An effective integration approach toward assembly sequence planning and evaluation

An integration strategy for assembly sequence planning and sequence scheme evaluation is proposed. This strategy can be used to plan a reasonable assembly sequence, to optimize a sequence scheme, and to predict whether a collision will occur between the assembly tool and assembled components by considering factors like target components and assembly resources. A hybrid method is presented for assembly sequence modeling that combines human-computer interactive operations to manually build a hierarchical assembly sequence main model and a hybrid graph method to automatically generate sub-assembly sequence schemes of the main model. An optimization algorithm based on time-cost is introduced to handle a best candidate components selection. This relieves the problem of limited capability found when handling large size assembly models with traditional methods. The essential issues involved in system implementation are discussed as well; these include a representation method for the assembly consequence model, an optimization model of assembly sequence planning, and an object-oriented system architecture model employed with multi-agent technology for visually evaluating the assembling process. This system, KM computer-aided assembly process planning, KMCAAPP, has been developed on the basis of our previous work, KMCAD3; KMCAAPP uses the presented approach. KMCAAPP can be integrated with CAD model from KMCAD3D. A case study shows that the presented approach can use large CAD assembly models and delivers a feasible and effective way to integrate the assembly sequence planning process with scheme evaluation by visually evaluating the assembling process. This allows the identification of design errors in a timely manner and mitigates economic loss.     

Quality assurance model in mechanical assembly

The uncertainty of mechanical assembly process usually brings great challenge to product quality control and assurance. The systematic approach should be employed in the research to find a comprehensive solution. The quality assurance model for assembly (QAMA), based on the analysis of the major problems in the mechanical assembly process and the factors that affect the quality of products, is established by means of three working models: the assembly process model (APM), the activity control model (ACM), and the quality data model (QDM). The APM formulates the assembly process starting with the analysis of process flow, logical scheme, and key factors. The ACM is built by defining the attributes of control activity, mapping relationship between the control activities and its objects in the multiview space and layers. The ACM presents the control flow with the logical relations among control activities as well as the control rules. Based on the two models mentioned above, the QDM supports the acquisition of quality data through data-collecting carriers along assembly process and defines the data structure to support the system model. All of the three models are grouped into a framework which integrates the technical approaches and solutions for quality assurance in mechanical assembly. Based on modeling studies, a computer-integrated and internet-based system called quality assurance system in mechanical assembly (QAS/MA) has been developed. And the development of QAS/MA proves that APM, ACM, QDM, and QAMA are practical and feasible.     

An intelligent modeling system to improve the machining process quality in CNC machine tools using adaptive fuzzy Petri nets

The paper first presents an AND/OR nets approach for planning of a computer numerical control (CNC) machining operation and then describes how an adaptive fuzzy Petri nets (AFPNs) can be used to model and control all activities and events within CNC machine tools. It also demonstrates how product quality specification such as surface roughness and machining process quality can be controlled by utilizing AFPNs. The paper presents an intelligent control architecture based on AFPNs with learning capability for modeling a CNC machining operation and control of machining process quality. In this paper it will be shown that several ideas and approaches proposed in the field of robotic assembly are applicable to the planning procedure modeling with minor modifications. Graph theories, Petri nets, and fuzzy logic are powerful tools which are employed in this research to model different feasible states for performing a process and to obtain the best process performance path using exertion of the process designer’s criteria.     

Concurrent tolerance charting for component and assembly using “modified tree” approach

The interaction between components in an assembly plays a major role in the performance and reliability of the assembly. However, in conventional design, the dimensions and tolerances of components are usually assigned intuitively to satisfy design constraints. Tolerance analysis is then performed to determine the critical tolerance accumulation that results from the assembly. The tolerances are then modified so that the interaction requirements are satisfied. This approach relies on the experience of the designers and the results may not be an optimum.This paper presents a direct method to determine the working dimensions and tolerances of components that allows control of the assembly interaction. The model is easy to construct and it provides a clear picture of the entire process and assembly links. Using the model, a set of linear equations is formulated directly based on the design constraints and interaction requirements. With appropriate constraints derived in terms of the process capabilities, the desired working dimensions and tolerances can be determined using LINDO, software for solving the linear programming problem. This approach allows the working dimensions and tolerances of all components in an assembly to be determined concurrently.     

