In response to the problems of mismatched human-robot motion and poor human-robot coupling (HRC) performance in ankle rehabilitation robots, this study proposes a cable-driven flexible ankle rehabilitation robot (FARR) based on biomimetic design and analyzes its HRC performance. Firstly, based on the motion characteristics and physiological structure of the ankle joint, an equivalent model of the ankle joint is proposed to solve the workspace of the ankle joint. Secondly, a design configuration of a cable-driven FARR with a flexible equivalent axis is proposed, and the structure of the FARR is designed. The robot's kinematics and dynamics are analyzed to solve the robot's workspace. By comparing the workspace of the ankle joint with the robot's workspace, the completeness of the robot's motion function is verified. Furthermore, the HRC dynamic theory model and the HRC dynamic simulation model are established to solve the HRC joint torque during the HRC system motion. The results show that the HRC joint torque is small in magnitude and stable in variation, indicating that the burden on the joint during robot motion is minimal. Finally, a prototype of the FARR is built, and motion function experiments of the HRC system are conducted to verify the matching between actual human motion and FARR motion under the HRC system.

The application of the parallel mechanism is still limited in the humanoid robot fields, and the existing parallel humanoid robot joint has not yet been reflected the characteristics of the parallel mechanism completely, also failed to solve the problem, such as small workspace, effectively. From the structural and functional bionic point of view, a three degrees of freedom(DOFs) spherical parallel mechanism for the shoulder complex of the humanoid robot is presented. According to the structure and kinetic characteristics analysis of the human shoulder complex, 3-PSS/S(P for prismatic pair, S for spherical pair) is chosen as the original configuration for the shouder complex. Using genetic algorithm, the optimization of the 3-PSS/S spherical parallel mechanism is performed, and the orientation workspace of the prototype mechanism is enlarged obviously. Combining the practical structure characteristics of the human shouder complex, an offset output mode, which means the output rod of the mechanism turn to any direction at the point a certain distance from the rotation center of the mechanism, is put forward, which provide possibility for the consistent of the workspace of the mechanism and the actual motion space of the human body shoulder joint. The relationship of the attitude angles between different coordinate system is derived, which establishs the foundation for the motion descriptions under different conditions and control development. The 3-PSS/S spherical parallel mechanism is proposed for the shoulder complex, and the consistence of the workspace of the mechanism and the human shoulder complex is realized by the stuctural parameter optimization and the offset output design.

The chaos phenomenon often exists in the dynamics system of the mechanism with clearance and friction, which has obvious effect on the stability of the mechanism, then it is worthy of attention for identifying the relationship between the friction coefficient and the stability of the mechanism. Two rotational degrees of freedom decoupled parallel mechanism RU-RPR is taken as the research object. Considering the clearance existing in the revolute pair, Lankarani–Nikravesh contact force model is used to calculate the normal contact force, and the Coulomb friction force model is used to calculate the tangential contact force. The dynamics model is established using Newton–Euler equations, and the Baumgarte stabilization method is used to keep the stability of the numerical analysis. Then, the equations are solved using the fourth adaptive Runge–Kutta method, and the effect of the revolute pair’s clearance on the dynamic behavior is analyzed. Poincare mapping is plotted, and the bifurcation diagrams are analyzed with varying the friction coefficient corresponding to different values of clearance size. The research contents possess a certain theoretical guidance significance and practical application value on the analysis of the chaotic motion and its stability in the dynamics of the parallel mechanism.