Design and position analysis of 2PUU-2PSS parallel mechanism for auxiliary assembly in DEMO vacuum vessel

This study proposes a 2PUU-2PSS parallel mechanism (PM) designed to meet the high-precision assembly requirements of the DEMO vacuum vessel. The mechanism, featuring four degrees of freedom (3T1R), is capable of moving to the working area with the assistance of a conveying device to perform high-precision cutting and welding operations on the inner walls of the vacuum vessel. Firstly, the number and characteristics of the mechanism's degrees of freedom are analyzed based on screw theory, where the correctness of the design is verified. Secondly, with the constraint of the mechanism's rod length, the forward and inverse kinematics equations are established. The forward kinematics equations are then transformed into unconstrained optimization equations, and a differential evolution algorithm is employed for solving. Furthermore, to enhance the optimization efficiency and robustness of the differential evolution algorithm, a mean-based dual mutation strategy and an individual selection strategy are proposed and integrated into the traditional differential evolution algorithm, forming a Mean-based Dual Mutation Differential Evolution (MDDE) algorithm with strong global search capabilities and convergence rates. To assess the performance of the MDDE algorithm, numerical experiments are conducted using two CEC Benchmarks functions and two mechanical examples. The experimental results demonstrate that the MDDE algorithm consistently outperforms the compared algorithms. Finally, numerical examples are provided to demonstrate the application of the MDDE algorithm in obtaining the forward kinematics solution, including the single-point position and orientation and continuous trajectory, for the 2PUU-2PSS PM, where the correctness of the forward kinematics solution is verified through the application of the inverse kinematics.