INTEGRATED APPROACH TO GENERATION OF PRECEDENCE RELATIONS AND PRECEDENCE GRAPHS FOR ASSEMBLY SEQUENCE PLANNING

An integrated approach to generation of precedence relations and precedence graphs for assembly sequence planning is presented, which contains more assembly flexibility. The approach involves two stages. Based on the assembly model, the components in the assembly can be divided into partially constrained components and completely constrained components in the first stage, and then geometric precedence relation for every component is generated automatically. According to the result of the first stage, the second stage determines and constructs all precedence graphs. The algorithms of these two stages proposed are verified by two assembly examples.     

产品装配顺序的层次化推理方法研究

提出一种装配优先关系分类模型，将装配顺序规划过程中的各种装配优先关系分为几种优先关系，确定性工艺优先关系和模糊性工艺优先关系3类。并在此基础上，提出层次化装配顺序推理方法，分别从装配顺序的几何可行性、工艺可行性和工艺优良性3个层次，对装配顺序进行递进推理，最终得到性能优良的装配顺序。开发的产品装配分析软件原型系统－－PAAS可以对复杂产品的装配顺序规划进行有效的分析和优化。     

Unification of the a priori inconsistencies checking among assembly constraints in assembly sequence planning

Sequence planning generation is an important problem in assembly line design. A good assembly sequence can help to reduce the cost and time of the manufacturing process. This paper focuses on assembly sequence planning (ASP) known as a hard combinatorial optimization problem. Although the ASP problem has been tackled via even more sophisticated optimization techniques, these techniques are often inefficient for proposing feasible assembly sequences that satisfy the assembly planners’ preferences. This paper presents an approach that makes easier to check the validity of operations in assembly process. It is based on a model of the assembly planners’ preferences by means of strategic constraints. It helps to check a priori the consistency of the assembly constraints (strategic and operative constraints) given by the assembly system designers before and while running an assembly plan generation algorithm. This approach reduces the solution space significantly. A case study is presented to demonstrate the relevance of the proposed approach.     

Automatic generation of robotic assembly sequences

This paper describes a programming system capable of automatically generating robotic assembly sequences. It is a generative robotic assembly process planner. A geometric model of the product to be assembled is defined interactively in the feature-based product database. Assembly relationships between components are modelled interactively in the graphical relation diagrams. An initial and a final relation diagram are used to describe the initial and final states of the assembly, respectively. The validity of the physical connections defined in the final relation diagram is checked by analysing the information contained in the feature-based product database A single robotic assembly sequence is generated automatically, using only the relational data defined in the final relation diagram. Subassemblies are generated automatically. The component (or subassembly) to be moved at each step of the sequence is also explicity provided in the assembly-sequence formulation.     

Sequencing mixed-model assembly lines by considering feeding lines

A mixed-model assembly line is a type of production line where different models of a product are assembled on. Mixed-model assembly lines can respond to unanticipated changes in product demands quickly without keeping so many inventories. Designing mixed-model assembly line involves solving the traditional problems of the assembly line design (consists of balancing problem, determining cycle time, and the number and sequence of stations) in addition of determining the sequence of products in assembly line. The main goal of this paper is presenting a method in order to determine the sequence of products in mixed-model assembly line by considering Just-in-Time systems. Moreover, supplying some required components from feeding lines is considered. A mathematical model is presented which is capable of specifying the sequence of products in the mixed-model assembly line by considering main criteria and keeping feeding lines balanced. Mathematical model can be used for solving small-size problems. Because the combinatorial nature of sequencing problems typically provides an intractable search space for problems of “real world” size, the search heuristics of simulated annealing and ant colony algorithms are presented and used to find solutions for several problem sets. Experimentations show that the simulated annealing approach outperforms the ant colony approach in objective function performance.     

