A compact 3-DOF shoulder mechanism constructed with scissors linkages for exoskeleton applications

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. Motioncaptured 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.

A Review on Design of Upper Limb Exoskeletons

“…Similarly, a DPL mechanism presented in Reference [7] supports 3-DOF shoulder movements, but the shoulder internal/external rotation was kept passive, which limits the use of this mechanism for other applications, i.e., rehabilitation and therapy. Both mechanisms [7,44] are not able to support shoulder elevation/depression and protraction/retraction. Kim et al [6] has presented a mechanism to actively support the COR of the glenohumeral joint during the shoulder girdle movements.…”

“…The exoskeleton structure with the DPL mechanism can actively support the shoulder extension/flexion and shoulder abduction/adduction, whereas the shoulder rotation was kept passive, which limits its use for several applications. Upper-limb exoskeleton research prototype: (A) parallel actuated shoulder exoskeleton adopted from Reference [41], (B) cmpliant robotic upper-extremity eXosuit (CRUX) adopted from Reference [42], (C) upper-limb exoskeleton for inferno adopted from Reference [43], (D) UB-EXO developed by Aalborg University [40], (E) compact 3 degrees of freedom (DOF) scissors linkages for upper-limb exoskeleton adopted from Reference [44], (F) NESM adopted from Reference [45], (G) Stuttgart Exo-Jacket adopted from Reference [46], and (H) CAREX 7 adopted from Reference [47].…”

“…It is required to replicate the actual shoulder and wrist movements that help to overcome the misalignment issues and to ensure the biomechanical compatibility of the design. Castro et al [44] proposed a 3-DOF scissor mechanism for the shoulder joint. The kinematic analysis of this spherical mechanism and motion capture data validation have been used to verify 3-DOF reachable workspace.…”

“…Wearable exoskeletons are not able to provide a wide range of motion in comparison to the human upper-limb torso [7]. For example, References [7,44] have presented the new mechanisms to assist three-DOF shoulder glenohumeral movements and to also maintain the instantaneous COR in supporting several ADLs. Since the mechanisms supporting shoulder GH movements only tend to reduce the actual upper-limb workspace but the effect is acceptable in some applications, so far, several studies have tried to address this limited workspace challenge in their design by considering shoulder girdle movements at the cost of complex and heavier mechanism [2,67].…”

“…This optimized design has ensured the uniform pressure distribution over the hand dorsum. References [7,44,51,59,154] have already tried to overcome the joint misalignment issues by considering the complex human biomechanics. Most of the active exoskeletons supporting shoulder and elbow movements found in the literature are not portable due to the high power-to-weight ratio.…”