Integrating Mimic Joints into Dynamics Algorithms: Exemplified by the Hybrid Recupera Exoskeleton

The design of various robots in industrial and academic contexts integrates closed loops to improve the mechanical stiffness in comparison with purely serial or tree-type topologies. In particular, planar kinematic loops as parallelograms or double parallelograms are employed in such hybrid robots. Since these systems are geometrically overconstrained in the group of spatial Euclidean motions, the computational performance and numerical accuracy of any model-based dynamics software is negatively affected. This paper introduces a novel method to avoid these numerical issues for any hybrid system with loops that can be characterized by the concept of linear mimic joints: these are passive joints which depend on an active joint in a closed loop in a linear manner. With the proposed approach, the loop closure functions are automatically composed from the robot description file and integrated into the analytical equations for solving the forward and the inverse dynamics problems. The paper illustrates the application of this method for a novel shoulder mechanism containing a planar six bar mechanism that has been designed for the Recupera whole-body exoskeleton.

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