Robotic redundancy via arm angle self-adaptation through nullspace resolution: Offset poses a challenge

Seven-degree-of-freedom offset manipulators are typically large-scale space robot systems featuring two long links. The offsets at the shoulder, elbow, and wrist joints extend the joint motion range, thereby enhancing the manipulator’s workspace and dexterity to perform more complex missions. The two long links render the physical significance of the manipulator’s elbow highly pronounced, with the arm angle being closely coupled to the elbow joint. Arm angle parameterization is well-suited for configuration control of such manipulators; however, offsets pose a geometric challenge that hinders analytical redundancy resolution. This article focuses on the Experimental Module Manipulator (EMM) of the China Space Station and investigates its nullspace redundancy resolution approach via arm angle self-adaptation. First, a novel semi-analytical inverse kinematics solution via arm angle parameterization is proposed for the EMM, offering high computational accuracy and fast solving speed. Moreover, convex hulls of the EMM and the China Space Station are modeled for high-precision collision detection based on the Gilbert–Johnson–Keerthi distance algorithm. Subsequently, an obstacle avoidance strategy via arm angle self-adaptation is developed for the EMM to perform tasks safely in complex environments. Additionally, the effectiveness and practicality of both the arm angle parameterized inverse kinematics solution and the obstacle avoidance strategy are verified through comprehensive simulations. Finally, this research is applied to the EMM for successful accomplishment of on-orbit servicing to support the Shenzhou-16 astronaut’s extravehicular activities aboard the China Space Station, thereby achieving on-orbit experimental validation.