Research On Autobody Assembly Variation Analyse And Fixture Compensation Method Oriented Function Evaluation

With the increasingly fierce competition at domestic and foreign car market, and customers'demands for product quality, improving the quality of body assembly is to strengthen the self-developed and manufacturing ability of our cars. Assessment of the production function test (Function Evaluation referred to as FE), one of the main technical links, is organically related to the design and manufacture and thus plays an active and independent role in ensuring normal production and reducing development costs. However, the process of frame planning, deviation diagnosis and fixture adjustment during FE process of car parts is currently more dependent on engineering experience and lacks systematic theory and method. So many design defects cannot fully be prevented during the product design stage. The accumulation and dissemination of deviation in the assembly process of parts cannot be under control. The trial body is time-consuming and labor-intensive. It is difficult to raise the passing rate quickly. All these affect the market cycle of new products and the quality of high-volume assembly.In response to above-mentioned problems, this paper integrates 3D digital modeling of body assembly deviation with engineering experience and carries out the analysis of assembly deviation and the deviation compensation for FE of body assembly. It focuses on establishment of the unified body reference point system (RPS) multi-level dimension chain, automatically generated method of assembly dimension chain, 3D modeling of body assembly deviation and sensitivity analysis, optimization of assembly fixture location for critical control features and other basic theory and technical methods. All these are applied to the engineering practice in the FE process in order to realize accurate description and rapid diagnosis of assembly deviation transmission in the FE process, and enhance effectively the technical level of the self-developed car body. The main research content and creative work include:(1) Establishment of RPS dimension chainAccording to the integration of thoughts with multi-level design, control and detection, the characteristics of FE and RPS systems are analysed. Considering the multi-level inheritance of reference system combined with a significant number of levels in body assembly, the dimension chains of body-level, assembly-level, sub-assembly and parts-level are set up oriented RPS. The RPS dimension chain in the FE process is established to effectively decompose and optimize the complex relationship among body assembly.(2) Generated method of assembly dimension chainThe automatically generated method of assembly dimension chain is set up oriented graph theory. This method is started from the original dimension and composition dimension. The binding characteristics of parts positioning, and reference system and nodes are analysed. The connection matrix used for expressing parts assembly is set up. The searching of the connection matrix can result in the optimum path of dimension chain and thus the generation of dimension chain. Compared to the traditional method (starting from designated closed loop, searching the composition of ring and then acquiring dimension chain oriented the principle of minimum dimension chain), the method used in this paper has some advantages such as a unified and global search, and high efficiency.(3) Sensitivity analysis of deviation oriented assembly dimension chainConsidering the complicated relationship between the deviation of fixture positioning and parts and critical dimension deviation, the deviation model of body assembly is set up oriented RPS of all levels. To settle the difficulties in solving this model with multiple parameters, multiple steps and nonlinearity, the analysis is started from an implicit equation of dimension chain. The sensitivity matrix of deviation analysis is derived from it so that composition dimension can be expressed explicitly by original dimension. According to the sensitivity matrix for the deviation, the combined effect of dimension deviation and fixture deviation on assembly deviation of body is explored. (4) Optimization of fixture location in the FE processAccording to 3D model of assembly deviation, non-linear relationship between fixture positioning program and the deformation of fixtures is established to analyse systematically adjustable position and deviation compensation in the framing process. Setting critical dimensions and design specifications close to the level as the objective function, the optimization model oriented the location of fixtures is set up. The genetic algorithm is improved to determine the adjustment strategy of the location of fixtures which is used to compensate the assembly deviation in the framing process of car body.(5) Verification of example oriented the FE of the front cover assemblyTo satisfy the engineering demand for the FE of a certain brand of car body, the matching of the front cover assembly is taken as an example of verification. Oriented the above theory and method, multi-level dimension chain system is set up. Assembly dimension chain is generated quickly and thus the sensitivity matrix of deviation of parts and fixtures is used to analyse the deviation. The verification shows that the structure deviation of theoretical calculation is accordance with the deviation of the actual measurement. The adjustment strategy of fixture location under multi-objective optimization calculation can reduce significantly the deviation of the assembly which realizes a desired effect.To sum up, this paper is oriented theoretical derivation, combined with the comparative analysis of production data. This paper develops the deviation analysis of body assembly and deviation compensation of fixtures for the FE process. It will enhance body design process from experience to mathematical reasoning and computation process and provide a new idea for improving the dimension quality of car body and other complex products.