Some Theoretial Issues On Analysis And Synthesis Of Lower-Mobility Parallel Mechanisms

With the development of parallel robot research, spatial lower-mobility parallel robots have attracted much attention due to their simple in structure, easy control, and low cost. Based on the analyses of the current situation of the lower-mobility parallel robots home and abroad, some important theoretical issues on the spatial parallel mechanisms are investigated in this dissertation. The research is financed by National PHD Foundation. Some kinematics problems of a novel 3-RRUR parallel mechanism, which has three translation degrees of freedom, are studied. The formulas and equation of the inverse kinematics of the 3-RRUR parallel mechanism are deduced. An ideal workspace under the limit length of link is researched, a group of ideal workspace sectional curves along upward direction and a solid model generated from these curves are drawn. Next, the first-order effect coefficient matrixes of the mechanism are deduced, the displacement and velocity curves are drawn. Based on the screw theory, the first-order effect coefficient matrixes of a 3-UPU parallel pyramid mechanism under initial position and movement along X-coordinate axis are established. Base on these results, the velocity and acceleration equations of the 3-UPU parallel pyramid mechanism are obtained. From these equations, their relation curves under movement along X-coordinate axis are drawn. Next, the kinematic characteristics of other 3-UPU parallel platform mechanism, which has two rotational degrees of freedom and one translational degree of freedom, are studied, and the first-order effect coefficient matrixes are established as its moving platform and base being parallel to each other, and principle screws and pitches of the mechanism are figured out. After that, the three-dimensional distribution of kinematic screws are drawn. By adopting a continuation approach, the position analyses of a novel 3-5R parallel mechanism, which has three translation degrees of freedom, are conducted. Next, an equation system of the mechanism structure constraint and an initial equation system are established. Next, a versatile software for solving polynomial equation system is compiled. As giving one displacement, all inverse kinematics are obtained from this software. The discrimination of input selecting reasonability of spatial parallel mechanism is an important problem for designing robot mechanism. Based on the concepts of reciprocal screws and their correlativity, input selecting reasonability of spatial parallel mechanism is discriminated. As discriminating the input selecting reasonability, not only the reasonability of the initial position is discussed, but also the reasonability at any moving position is discussed. Next, a criteria for input selecting is discussed. Based on these results, the input selecting of many novel lower-mobility parallel mechanisms are analyzed. Based on input selecting theory, some reasons of unreasonable input of mechanism are discussed and solutions are given. By modifying shape of the moving platform and the base, varying arrangement of limbs, or synthesizing mechanism, let the mechanisms have the reasonable input as kinematic characteristics of mechanism aren't changed. Two novel 4-DOF parallel mechanism are synthesized by combining different limps. One kind of 21 symmetrical 4-DOF parallel mechanisms, which have one rotational degree of freedom and three translational degrees of freedom, is systematically obtained by using a constraint screw synthesis method. The most of them are put forwarded first. Next, the other kinds of the symmetrical parallel mechanisms with finite motion are synthesized. The mechanisms have two rotational degrees of freedom and three translational degrees of freedom, and 20 types in all. It is obvious that the constraint screw synthesis method is effective. Research works in this dissertation enrich lower-mobility parallel mechanisms. From analysis the above problems, a conclusion is obtained below: There are five force vectors. If three of them are parallel each other but they are not in the s...