The Statics Study Of Two-dof Spherical Parallel Ankle Joint With A Centered Spherical Hinge

Body machine structure is the key and basic in the research of humanoid robot. It directly determines the functions of the robot and limits its potential future development in controls and intelligence. And joint structure is the core of humanoid robot structure.In terms of bionics, combing with the structure motion and function of the real ankle joint, regarding 2-DOF parallel spherical UP+R mechanisms as the prototype, a new 2-DOF humanoid spherical parallel bionic ankle joint with a centered spherical hinge is put forward, which is similar to real ankle joint in shape, structure, workspace, motions, carrying ability and many other aspects. The performances of the mechanism are studied in detail.Based on the static solution of the 2-DOF parallel spherical UP+R mechanisms, the joint force and the deformational characteristics are studied. The research result shows that the stiffness of the structure is imbalances. The linear displacement at the center of the moving platform is too big. So the prototype mechanism can’t be used as the ankle joint directly.In the views of mechanism and bionics, a new three branches 2-DOF humanoid spherical parallel bionic ankle joint with a centered spherical hinge is designed based on the idea that apply a spherical hinge to the moving platform to unload and increase the linear displacement stiffness of the structure.The performances of the new ankle joint are studied. The kinematics model of the joint is established. The forward and reverse position is solved. The workplace of the structure is expressed. The degree of freedom is calculated based on constraint screw principle.The new humanoid spherical parallel bionic ankle joint is a statically indeterminate structure. The static mechanics is analyzed. Static equilibrium equations of each component of the structure are developed in the traditional way. The analytic expressions for calculating the sphere displacement of the three branches is proceed based on the principle of small deformation superposition. Based on the deformation equations of the sphere, eight deformation compatibility equations are complemented. Based on those, the simultaneous equations of the statically indeterminate structure are obtained and the solution is done.The whole force and torque on each component is calculated under three kind of external load conditions. They are force only, torque only, both force and torque. The pictures to express the relationship between the load and the pose under three kinds of external load are drawn.The total force and torque on every hinge point and the total deformational of the moving platform are calculated. Compared with the mechanism prototype, The new ankle joint structure can unload almost the whole force from the external load. The linear displacement stiffness of the structure rises greatly. The problem that the stiffness of the structure is imbalances is improved. Finally, the simulation is completed. Overall, the result is exactly as expected and the structure of the new ankle joint can meets the requirements.