Combined optimization of part topology,support structure layout and build orientation for additive manufacturing

Additive manufacturing (AM) enables the fabrication of parts of unprecedented complexity. Dedicated topology optimization approaches, that account for specific AM restrictions, are instrumental in fully exploiting this capability. In popular powder-bed-based AM processes, the critical overhang angle of downward facing surfaces limits printability of parts. This can be addressed by changing build orientation, part adaptation, or addition of sacrificial support structures. Thus far, each of these measures have been studied separately and applied sequentially, which leads to suboptimal solutions or excessive computation cost. This paper presents and studies, based on 2D test problems, an approach enabling simultaneous optimization of part geometry, support layout and build orientation. This allows designers to find a rational tradeoff between manufacturing cost and part performance. The relative computational cost of the approach is modest, and in numerical tests it consistently obtains high quality solutions.     

基于开源场景图形的三维可视化与信息管理系统设计

对于产品虚拟装配的三维呈现过程中难以实现零件信息管理问题,结合电动车模型的拆装,提出三维可视化与信息管理技术融为一体的设计方案。首先,建立三维模型库,并根据电动车模型的拓扑结构和辅助信息,如零件的材质、型号等,建立信息库;其次,读取信息库中零件与子装配体间的父子关系信息建立目录树,根据子装配体与场景树均为“多叉树”组成结构的原理进行子装配体的三维呈现,再对子装配体的各个节点设置动画完成拆装呈现;最后,采用多线程方法将电动车配件信息管理与可视化有机结合,实现支持三维的拾取交互查询和检索定位查询等功能。系统以世纪鸟电动车进行验证,实现了三维可视化技术与信息系统相融合,能快速有效地为电动车的三维呈现和虚拟装配提供技术支持。实例验证表明,所设计系统可以有效地将零件的信息管理融入到虚拟装配的三维可视化中。     

Optimum part orientation in Rapid Prototyping using genetic algorithm

Part orientation is an important parameter in the planning of a Rapid Prototyping (RP) process as it directly governs productivity, part quality and cost of manufacturing. This paper reports the design and implementation of a system for obtaining optimum orientation of a part for RP. Developed in a modular fashion, the system comprises of functional modules for CAD model preprocessing, shelling (hollowing), part orientation and optimization. CAD part model in STL format is an input to the system. The oriented CAD model is sliced and hollowed with desired shell thickness. Genetic algorithm based strategy is used to obtain optimum orientation of the parts for RP process. The objective criteria for optimization is considered to be a weighted average of the performance measures such as build time, part quality and the material used in the hollowed model. The developed system has been tested with several case studies considering SLS process.     

Design of cellular manufacturing systems with assembly considerations

This research proposes incorporating assembly aspects associated with a product into the design of Cellular manufacturing System (CMS). The literature on CMS design implicitly assumes that finished part is the end product by itself. In practice, often, manufacturers produce parts which are assembled into a finished product. The methodology employs a part–subassembly matrix derived from the product structure in addition to the part–machine matrix. A mathematical programming model is developed which determines an assignment of parts, machines and subassemblies to manufacturing cells. The proposed model employs a new similarity coefficient between part, machine and subassembly. The model resulted in a nonlinear program with 0-1 variables. A case study has been analyzed based on a published part–machine matrix and a randomly generated product structure. The analysis reveals that it may be required to forego some of the efficiencies of Group Technology (GT) in order to achieve integration of assembly operations with production of parts. From a practical stand point of view it is preferred to have a system design which has a mix of GT and integration efficiencies, compared to a design which outperforms on GT criteria and completely lacks integration of assembly operations with production of parts.     

