Research on a biomimetic flexible ball joint with variable stiffness for robots

The human ball joint is a compact and flexible 3-DOF composite joint. The ball joints commonly used in robot design lack active driving capability. Ball joints with active drive generally consist of three single-degree-of-freedom joints connected in series, which is a non-compacted structure and easily leads to singular postures. In order to meet the demand for high performance composite joint modules for service robots, this paper designs a flexible biomimetic spherical robot joint with variable stiffness characteristics: the mechanism of muscle parallel antagonistic drive and ligament wrapping constraint is simulated, three parallel branch chains are used to drive three composite degrees of freedom, ropes, soft airbags and series elastic drive gears are used to form a flexible transmission system, the contour of the rope winch has been optimized with the aim of transmission stability, and a pneumatic variable stiffness soft structure has been designed and fabricated. A compliance control algorithm for joints was developed based on the principle of impedance control. The research results indicate that the biomimetic ball and socket joint has a compact structure, a wide range of motion and good motion tracking performance, variable stiffness performance and flexible interaction ability.