Configuration Change and Mobility Analysis of a New Metamorphic Parallel Mechanism Used for Bionic Joint

Traditional bionic joints including wrist, waist, ankle and shoulder have three revolute degrees of freedom, which can be treated as spherical joints, and its structure is prone to be damaged with impact force through the center of spherical joint. In this paper, a novel metamorphic parallel mechanism with two configurations used for bionic joint design is presented. The origin kinematic chain for this metamorphic mechanism, which is also the first configuration mechanism, consists of a moving platform, a base plate and four connecting legs between moving platform and base plate. In contrast with traditional ankle joint, a constrained translational degree of freedom can be activated in origin kinematic chain to avoid impact damage. The origin kinematic chain can transform to the second configuration mechanism which is in possession of three revolute degrees of freedom and equivalent to normal spherical joint. The transforming process is represented by new incidence matrix and the motion characteristics of this metamorphic mechanism in different configuration corresponding to each work-stage are analysed using screw theory.