Abstract: Metamorphic mechanism is characterized by variable function and topology structures. It’s a challenge to perform the syntheses of kinematic chain corresponding with metamorphic conditions. Conceptions of metamorphic mechanisms and metamorphic sub-chains are discussed and their structural formulation is analyzed based on principle of metamorphosis. According to the number of metamorphic sub-chains, a metamorphic mechanism is divided into three types including single-type, multy-type and all-type. The essence of variable topology characteristics is change of constraint, and consequently, type synthesis of a metamorphic mechanism is converted into synthesis of an original mechanism with variable constraint sub-chains. Constraint screw theory is introduced and mathematical analysis is used to illustrate the relationship between a source metamorphic mechanism and sub-configuration with mathematical expression. Variable constraint synthesis theory is proposed and a general process is formed aiming at metamorphic mechanism design. In order to validate the proposed theory, a spatial metamorphic parallel mechanism used for bionic joint is presented and the mechanisms which meet the requirements are obtained with variable constraint synthesis theory.

Key words: metamorphic mechanisms;variable constraint;screw theory;type synthesis