Computer aided geometric feasible assembly sequence planning and optimizing

Assembly sequence planning (ASP) is the foundation of the assembly process planning and design for assembly (DFA). In ASP, geometric feasibility is the prerequisite in the valid assembly sequences searching. The assembly precedence relations’ (APRs) deriving and fulfilling are the essential tasks in the geometric feasible assembly sequence planning. In this paper, a systematical approach called geometric constraint analysis (GCA) is proposed and the corresponding software system is developed and integrated with CAD system. Using this system, only with a few mouse clicks on CAD draft, assembly precedence relations (APRs) can be derived correctly and completely. Then, all the geometric feasible assembly sequences can be inferred out automatically. Moreover, an optimal algorithm is designed and realized in the GCA method, by which, the most optimal assembly sequence in terms of the operation convenience can be found out from the immense geometric feasible sequences. An erratum to this article can be found at     

A genetic algorithm approach to optimising component placement and retrieval sequence for chip shooter machines

A chip shooter machine in printed circuit board (PCB) assembly has three movable mechanisms: an X-Y table carrying a PCB, a feeder carrier with several feeders holding components and a rotary turret with multiple assembly heads to pick up and place components. In order to get the minimal placement or assembly time for a PCB on the machine, all the components on the board should be placed in a perfect sequence, and the components should be set up on a right feeder, or feeders since two feeders can hold the same type of components, and additionally, the assembly head should retrieve or pick up a component from a right feeder. The entire problem is very complicated, and this paper presents a genetic algorithm approach to tackle it.     

Assembly sequences merging based on assembly unit partitioning

Assembly sequence planning is a typical of combinatorial optimization problem which is difficult to be tackled when the number of parts of assembly becomes large. To reduce the searching space of assembly sequence planning of complex products, assembly sequences merging based on assembly unit partitioning is suggested. Assembly unit partitioning is presented to decompose the complex products into a group of assembly units containing a reduced number of parts or components, and the assembly design constraints and the assembly process constraints are comprehensively taken into account. The global optimal assembly sequences can be acquired through three steps. Firstly, the assembly units and decision graph of assembly unit are generated utilizing fuzzy analytical hierarchy process approach. Secondly, the optimal or near-optimal subsequences of assembly units can be obtained with current efficient methods of assembly sequence planning. Thirdly, under the assembly interference of assembly relations (geometrical constraints) of the whole products and the assembly precedence concluded by subsequences of assembly units, the assembly sequence merging is implemented to generate the global assembly sequences, and the optimal sequence is obtained through assembly sequences evaluation. The assembly constraints considered at the two previous steps is represented by the evaluation function. The effectiveness of the method is verified by an illustrative example and the results show that the searching space of assembly sequence merging of complex products is reduced remarkably and the optimal assembly sequence of the whole produces is obtained.     

面向大型产品装配序列规划的分类方法研究

讨论了在对大型产品进行装配序列规划时 ,为了简化装配序列产生过程 ,需要对待装配大型产品按操作复杂度分成两类 ,即复杂装配部分和简单装配部分 ,复杂部分进行虚拟装配规划 ,简单部分进行自动装配规划。分类时 ,首先对装配产品进行子装配体识别以得到组件 ,然后利用分类算法将组件分为两类。     

Optimal tolerance allocation for precision machine tools in consideration of measurement and adjustment processes in assembly

The high geometric accuracy requirement of precision machine tools represents a challenge for tolerance design and assembly process planning that guarantee the final assembly accuracy. Component tolerances should be allocated in association with assembly processes. However, tolerance design and assembly process planning are usually considered separately and lack quantitative analysis. In this paper, to integrate the geometric tolerance of components and variation propagation in assembly process, a state space model is developed. The measurement and adjustment process are expressed as observation matrix and control inputs. An optimal control problem is formulated to determine the adjustment process in consideration of the loss of final assembly accuracy and costs of remachining adjustment process. Tolerances of components can be optimally allocated based on the variation propagation in this deterministic assembly process. The generality and effectiveness of this approach are validated by applying the model on a four-axis horizontal machining center.     