Numerical inverse kinematics for modular reconfigurable robots

The inverse kinematics solutions of a reconfigurable robot system built upon a collection of standardized components is difficult to obtain because of its varying configurations. This article addresses the formulation of a generic numerical inverse kinematics model and automatic generation of the model for arbitrary robot geometry including serial and tree‐typed geometries. Both revolute and prismatic types of joints are considered. The inverse kinematics is obtained through the differential kinematics equations based on the product‐of‐exponential (POE) formulas. The Newton–Raphson iteration method is employed for solution. The automated model generation is accomplished by using the kinematic graph representation of a modular robot assembly configuration and the related accessibility matrix and path matrix. Examples of the inverse kinematics solutions for different types of modular robots are given to demonstrate the applicability and effectiveness of the proposed algorithm. ©1999 John Wiley & Sons, Inc.     

Human-object integrated assembly intention recognition for context-aware human-robot collaborative assembly

Human-robot collaborative (HRC) assembly combines the advantages of robot's operation consistency with human's cognitive ability and adaptivity, which provides an efficient and flexible way for complex assembly tasks. In the process of HRC assembly, the robot needs to understand the operator's intention accurately to assist the collaborative assembly tasks. At present, operator intention recognition considering context information such as assembly objects in a complex environment remains challenging. In this paper, we propose a human-object integrated approach for context-aware assembly intention recognition in the HRC, which integrates the recognition of assembly actions and assembly parts to improve the accuracy of the operator's intention recognition. Specifically, considering the real-time requirements of HRC assembly, spatial-temporal graph convolutional networks (ST-GCN) model based on skeleton features is utilized to recognize the assembly action to reduce unnecessary redundant information. Considering the disorder and occlusion of assembly parts, an improved YOLOX model is proposed to improve the focusing capability of network structure on the assembly parts that are difficult to recognize. Afterwards, taking decelerator assembly tasks as an example, a rule-based reasoning method that contains the recognition information of assembly actions and assembly parts is designed to recognize the current assembly intention. Finally, the feasibility and effectiveness of the proposed approach for recognizing human intentions are verified. The integration of assembly action recognition and assembly part recognition can facilitate the accurate operator's intention recognition in the complex and flexible HRC assembly environment.     

Singularity-free time integration of rotational quaternions using non-redundant ordinary differential equations

A novel ODE time stepping scheme for solving rotational kinematics in terms of unit quaternions is presented in the paper. This scheme inherently respects the unit-length condition without including it explicitly as a constraint equation, as it is common practice. In the standard algorithms, the unit-length condition is included as an additional equation leading to kinematical equations in the form of a system of differential-algebraic equations (DAEs). On the contrary, the proposed method is based on numerical integration of the kinematic relations in terms of the instantaneous rotation vector that form a system of ordinary differential equations (ODEs) on the Lie algebra \(\mathit{so}(3)\) of the rotation group \(\mathit{SO}(3)\). This rotation vector defines an incremental rotation (and thus the associated incremental unit quaternion), and the rotation update is determined by the exponential mapping on the quaternion group. Since the kinematic ODE on \(\mathit{so}(3)\) can be solved by using any standard (possibly higher-order) ODE integration scheme, the proposed method yields a non-redundant integration algorithm for the rotational kinematics in terms of unit quaternions, avoiding integration of DAE equations. Besides being ‘more elegant’—in the opinion of the authors—this integration procedure also exhibits numerical advantages in terms of better accuracy when longer integration steps are applied during simulation. As presented in the paper, the numerical integration of three non-linear ODEs in terms of the rotation vector as canonical coordinates achieves a higher accuracy compared to integrating the four (linear in ODE part) standard-quaternion DAE system. In summary, this paper solves the long-standing problem of the necessity of imposing the unit-length constraint equation during integration of quaternions, i.e. the need to deal with DAE’s in the context of such kinematical model, which has been a major drawback of using quaternions, and a numerical scheme is presented that also allows for longer integration steps during kinematic reconstruction of large three-dimensional rotations.     

