Research On Forward Kinematics Configuration Bifurcation And Dimensional Synthesis Of The Stewart Platform

In comparison with a serial manipulator, the Stewart parallel manipulator, capable of providing 6-DOF movement, has advantages of high structural rigidity, high accuracy, fast dynamic response, and large load-to-weight ratio, which makes it find a wide application in flight simulation, spaceship aligning, radar and satellite antenna orientation, industry robot, parallel machine tools etc. However, due to the closed-loop structure of parallel robots, the characteristic of theses mechanisms are rather complex. This dissertation researches on forward kinematics, configuration bifurcation and dimensional synthesis of the 6-DOF Stewart parallel manipulator.The forward kinematics problem is usually complex due to lack of a closed-form solution and the high degree of nonlinearity in the kinematics formulations. In this dissertation, wavelet network method solution for the forward kinematics of parallel manipulator is proposed. The network is optimized by analyzing the sparseness of input data and selecting the fitting wavelets by orthogonalization method according to the output data. Then it is applied to solve the forward kinematics of a 6-DOF parallel manipulator as a numerical example. For comparison purposes, a BP neural network is also used for this problem. Simulation results show that the wavelet network performs better than BP neural network. In addition, a numerical example indicates that it can bring about good results when the result solved with wavelet network is made as an initial value of numerical method.For a given set of motion input parameters, the parallel robot is possibly at the state of configuration bifurcation. The dissertation discusses the configuration bifurcation about Stewart platform. Eight kinds of assembly configurations are obtained, which is solved by homotopy method. Based on the idea of analyzing the movement state of parallel manipulator in link space, sixteen kinds of extreme poses for 6-DOF parallel manipulator are described with the vector expressions of link lengths. The bifurcation characteristics of typical extreme poses are analyzed by static bifurcation condition. So, the length of links not occurring configuration bifurcation is obtained.The dissertation designs a safety mechanism which can move at will when the length of links ranges in its whole extension region. The configuration bifurcation, joint restriction, link interference are first analyzed. Then, the arithmetic of safety mechanism is established by considering the constraint conditions about extreme configuration and the static forces acting on the six links. The performance of the manipulator constrainted by the extreme pose condition is better to some extent. Then the rationality of the static forces acting on the six links is tested when the links move in their travels. Thus this manipulator has no singular points in its whole workspace. And the safety travel of links is obtained about general 6-DOF Stewart parallel manipulator. The analysis about the relation between the safety travel and the structural parameters provides us a reference to design the parallel mechanism. In order to analyze the performance of the safety mechanism, ADAMS (Automatic Dynamic Analysis of Mechanical Systems) is used to simulate the manipulator.After the analysis about the performance indexes and workspace of parallel manipulator, the dissertation designs two kinds of Stewart safety mechanism movement simulators based on the technology performance indexes. These two movement simulators have better property of global movement or bigger workspace, and are designed by optimizing all structural parameters with genetic algorithm, which aim at minimizing the average condition number of extreme poses or maximizing the workspace.