A systematic approach for automatic assembly sequence plan generation

This paper presents a systematic approach for automatic assembly sequence plan generation (ASPG) by using an integrated framework of the part liaison matrix and precedence Boolean relations. The objective of this study is to propose a unified and integrated mathematical representation, manageable by computer programs, that allows an optimal assembly sequence be generated for various different production conditions and environment. To meet the aforementioned objective, a five-step modelling procedure is presented. In the first step, the product design and production information associated with part allocations and assembling sequences are logically arranged, simplified and systematically coded into the liaison matrix and precedence Boolean algebraic expression. In the second step, the position and assembly sequence relations of the original product are simplified into the so-called sub-assembly by using a group-like technology method. By doing so, the liaison matrix and precedence Boolean relation of the grouped sub-assemblies can then be easily obtained in the third modelling step. In the fourth modelling step, the constrained precedence Boolean relations of the grouped sub-assemblies are characterised and generalised for use in the proposed model. By simultaneously solving the aforementioned general and constrained position and precedence Boolean relations, the optimised assembly sequence of the underlying product can be quickly obtained in the fifth modelling step. A product assembly example is presented to illustrate the effectiveness of the proposed modelling approach. The enhancement of assembly efficiency and quality are also carefully evaluated .     

A new robotic assembly modeling and trajectory planning method using synchronized Petri nets

In this paper, a new approach to trajectory planning, performance Evaluation, and control for robotic assembly is presented. The assembly process is modeled as a discrete event system using synchronized Petri nets (SynPN). In a SynPN model, transitions are associated with firing conditions depending on external events. Due to this characteristic, SynPN is commonly used to model the connection between system states and external signals. The most important force/moment and position/orientation information in the assembly process serve as external events. Using the external events, the assembly states can be recognized and the system will be directed to the desired end state. Furthermore, trajectory planning algorithms and performance evaluation criteria of a discrete event system are also studied based on SynPN model. Finally, simulation and simple experiments are discussed in which a successful peg-in-hole assembly is fulfilled.     

Integrated assembly and machining planning for electronic products using a genetic algorithm approach

To produce an electronic product, both assembly operations and machining operations are required in the process plan. In most cases, the assembly operations and machining operations need to be combined in a continued order with an integrated sequence. This is different from the traditional process planning approaches in which machining operations and assembly operations are separated as two independent tasks with no interactions. For an electronic product, the two types of operations and the associated costs may affect each other in an interactive way. Therefore, the sequence planning of assembly operations and machining operations must be analyzed with an integrated model. In this research, a graph-based model is presented to represent the assembly and machining operations in an integrated model. The related operation cost functions are developed to evaluate the costs for the integrated assembly and machining sequences. The integrated sequence planning problem is solved using a genetic algorithm approach with an objective of lowest operation costs. As a result, the assembly operations and machining operations can be planned in an integrated sequence suitable for producing electronic products. The result shows that the developed method using the genetic algorithm approach is efficient for solving the integrated sequence planning problem. Example products are demonstrated and discussed.     

基于约束解除的装配序列并行化优化

为了提高产品装配的效率，提高装配序列的并行性，提出了基于约束解除和简化配合关联图的装配序列并行优化方法。给出了装配效率的评价指标一并行性评价因子的数学模型。将模糊逻辑用于装配约束的量化过程，通过约束归约方法将无约束条件下可自由运动零件的自身约束与零件间通过点、线、面之间几何约束关系所表达的装配约束进行求交，建立零件间的等价约束评价值即联接度，联接度概念的提出清晰地表达了配合关联图中的薄弱环节；在此基础上，建立装配体的赋权配合关联图，并根据联接度值的大小对零件间的虚联接进行简化，给出了一个基于阈值的追求装配最高并发度的配合关联图分解与重组策略，实现了装配序列并行优化，并通过实例说明了该算法的有效性。     

基于有向约束图的装配序列并行化方法研究

并行装配与串行装配相比可以缩短装配时间,提高装配效率。为了实现产品的并行装配,提出了基于有向约束图的装配序列并行化方法。定义了装配约束矩阵和零件约束度的数学模型来描述装配体在装配过程中各零件的干涉状态,建立了装配体的有向约束图描述零件间的约束关系,给出了基于关联解除的有向约束图分解方法生成装配序列,采用装配序列并行化策略产生了基于多叉树结构的并行装配序列。通过机用虎钳装配实例验证结果表明了该方法的可行性。     

装配顺序自动规划及其评价的研究

基于现有的商用CAD实体造型系统,提出了经动态干涉检测而得到装配体的干涉矩阵,详细分析了干涉矩阵的性质并通过干涉矩阵得到装配体优化后的装配顺序,同时提出了评价装配顺序的指标体系。