Type Synthesis Theory Of Lower-Mobility Free-Singularity Fully-Isotropic Parallel Manipulators

Besides the inherent advantages of six degree-of-freedom (DOF) parallel manipulators, lower-mobility parallel manipulators (LMPM) also possess other merits in terms of simpler structures, lower cost in manufacturing and control, larger workspace. Therefore, LMPMs have been one of the most popular subjects in the international mechanisms and robots fields in recent years. For general parallel mechanisms, one of their drawbacks is the strong kinematic coupling, which leads to many difficulties on path planning and control. Although a lot of new parallel manipulators are constructed, a few of them have been applied in practice, where the strong kinematic coupling is one of the most important factors. So how to design decoupled, uncoupled, or even fully-isotropic parallel mechanisms has become a new subject in this field.This dissertation mainly focuses on type synthesis of free-singularity fully-isotropic parallel manipulators. The material contributions are as follows:Expressions relating input and output velocities of parallel manipulators are described firstly based on screw theory. The basic theory system of the type synthesis for free-singularity fully-isotropic parallel manipulators is proposed, which provides a theoretic frame for mechanism structure synthesis.A systemic approach for type synthesis of lower-mobility free-singularity fully-isotropic parallel manipulators is presented based on reciprocal screw theory. The type synthesis of free-singularity fully-isotropic spatial translational parallel manipulators,2-dimensional translation and 1-dimensional rotation (2T1R-type) spatial and planar parallel manipulators is performed using the approach proposed here and many novel mechanisms are obtained. The progress of type synthesis is as follows:firstly, according to the characteristics that both the forward and the inverse matrices of the fully-isotropic parallel manipulators are constant diagonal matrices, the types of each limb's actuation screw and actuated screw is determined; then, the types and the assembling configuration of the unactuated screws of the limbs are defined in terms of the property that the reciprocal product between the actuated screw and all unactuated screws within the same limb is identically equal to zero; thirdly, the kinematic chains structures are synthesized by means of the different limb connectivity; finally, the expected parallel manipulators are obtained by connecting the moving platform with the fixed base using the limbs synthesized previously. Moreover, the selecting principles of the actuated joints of mechanisms are proved using this type synthesis approach. According to the concept of hybrid mechanism, fully-isotropic 2T2R 4-DOF hybrid mechanisms can be achieved by combining any one of the fully-isotropic 2T1R planar manipulators synthesized here and a revolute joint. A systemic method for type synthesis of uncoupled 2T2R parallel manipulators is proposed based on the kinematics analysis of fully-isotropic 2T2R hybrid mechanisms. Then the type synthesis of fully-isotropic 2T2R parallel manipulators is performed by replacing the actuated prismatic pair in the fourth limb of the uncoupled 2T2R parallel manipulators with two revolute joints whose axes are parallel to each other or a parallelogram structure comprised of 4 revolute joints.543025 novel uncoupled and fully-isotropic 2T2R parallel manipulators are synthesized in theory.The type synthesis of free-singularity fully-isotropic 2T1R spatial manipulators is also performed based on the theory of linear transformations. Both kinds of uncoupled and fully-isotropic mechanisms are obtained. The compared analysis for two synthesis approaches used in this dissertation is completed simply.Kinematics analysis of a novel 3-CRP translational parallel manipulator synthesized in the paper is addressed. Mobility analysis and determination of the actuated joints for the manipulator are studied based on the screw theory. Kinematic problems and performances of the mechanism are investigated respectively in terms of different actuated inputs, i.e., rotary actuation or linear actuation method. When the actuated joints are rotary actuation, Jacobian matrix of the manipulator is a diagonal matrix, so it is an uncoupled mechanism. As the actuated joint is linear one, the Jacobian matrix is an identity 3×3 matrix and its determinant is equal to one. In a result, the manipulator is singularity-free fully-isotropic throughout its entire workspace, which further proves that the type synthesis method proposed in this dissertation is valid and feasible.