Type Synthesis Theory Of Lower-Mobility Parallel Mechanisms And Synthesis Of New Architectures

Compared with the general 6-DOF (degrees of freedom) parallel mechanism, lower-mobility parallel mechanisms have advantages of simple structure, low cost in design, manufacturing and control. Particularly the symmetrical lower-mobility parallel mechanism, which characterizes identical limbs, symmetrical arrangement and isotropy, has great potentials in practical application. However, the problem of type synthesis of lower-mobility parallel mechanisms has not been well solved over the last 20 years. A universal and effective type synthesis theory has not been established and consequently there is a lack of symmetrical 4-DOF and 5-DOF parallel mechanisms. The invention of new architecture determines the possession of knowledge properties. Thus, type synthesis of lower-mobility parallel mechanisms has been a hot subject and advancing edge in international academic and industrial field. The competition in this filed is fairly fierce.This paper focuses on type synthesis of symmetrical lower-mobility parallel mechanisms and aims to establish a universal and effective theory and to synthesize new architectures of lower-mobility parallel mechanisms with independent knowledge properties. The main contributions are as follows:Based on the linear dependency of constraint screws under different geometrical conditions, a straightforward classification of constraint screw system is proposed and the corresponding constrained motion is obtained by reciprocal relations between twists and wrenches. Further, the constraint effect of the limb acting on the moving platform is described by limb constraint system and the combined effect of all limb constraints is described by mechanism constraint system.The concept and geometrical condition of common constraint and redundant constraint are obtained using screw theory. Then the criterions of common constraint and redundant constraint are proposed. Then a revised Grubler-Kutzbach mobility criterion is proposed, which is universally effective in mobility analysis of lower-mobility parallel mechanisms. A mobility criterion based on constraint analysis is proposed. By introducing the concept of generalized kinematic pair, the mobility of parallel mechanism with closed-loop in limb can be obtained by the revised Grubler-Kutzbach mobility criterion.Based on the kinds of mobility, rotational or translational, lower-mobility parallelmechanisms are sorted into nine classes. The limb constraint system and the mechanism constraint system of the nine kinds of parallel mechanism are enumerated with the geometrical conditions which guarantee the combined effect of all the limb constraint screws equal to the desired mechanism constraint system. Based on this, the full constraint synthesis theory of lower-mobility parallel mechanism is established, which is the only one applicable to all the nine kinds of lower-mobility parallel mechanisms.The type synthesis of two kinds of symmetrical 5-DOF parallel mechanisms is performed using constraint synthesis method. Symmetrical 5-DOF parallel mechanisms which can undergo finite motion are synthesized for the first time, including many novel mechanisms. The structural characteristics of symmetrical 5-DOF parallel mechanisms are revealed systematically.The type synthesis of three kinds of symmetrical 4-DOF parallel mechanisms is performed using constraint synthesis method. Many novel mechanisms are obtained and the structural characteristics of symmetrical 4-DOF parallel mechanisms are revealed systematically.The type synthesis of three kinds of symmetrical 3-DOF parallel mechanisms is performed using constraint synthesis method. Many novel mechanisms are obtained and the structural characteristics of symmetrical 3-DOF parallel mechanisms are revealed systematically.Based on the Lie group theory, a displacement manifold synthesis theory is proposed. Limb displacement manifold is used to describe the motion of the limb end while mechanism displacement manifold is used to describe the motion of the moving platform. Using the structural cha...