Symbolic OBDD representations for mechanical assembly sequences

Assembly sequence planning is one typical combinatorial optimization problem, where the size of parts involved is a significant and often prohibitive difficulty. The compact storage and efficient evaluation of all the feasible assembly sequences is one crucial concern. Ordered binary decision diagram (OBDD) is a canonical form to represent and manipulate the Boolean functions efficiently, and appears to give improved results for large-scale combinatorial optimization problems. In this paper, subassemblies, assembly states and assembly tasks are represented as Boolean characteristic functions, and the symbolic OBDD representation of assembly sequences is proposed. In this framework, the procedures to transform directed graph and AND/OR graph into OBDDs are presented. The great advantage of OBDD-based scheme is that the storage space of OBDD-based representation of all the feasible assembly sequences does not increase with the part count of assembly dramatically so quickly as that of both directed graph and AND/OR graph do. We undertake many experimental tests using Visual C++ and CUDD package. It was shown that the OBDD scheme represented all the feasible assembly sequences correctly and completely, and outperforms either directed graph or AND/OR graph in storage efficiency.     

Improving assembly precedence constraint generation by utilizing motion planning and part interaction clusters

In this paper, we present a technique that combines motion planning and part interaction clusters to improve generation of assembly precedence constraints. In particular, this technique automatically finds, and clusters, parts that can mutually affect each other’s accessibility, and hence may impose assembly constraints. This enables the generation of accurate precedence constraints without needing to examine all possible assembly sequences. Given an assembly model, our technique generates potential disassembly layers: spatial clustering is used to generate part sets. Next, motion planning based on rapidly-exploring random trees (RRT) with multiple trees is used to evaluate the interaction between these part sets. Specifically, motion planning is used to determine which part sets can be removed from the assembly. These sets are added to the first disassembly layer and removed from the assembly. Part sets that can be removed from the simplified assembly are then added to the second layer. If the process gets stuck, parts in the parent set are regrouped, and the process continues until all disassembly layers are found. The resulting structure reveals precedence relationships among part sets, which can be used to generate feasible assembly sequences for each part set and the whole assembly. We present theoretical results related to the algorithms developed in the paper. Computational results from tests on a variety of assemblies are presented to illustrate our approach.     

Assembly unit partitioning for hull structure in shipbuilding

In order to solve the problem that low automatic degree to the block assembly process design will negatively influence construction cycle time and construction quality in shipbuilding, this paper presents a newly developed determination system of assembly units for the hull structure. Firstly, the assembly information model of a hull block which includes the part information and the linkage information between parts is proposed following the structural features and assembly process of the hull structure. Secondly, the assembly relation matrix is established on the basis of the fuzzy assessment rule and then the fuzzy clustering method to analyze partition of assembly units is given out. Thirdly, through analyzing the assembly ability of hull structures, the set of evaluating indexes is founded whose weights are decided by an analytic hierarchy process (AHP). Meanwhile, the model for the assessment is provided by fuzzy synthetic evaluation (FSE) and the comprehensive assessment values of assembly partition schemes can be computed to judge the final optimization scheme. A block assembly is taken as an example to verify the proposed method, and the results show that it is an effective method for solving the partition problem of hull structures.     

求解存在运输空间约束多单元协作调度问题的拍卖算法

针对存在运输空间约束的多单元协作调度问题,提出合理的运输模式,建立非线性整数规划模型,对问题进行描述.通过拍卖的方式,将设备资源和运输资源分配给每一个工件,得到问题的可行解,并基于改进的(非)连通图对可行解进一步优化.通过拍卖方式,可以恰当地针对不同时间段的资源进行价值评估,从而提高资源的利用率,减少总的生产时间.拍卖过程分为两部分:车辆资源拍卖和设备资源拍卖.在整个拍卖过程中,车辆和设备分别扮演拍卖者,每个工件扮演竞拍者.通过对比实验,验证了所提出运输模式的合理性以及算法的有效性.     

