Research On Parallel Robot Based Flexible Fixtures For Automotive Sheet Metal Assembly

On account of the application requirements of flexible manufacturing and hybrid assembly for sheet metals, this thesis proposed parallel robot based flexible assembly fixtures for automotive sheet metals; and completed a series of research on the parallel fixture mechanisms, including structure synthesis, design, analysis, manufacturing and experiments; in the process a flexible assembly fixturing system based on the"N-2-1"principle is constructed, and the deviation analysis and robustness opertimazition for the system is developed. The study includes the following aspects.Firstly, the design requirements of automotive sheet metal assembly fixtures are analyzed, a total design scheme of the parallel flexible fixturing system is proposed. Then a new method of parallel structural synthesis and derived design, based on screw theory and Lie theory, is proposed, and the synthesis and derivation of a new class of 6-DOF parallel mechanisms with three limbs are investigated. Among them, three typical mechanisms are selected; a modular design for these mechanisms is carried out.According to screw analysis, the DOFs and actuated joints of the tipical mechanisms are confirmed. Taking advantages of the geometric features of the mechanisms, the inverse kinematics are developed. The Jacobian matrices are established on the basis of the screw analysis, which makes the physical meaning of the expressions more explicit. Based on the geometrical constraint conditions and the assembly requirements, the dimensional design space of the parallel fixturing mechanisms are obtained; and the issue of multiple parameters is solved by modeling of the secondary space. The establishment of inverse kinematics, Jacobian matrices and design space provide the basis of analyzing the performance indeces of the mechanisms.From the view of the mechanism applications, the performance of workspace, singularity and stiffness of the parallel fixturing mechanisms have been systematically studied. Firstly, the relationship between shape, area and parameters has been discussed; secondly, on the basis of previous studies, a new singularity classification based on the screw analysis is proposed; finally, a method for stiffness modeling of the parallel mechanisms is proposed, which utilizes screw theory to conduct force analysis, deformation analysis and stiffness modeling of the mechanisms. After the establishment of the global evaluation indices of all the performance, the performance atlases are searched in the design space, it provides reference for the dimensional optimization of the mechanisms.Through the analysis of the automotive sheet metal assembly process, 5 variations are taken into consideration, which are workpart variation, fixture variation, positioning variation, fixturing variation and welding variation. Variation modeling of sheet metal assembly is completed, which include the establishments of the springback equation and the modeling of the composed variations, particulary to the modeling of the positioning variation and fixturing variation, the two variations are separate based on screw theory and the methods of multi-point constraints and shape function difference. Then a robustness optimization index based on the variation model is proposed, which can be used for getting the robust fixtuing scheme.Finally, the dimensional optimization criterion for the parallel fixture mechanisms is given, based on the analysis of performance index. After that the dimensional optimization of the mechanisms is conducted, the detailed design of the experimental prototype is developed. Upon the integration of the control system hardware and software design for the parallel fixturing mechanisms, the basic experiments about clamping test, measurements of stiffness and repeatability of the mechanisms are carried out.The work of this thesis has great significance in improving the productivity and the rate of automobile production in response to market changes.