Optimization and analysis of a redundant 4R spherical wrist mechanism for a shoulder exoskeleton

Abstract

This paper presents a redundant 4-revolute (4R) spherical wrist mechanism for a shoulder exoskeleton, which overcomes several major issues with the 3R mechanisms used in the past. An analysis of the 3R mechanism is done to highlight the limitations in its range of motion and problems caused by operating near singular configurations. To ensure that the redundancy in the 4R mechanism is efficiently utilized, genetic algorithm is used to optimize the mechanism design and identify the optimal operating configurations of the mechanism. The capability to reach the entire shoulder workspace is guaranteed and the joint velocities are minimized by considering the joint displacements required to move the end-effector throughout the workspace and the condition number of joint configurations for reaching 89 positions in the workspace. Analysis of the 4R mechanism obtained from the optimization process indicates that it can move throughout the entire shoulder workspace with feasibly low joint velocities.

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

“…Designing mechanical shoulder joints that are singularity-free in the anatomical shoulder joint workspace is a problem that has been addressed repeatedly, either by repositioning the axes of the typical curved serial linkages shoulder configuration or by using redundant linkages. The problem for one particular case was comprehensively dissected by Lo and Xie [17]. For the configuration presented in Figure 2-A, the existing singularity of the system coincides with the posture of a 90º shoulder flexion as represented in Figure 2-B.…”

Kinematic Design Optimization of an Eight Degree-of-Freedom Upper-Limb Exoskeleton

“…The constrained optimization is formulated by defining a forbidden region for the structure when inverse kinematics has no solution, or the structure approaches singular configurations, or physical interference could occur. 35 Similarly, an optimization problem to minimize the global conditioning number of a parallel ankle rehabilitation robot is formulated and solved in Ref. [36] using a variation of GA.…”

Kinematics of the 3(RPSP)-S Fully Spherical Parallel Manipulator by Means of Screw Theory

“…A spherical parallel manipulator (SPM) is a mechanical device composed of a moving platform and a fixed platform connected to each other by means of two or more kinematic chains, the main virtue of which is that all points of the moving platform move on concentric spheres according to a point of the moving platform named the reference pole. Due to their large orientation workspace and compactness, SPMs have been widely employed as wrist joints [13,14], camera-orienting devices [15,16] and surgical robots [17][18][19]. On the other hand, the kinematics of spherical parallel manipulators has been extensively well studied and, without a doubt, approaching topics like inverse-forward displacement analysis, dexterity, workspace, detection of singularities, and so on [20][21][22][23][24][25][26][27][28][29].…”