Abstract: For the sub-6 DOF parallel kinematic machines (PKM), it’s necessary to separate the geometric error sources which influence the compensatable and uncompensatable errors of the mobile platform by error modeling for the purpose of supervising the accuracy design and kinematics calibration. By taking the TriVariant, a 5-DOF hybrid robot, as an example, a novel approach for error modeling is presented for such robot. The geometric error sources affecting the uncompensatable position and orientation errors of the mobile platform can be identified through this approach. It can be concluded that the uncompensatable errors of the mobile platform are primarily dominated by the errors of the proper-constraint limb included by both the manufacturing and assembling errors of U joint and the twist errors of the tracks. The effects of the geometric error sources on the uncompensatable errors are also discussed by means of the sensitivity analysis. The analysis results show that the error brought by the deviation of the intersection of the two axes of the U joint takes the greatest effects on the uncompen-satable errors of the mobile platform.

Key words: Error modeling, Parallel kinematic machines, Sensitivity analysis