An approach to automatic adaptation of assembly models

Adaptation plays a fundamental role in case-based design. However, after decades of efforts, automatic adaptation is still an open issue. In works of case-based design, a designer usually chooses a start-up product model (a candidate model) of moderate complexity based on a query model possessing primary new design requirements (kinematic semantics and geometry), then achieves the target design by adapting the candidate model according to the new design requirements and human interventions are often indispensable. To smartly adapt the candidate model to fit the new design requirements, a novel approach to automatic adaptation of assembly models is proposed in this paper. First, in order to effectively identify the corresponding links and interfaces between two non-preregistered assembly models as relevant elements, an attributed kinematic graph is put forward and adopted. Second, based on the attributed kinematic graph, the kinematic semantics of the candidate model is automatically adapted to that of the query model. Third, through performing interface layout transferring, the geometry of the candidate model is automatically adapted to that of the query model based on the corresponding links and interfaces. A prototype system is also implemented to verify the effectiveness of the proposed approach.     

基于属性邻接图匹配的装配体模型搜索方法

为了重用装配体模型中包含的设计、制造和装配工艺等信息,需要在装配体模型数据库中搜索相似的装配体模型作为参考.为此提出一种基于属性邻接图匹配的装配体模型搜索方法.首先提取装配体模型中零件的接触面信息,将装配体模型及其零件模型分别转化为零件属性邻接图和接触面属性邻接图;然后计算用于零件模型匹配的成功匹配接触面数量、接触面关系编码数量以及用于装配体模型搜索的零件相似度矩阵,将其作为零件模型和装配体模型相似度度量;最后由最优匹配Kuhn-Munkres算法得到零件相似度矩阵最优匹配加权和,作为装配体模型相似度.实验结果表明,该方法能有效地搜索到相似的装配体模型,并且对模型的相似度进行了排序.     

45 t岸边集装箱起重机可视化装配仿真

为缩短岸边集装箱起重机生产制造过程中的装配周期并降低成本,针对某45 t岸边集装箱起重机,基于面向装配设计的方法引入可视化虚拟装配技术,运用CATIA对其进行虚拟装配仿真分析,确定零部件装配层次关系,设计零件的部装、预拼装和总装等部分的整体装配流程.在CATIADMU平台建立基于仿真试拆卸的装配路径规划流程,并以运行小车滑轮组为例分析装配路径规划并进行装配干涉检查.该方法可以直观地显示出各零部件的可装配性,优化岸边集装箱起重机装配结构.     

装配规划与方案评价集成系统的研究与开发

为了将装配顺序规划与方案评价有效集成,规划合理的装配顺序,检验装配过程中存在的干涉情况,提出了人机交互定义分层装配顺序主模型与自动生成子装配的两级装配顺序规划方法．该方法适用于大型装配模型的装配顺序规划场合;在原有的研究项目KMCAD3D基础之上,开发了一套实用的装配顺序规划与方案评价原型系统;通过对二级蜗轮减速器实例的装配顺序规划讨论,说明了该方法的可行性．     

The symbolic OBDD scheme for generating mechanical assembly sequences

Assembly sequence planning is one of typical combinatorial optimization problems, where the size of parts involved is a significant and often prohibitive difficulty. The compact storage and efficient evaluation of feasible assembly sequences is one crucial concern. Ordered binary decision diagram (OBDD) is a canonical form to represent and manipulate the Boolean functions efficiently, and appears to give improved results for large-scale combinatorial optimization problems. In this paper, assembly knowledge models of liaison graph and translation function are formulated by OBDDs, and OBDD-based representation of assembly sequences is proposed. A novel OBDD-based procedure was presented to generate all geometrically feasible assembly sequences from the OBDDs of liaison graph and translation relation. This procedure can be used conveniently on the computer and all the feasible sequences can be derived. The great advantage of OBDD-based scheme is that the storage space of OBDD-based representation of feasible assembly sequences does not increase with the part count of assembly dramatically so quickly as that of AND/OR graph does. We developed the prototype tool for generating assembly sequence using Visual C++ and CUDD package, and undertake some experimental tests. It was shown that the OBDD scheme generated feasible assembly sequences correctly and completely.     

