibrate the 5-dof parallel kinematics and the HITA-STT machine tool in particular. This calibration method could be named “Method of calibration by decomposition in a sum of elementary defects” (DSED). It is based on the observation that the error of position and orientation of the spindle, generated by several defects, is equal to the sum of the errors generated by only one defect at the same time. This calibration method will be integrated to the existing controller so that the calibration process will be totally automatic and will require shortest time to calibrate the machine automatically in manufacturing conditions.