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Digital twin is revolutionizing industry. Fired by sensor updates and history data, the sophisticated models can mirror almost every facet of a product, process or service. In the future, everything in the physical world would be replicated in the digital space through digital twin technology. As a cutting-edge technology, digital twin has received a lot of attention. However, digital twin is far from realizing their potential, which is a complex system and long-drawn process. Researchers must model all the different parts of the objects or systems. Varied types of data needed to be collected and merged. Many researchers and participators in engineering are not clear which technologies and tools should be used. 5-dimension digital twin model provides reference guidance for understanding and implementing digital twin. From the perspective of 5-dimension digital twin model, this paper tries to investigate and summarize the frequently-used enabling technologies and tools for digital twin to provide technologies and tools references for the applications of digital twin in the future.

Reverse engineering can be considered as the methodological process of analyzing and reconstructing a digital model of a physical asset. It has gained considerable interest with the advent of sophisticated sensors and data processing technologies hence becoming an important enabler for the product digital twin. However, while existing approaches to reverse engineering focus on the mere geometric reconstruction, this paper presents a novel paradigm called function-oriented surface reconstruction, that is capable of reconstructing the underlying part and surface function and thus outperforms existing methods. The applicability of the presented method is demonstrated through a case study of a gearbox.

An increasing interest in lattice structures is developing in various industrial sectors. These structures, composed of several cavities or voids, are used in a wide range of applications. They allow to lighten workpieces while maintaining good mechanical characteristics. Although these structures offer new design opportunities, they are difficult to manufacture because of their complex geometry and internal surfaces that are difficult to access. The special characteristics of lattice structures almost always require the use of additive manufacturing processes. By using lattice structures, new problems arise in additive manufacturing, such as lattice defects characterization and measurement issues. The objective of this study is to characterize the geometric defects of lattice structures obtained using L-PBF additive manufacturing process. Lattice structures exhibit not only common defects inherent to L-PBF process, but other geometric defects specific to their unit cell that require appropriate measurement systems and new processing techniques. This paper deals with a systematic approach to evaluate geometric defects obtained from Additive Manufacturing simulation software. First, lattice structures are generated using Rhino-Grasshoper visual programming capabilities. Then, virtual parts are built using Ansys Additive software. Registration methods are used thereafter to compare the simulated and the nominal models. The advantage of non-rigid methods such as CPD and BCPD for registration is highlighted. Finally, the obtained data can be used in the future to enhance the predictive modeling of geometric defects in L-PBF.

Over the past 70 years, product design has undergone many important changes due to the impact of contemporary digital technologies (i.e. digital design). To support digital design and information flow throughout the product lifecycle, the digital-driven technologies currently in use rely on the evolution of CAD and PLM systems to address new design and manufacturing challenges generated by the new era of 4.0 digital transformation. This paper will discuss the past and present coevolution and shortcomings within industrial organisations, the digital technologies employed in the product development cycle and will illustrate the current challenges and future prospects of the digital thread for design.

International audience; The geometrical quality of AM products is an indispensable concern when conducting Design for Additive Manufacturing (DfAM), since it dominates the geometrical consistency between the manufactured samples and the design intent and has an impact on the functionality of the products. Therefore, effective prediction of the geometric deviations prior to the mass production will provide useful information for designers in order for design optimization. Data-driven methods open up new possibilities to gain high-fidelity prediction based on existing observable data. In this paper, a Convolutional Neural Network based deep learning method is proposed which enables the prediction of deviations for different shapes and process settings. A data augmentation technique is also introduced to generate samples for network training based on a small number of available data. Through a case study, it's demonstrated that the trained network manages to accurately predict the geometric deviations of shapes manufactured with varied size and process parameter settings. The predicted deviations could substantially benefit DfAM in evaluation of geometrical consistency. Moreover, reverse compensation can be accordingly applied to the CAD model prior to the manufacturing process, thus increasing the geometrical accuracy of the manufactured parts. c 2020 The Authors. Published by Elsevier B.V. Peer-review under responsibility of the scientific committee of the CIRP Design Conference 2020.

