This work deals with the analysis of three degree of freedom spherical shoulder mechanisms which have force redundancies in their parallel structure. First, a comparative study between a fully parallel shoulder mechanism (three legs with three inputs) and a redundantly actuated parallel shoulder mechanism (four legs with four inputs) is performed in terms of force transmission capability. The mode in which all joints of the shoulder (three- and four-legged) are actuated is studied next, in terms of active stiffness transmission capability. It is shown that the four-legged shoulder is superior to the three-legged shoulder in both force and active stiffness transmission, and that force and active stiffness transmission capability are operational trade-offs. Design issues associated with general redundantly actuated systems are mentioned.

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