摘要
轿车车身尺寸质量直接关系到整车外观、风噪声、关门效果甚至行驶平稳性,影响轿车车身尺寸质量的主要有零部件设计、装配工艺及制造偏差等因素。随着汽车技术的不断发展,车身制造水平已经接近制造质量控制的极限,车身尺寸质量的进一步提高依赖于装配方案等工艺设计水平的改善。在车身薄板零件的装配过程中,装配顺序、焊接配置和夹具定位策略等装配方案影响着车身产品特征的装配偏差。以装配尺寸质量为目标的装配顺序优化研究方面,多数学者主要采用一维的偏差分析模型,对零件间的对接、搭接等接头形式进行定性分析。但是轿车车身作为具有复杂装配关系的薄板产品,在焊接过程中零件易于变形,传统的刚体偏差分析模型不再适用。
本文针对柔性薄板零件的装配顺序优化进行了研究:首先提出了基于多属性有向图的装配顺序规划算法,根据装配约束剔除非工程可行的装配顺序;然后建立多工位柔性薄板偏差分析模型,可计算不同装配顺序下的关键特征的装配公差;最后提出了装配顺序的两级优化算法,实现了装配顺序的优选。在此基础上,为了提高装配顺序优化的效率,开发了装配顺序生成和优化的原型软件,并通过车身侧围、尾灯支架等案例对本文提出的方法进行了验证。
本文的主要内容和创新点为:
(1)提出基于多属性有向图的工程可行装配顺序规划算法
针对通常以装配关系为基础的装配顺序规划算法,所生成的装配顺序难于完全保证工程可行,本文提出了基于多属性有向图的装配顺序规划算法。在装配模型中,不仅描述了零件间的装配关系,而且还添加了装配匹配特征信息和工程实际中的装配约束。并且引入环联分总成和对串、并、环联分总成的详细分类,完善了分总成类型。通过所提出的装配顺序自动规划算法可剔除非工程可行的装配顺序,提高了装配顺序生成的效率。
(2)建立多工位柔性装配偏差分析模型
传统柔性装配偏差分析模型着重于建立零件偏差和装配偏差间的关系,而在实际柔性薄板零件的装配过程中,夹具偏差起着重要的作用,本文建立了多工位间偏差传递和累积的柔性装配偏差分析模型。在“N-2-1”和“3-2-1”两种夹具定位原则下,夹具偏差、零件偏差和装配偏差在装配过程中有着不同的偏差变化,本文建立了这两种夹具定位规则下的装配前后偏差关系。在多工位装配过程中,按照夹具定位点的释放模式不同,建立了过定位释放和完全释放两种模式下的装配偏差分析模型。该模型可用于分析不同装配顺序下的装配偏差的传递与累积过程,提高了装配偏差计算的精度。
(3)提出装配顺序的两级优化算法
针对柔性薄板零件的装配偏差分析需通过大量的有限元计算,本文提出了装配顺序的两级优化算法:首先以刚体模型进行装配顺序的初步优化,得出一条优化的装配顺序;然后优化所得到装配顺序下的夹具定位策略和装配焊点组合,用于减少零件变形和降低装配偏差。在以刚体模型为基础的优化过程中,采用遗传算法优化零件间的装配匹配特征,获得优化的装配顺序。在以柔性装配模型为基础的优化中,提出了启发式正交方法优化夹具定位策略和装配焊点组合。为了进一步减少有限元分析次数,提高优化效率,建立了以径向函数为基础的响应面模型拟合优化变量与评估函数之间的关系。本文所提出的装配顺序的两级优化,克服了以往需要大规模有限元分析的缺点,从而提高了装配顺序优化的效率,增加了柔性薄板零件装配顺序优化的可行性。
(4)开发装配顺序优化的原型软件
在本文理论研究的基础上,为提高装配顺序生成与优化的效率,开发了装配顺序优化的原型软件。该软件以ACCESS数据库为基础,采用VC++作为开发工具,并应用OpenGL技术在软件中显示零件的三维数学模型。该软件根据所研究的主要内容分为装配模型、装配顺序生成、装配偏差分析和装配顺序优化等模块,可以自动生成和优化装配顺序。
The dimensional quality of car body directly related to vehicle appearance, wind noise, shut door effection and drive stationarity, is influenced by parts design, assembly techniques and manufacture variations. Further improvement of quality of auto-body product depends on leverage of better design methods rather than merely support from advanced manufacturing tools and technology, of which the capability limit has been reached. During assembly processes of compliant parts, assembly sequence, welding position and fixture configuration influence assembly variation of auto-body product character. Most scholars mainly adopted one dimension variation analysis model, and took qualitative analysis on the butt joint, lap joint of the parts in the research field of assembly sequence optimization which targets was dimension quality. But the sheet metals of auto-body have complex assembly relationship, and they are easy to distort, so the traditional rigid body variation analysis model is on longer fit for flexible sheet medal parts.
For compliant assemblies, an algorithm of assembly sequence optimization was presented. Assembly sequence planning is proposed based on multi-property directed graph, which can remove unfeasible sequences in engineering according to assembly constraints. Afterwards, the assembly variation analysis modeling is established, which can applied to analyze assembly tolerance of key characteristics in different sequences. The paper proposes assembly sequence optimization with two stages. For improving the efficiency of assembly sequence optimization, a software is developed for assembly sequence generation and optimization. Some cases are used to illustrate the algorithm such as the auto-body side and the rear lamp bracket assemblies. The main researches are as follows:
(1) Proposing an algorithm of assembly sequence planning based on multi-property directed graph
The current algorithms of assembly sequence planning generate assembly sequences based on precedence relationships among components, which can not assure they are all used in engineering. The paper proposes an algorithm based on multi-property directed graph. Assembly modeling not only describes the relationships among components, but also appends matching features and assembly constraints. Moreover, the paper completes the subassembly types through importing loop subassemblies and classifying the three types into subgroups. Finally the algorithm removes unfeasible assembly sequences in engineering, which can improve the efficiency of assembly sequence generation.
(2) Establishing modeling of multi-station compliant assembly variation analysis
Traditional assembly variation analysis modeling mainly focus on the relationship between part variation and assembly variation; fixture variation is very important to assembly variation during the assembly processes of compliant parts. The paper establishes modeling of multi-station compliant assembly variation analysis. The sensitivity matrices are given according to the process of variation propagation for two locating principles of“N-2-1”and“3-2-1”. Moreover, different assembly models are deduced for adapting each fixture layout and locator release.The modeling can be used to analyze prorogation of assembly variation in different sequences.
(3) Proposing assembly sequence optimization with two stages
For reducing the complexity of assembly sequence optimization, two stages are employed to optimize assembly sequences, which include: (1) optimizing assembly sequences based on rigid model; (2) optimizing schemes of fixture and welding points. In the first stage, the genetic algorithm is used to optimize matching features among parts. In the second stage, the paper proposes the response surface modeling based on radial basis function, which can well simulate nonlinear problem of deformable sheet metals. For improving the efficiency of fixture locating design, the paper uses the heuristic algorithm and the orthogonal design to optimize locating layout. The heuristic and orthogonal design algorithms are presented to select optimal assembly operations among components, whose advantages are that the search efficiency can be improved.
(4) Developing the software of assembly sequence optimization
Based on the above researches, a software is developed for improving the efficiency of assembly sequence generation and optimization. The software uses the ACCESS database, utilizes the developing tool of VC++ and shows 3D data model of parts through the model of OpenGL. The software includes assembly modeling, assembly sequence planning, assembly variation analysis and assembly sequence optimization, which can automatically generate and optimize assembly sequences.
引文
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