Error Model and Kinematic Calibration of a 5-axis Hybrid Machine Tool

In this paper, a new 5-axis hybrid-kinematic machine tool is introduced, which is made up of a 3-DOF parallel manipulator and a 2-DOF serial manipulator. The machine tool maintains high ratio of stiffness to mass due to the parallel structure and high orientation capability due to the serial-type wrist. In order to guarantee high accuracy, the methodology of measuring the output shafts by additional sensors in stead of using encoder outputs at the motor shafts is proposed. In the kinematic view point, the hybrid manipulator reduces to a serial one, since the passive joints in the U-P serial chain at the center of the parallel manipulator are directly measured by additional sensors. Using the method of successive screw displacements, the kinematic error model is derived. Since a ball-bar is less expensive than a full position measurement device and sufficiently accurate for calibration, the kinematic calibration method of using a ball-bar is presented. The effectiveness of the calibration method will be verified through the simulations