A compact 3-DOF shoulder mechanism constructed with scissors linkages for exoskeleton applications. (February 2019) - Castro, Miguel Nobre
Highlights: A novel compact 3-degree-of freedom spherical mechanism is described. The mechanism is singularity-free in the anatomical shoulder joint workspace. The derivation of the kinematics of the spherical mechanism is presented. An application case is given as a shoulder joint for an exoskeleton device. The exoskeleton does not compromise upper extremity function. Abstract: A novel 3-degrees-of-freedom (DOF) spherical mechanism, singularity-free in the anatomical shoulder joint workspace, is described. The use of curved scissors linkages interconnected by revolute joints, whose axes share the same remote centre-of-motion, achieves the most compact design of its kind. The kinematics of this scissors shoulder mechanism (SSM) are derived and presented. A design equation restricting the linkage's curvature by the central/pitch angle of the fully stretched scissors is obtained. Motion-captured data are used for validating the reachable 3-d workspace while a test-subject is wearing a null protraction/retraction constrained exoskeleton. The embodiment of the SSM as a shoulder joint for an exoskeleton device does not compromise the upper extremity function within the anatomical reachable 3-d workspace. It operates within a volume of 0.236 m 3, corresponding to 68.09% and 94.97% of the volumes of the full active (0.350 m 3 ) and null protraction/retraction constrained active (0.223 m 3 ) reachable workspaces of the test-subject, respectively. Thus, the SSM represents a simplification of a spatial spherical mechanism design and overcomes the need for the use of redundant links and optimization routines. Graphical abstract: Image, graphical abstract