Design optimization of a 2-DOF parallel mechanism for ankle rehabilitation considering structural layouts of RUS limbs

The mechanism design is crucial in platform-based ankle rehabilitation robots. In this study, we propose a new two-degree-of-freedom platform-based parallel mechanism for ankle rehabilitation, featuring both nonredundant and redundant actuation. Compared to existing parallel ankle robots, the proposed mechanism with several revolute-universal-spherical (RUS) limbs offers the advantages of a simpler structure and reduced inertia. However, the adopted RUS limbs have complicated and various structural layouts, involving the axes of the active R joints, the locations of the S joints, and assembly modes. The challenge lies in designing the structural layouts of RUS limbs and selecting the optimal layout to achieve peak performance. To cope with it, an arrangement method for the symmetrical plane of an RUS limb is proposed based on the characteristics of ankle motion. Secondly, an optimization method blending structural arrangements and dimensional parameters is developed. The optimization problem of maximizing a kinematic performance index is solved by using an evolutionary algorithm, and one optimal layout is selected to build a redundant prototype. The optimization and experimental results show that: (1) the nonredundant and redundant cases have their respective optimal layouts; (2) the kinematic performances are significantly improved under the optimal layouts; (3) the prototype can rotate continuously to the maximum angles and perform the ankle motions smoothly in given trajectories.