Optimization design of prosthetic shoulder joint based on three-RRR asymmetric spherical parallel mechanism

Due to its compact spatial layout and good structural stiffness, the three-DOF spherical parallel mechanism has potential advantages in designing multidegree of freedom joints, such as the shoulder and wrist joints. However, existing spherical mechanism design methods do not consider the characteristics of the asymmetric shape of the human shoulder workspace and the asymmetric distribution of dexterous motion ability. Considering the three-RRR asymmetric spherical parallel mechanism as the prototype, this paper investigates the parameter optimization of the prosthetic shoulder joint mechanism to improve the kinematic performance of the asymmetric spherical parallel prosthetic shoulder joint mechanism. First, the parametric model of the mechanism is determined. Based on the derivation of the forward and inverse kinematics solution and the Jacobian velocity matrix, the multiobjective optimization design of the mechanism parameters is with the goal of the maximum coverage of the mechanism workspace to the human shoulder workspace, the maximum dexterity of the mechanism, and the minimum torque of the input shaft. The weight of each index is calculated by the fuzzy analytic hierarchy process based on a triangular fuzzy number, and the optimal mechanism parameters are selected from the Pareto solution set by the weighted summation method. The working space of the bionic shoulder joint mechanism designed according to the optimal mechanism parameters can effectively cover the working space of the human shoulder joint. Compared with the existing parallel mechanisms, its dexterity is improved, and the maximum joint torque is reduced. Thus, this study can provide a theoretical fundament for the design and application of prosthetic shoulder joints.