Cyber coordinated simulation for distributed multi-stage additive manufacturing systems

Additive Manufacturing (AM) processes have been increasingly used to manufacture energy storage products with dedicated material preparation and post-processing stages to enhance product properties. Most researchers focus on selecting materials and improving processes, yet the system modeling and management has not been investigated so far. This paper extends the conventional single-stage AM processes to multi-STage distRibutEd AM (STREAM) systems. In STREAM, a batch of material produced at the pre-processing stage is jointly consumed by distributed AM printers, and then the printed parts are collected for the post-processing stage. Modeling and managing such complex systems have been challenging. We propose a novel framework for “cyber-coordinated simulation” to manage the hierarchical information in STREAM. This is important because simulation can be used to infuse data into predictive analytics, thus providing guidance for the optimization and control of STREAM operations. The proposed framework is hierarchical in nature, where the single-stage, multi-stage, and distributed productions are modeled through the integration of different simulators. We demonstrate the proposed framework with simulation data from Freeze Nano Printing (FNP) AM for the fabrication of energy storage products.     

基于遗传算法的复杂型腔类零件刀具优化

针对复杂型腔数控加工的刀具组合优选问题,提出了一种基于遗传算法的复杂型腔类零件刀具优化方法.首先通过偏置环算法构建可行的型腔加工刀具集;其次为解决型腔类零件每个特征与整体最优刀具组合的差异性问题,建立以加工效率和刀具成本为优化目标的复杂型腔零件刀具组合优化模型,并通过有向图和改进的遗传算法对多刀具组合优化模型进行求解;最终,以某复杂型腔零件数控加工刀具组合优选为例进行实验,验证了方法的可行性.     

An assembly precision analysis method based on a general part digital twin model

Assembly precision analysis is one of the fundamental technologies used for controlling the assembly quality of products. Existing assembly precision analysis methods focus on identifying the assembly deviation caused by manufacturing errors of parts. They place less emphasis on the influencing factors in the assembly process, which significantly affect the reliability of the analytical results. Additionally, the lack of assembly knowledge for part models leads to a low efficiency of the assembly simulation. To address these problems, this paper presents an assembly precision analysis method based on a general part digital twin model (PDTM). The proposed PDTM integrates multi-source heterogeneous geometric models and maps assembly information from assembly semantics to geometry elements, allowing automatic assembly positioning of parts and improving the efficiency of assembly simulation. In addition to the manufacturing errors, the assembly-positioning error and mating-surface deformation are considered to quantify the impact on the key characteristics of the assembled product. Based on the real mating status simulation for the mating surfaces, the uncertainty of assembly positioning in an actual assembly is simulated by combining the small displacement torsor (SDT) theory and the Monte Carlo method. Furthermore, the mating-surface deformation can be superposed to the result of the assembly-gap calculation, improving the reliability of the analytical results. Finally, a prototype system and a case study involving a load-sensitive multi-way valve assembly process are introduced to demonstrate the applicability of the proposed method.     

A graph model for mutual information based clustering

We propose a graph model for mutual information based clustering problem. This problem was originally formulated as a constrained optimization problem with respect to the conditional probability distribution of clusters. Based on the stationary distribution induced from the problem setting, we propose a function which measures the relevance among data objects under the problem setting. This function is utilized to capture the relation among data objects, and the entire objects are represented as an edge-weighted graph where pairs of objects are connected with edges with their relevance. We show that, in hard assignment, the clustering problem can be approximated as a combinatorial problem over the proposed graph model when data is uniformly distributed. By representing the data objects as a graph based on our graph model, various graph based algorithms can be utilized to solve the clustering problem over the graph. The proposed approach is evaluated on the text clustering problem over 20 Newsgroup and TREC datasets. The results are encouraging and indicate the effectiveness of our approach.