Structural error and friction compensation control of a 2(3PUS + S) parallel manipulator

To reduce the adverse effects of structural error and joint friction on a 2(3PUS + S) parallel manipulator, studies on the compensation method are conducted. The static structural errors such as machining error and assembly error are compensated through the kinematic model which is corrected based on the error modeling. Dynamic structural error, which is caused by the elastic deformation and clearance of the joint, is related to the motion state and external load. Owing to the difficulty of calculating and identifying dynamic structural error directly, it is compensated in the working space based on the predicted posture errors of the moving platform. Regarding the friction in the artificial hip joint, thrust ball bearing and linear module, friction modeling and parameter identification are conducted, and then they are compensated through a feedforward compensation method. With the structural error and friction compensated simultaneously, a compensation control strategy for the 2(3PUS + S) parallel manipulator is designed based on the Augmented PD control method and experiments are performed. The results from the experiment indicate that the motion accuracy of the moving platform is improved through the structural error and friction compensation.