Modeling and Identification of Kinematic Model with Joint Clearance for a 6-PSU Parallel Manipulator

6-PSU parallel manipulator with six degrees of freedom has the characteristics of low inertia of moving parts and high precision and frequency response, so it is suitable for many high-precision operation tasks. Accurate kinematics model is the basis of high-precision motion control. However, due to manufacturing and assembly reasons, there are errors between theoretical model and actual model, so kinematics modeling and identification are significant. In order to ensure the rigidity, S-joint is usually realized by 3 orthogonal R-joints and U-joint by 2 vertical R-joints. In fact, there is joint clearance in the R-joint that makes up the S-joint and the U-joint. Ignoring this clearance has a great influence on the accuracy of kinematic model. At present, there is rare research on kinematic modeling and identification of 6-PSU parallel manipulator with joint clearance. In this paper, a kinematic modeling and identification method for six-DOF 6-PSU parallel manipulator with joint clearance is proposed. There is a clearance between the shaft and the hole of the R-joint, but the position of the clearance in the hole is related to the acceleration of the P-joint. Therefore, the position of all clearances in the holes can be determined by acceleration of the end-effector according to inverse kinematics. Furthermore, a kinematic model of 6-PSU parallel manipulator considering joint clearance is established. When both the shaft and the hole are circular, the joint clearance can be equivalent to the variation of link length, which greatly simplifies the complexity of joint clearance identification. The correctness and validity of the proposed modeling and identification method are verified by experiment.