Polytope-based tolerance analysis has shown a great potential in the Computer Aided Tolerancing (CAT) field. Previous works have considered form defects into this model, which overcomes the drawback of considering toleranced features as surfaces of perfect form. To enrich the capability and applicability of conventional polytope model, this paper further integrates the combined effects of form defects and surface deformations into polytope-based tolerance analysis. Specifically, form defects are considered based on the concept of Skin Model Shapes and surface deformations are calculated based on a Conjugate Gradient Fast Fourier Transform (CG-FFT) method. Thereafter, the polytope-based tolerance analysis is applied to an over-constrained mechanism. With the consideration of non-ideal surface contact status, this new polytope model provides more realistic and accurate analysis results.

Geometrical features are fundamental elements for the fulfilment of the functional product requirements. Therefore, their inevitable deviations caused by manufacturing inaccuracies are the main issues in the field of tolerance management. To close the gap between physical part description and nominal model representation, the framework of discrete geometrical representatives via Skin Model Shapes has been developed and continuously enhanced. As the underlying concepts are based on final virtual parts, consistent inferences to every single manufacturing step are not fully investigated in literature. This paper presents the development of Skin Model Shapes based on the virtual representation of intermediate part outcome (in-process geometry) and the integration of machining processes. Thus, integration and simulation of different kinds of deviations are enabled at each manufacturing stage considering error accumulation due to machine specific inaccuracies, such as tool movement and fixturing. Furthermore, the effect of inaccuracies on the final conformance result can be quantified at each manufacturing stage. A case study is presented to illustrate the entire approach and to demonstrate the validity of the proposed methods.

Geometric tolerance characteristics of metal additive manufactured (AM) parts play a significant role in ensuring the part functionality. In such cases, prior estimation of geometric tolerances, i.e. geometric dimensioning and tolerancing (GD&T) characteristics, can prove vital to reduce part rejection and to minimize material wastage and cost. This article presents a framework to estimate geometric tolerances in laser powder bed fusion (LPBF) processes. For a given geometry, skin model shapes are generated based on material shrinkage and thermo-mechanical simulation. Samples from skin model shapes are utilized for geometric tolerance estimation. A case study is presented to validate the developed framework and demonstrate its applicability in metal AM.

Highly qualified labour force is a key resource for growth. In modern manufacturing, the competent use of advanced measuring equipment for inspection and digitization of parts is an essential competence that is needed for both advanced product/process engineering and quality control. Coordinate Metrology (including 3D digital measuring technologies) is by far the most important tool for these specialized activities. As widely reported, the individuals operating the measuring systems - with their decisions - are frequently one of the most relevant error sources in Coordinate Metrology operations, especially when dealing with new measuring technologies supporting Digital Manufacturing (e.g. Computed Tomography, Fringe-projection, Reverse Engineering). The paper reports the intermediate results of an initiative aiming at innovating training in Coordinate Metrology, focused on supporting the needs of SMEs in the supply chain of the automotive industry. The main target group are industry employees operating in SMEs that are newcomers on 3D measuring technologies. An integrated concept for training in Coordinate Metrology has been developed using a blended learning approach, based on a 10-steps structure and incorporating the learning outcomes required to operate different measuring systems in a consistent way.

Effective modeling of geometric deviations is an important issue in Design for Additive Manufacturing (DfAM), since it enables the evaluation of geometric consistency and the optimization of geometric design. Motivated by the awareness that process-related factors have non-trivial effects on geometric deviations, a new method is proposed in this paper which combines Statistical Shape Analysis with Gaussian Process to enable the modeling of deviations with consideration of process parameters. By learning from a number of simulated samples, the method could achieve effective prediction of deviations for new parts. Its applications in surface deformation evaluation and geometric compensation are also discussed, which will bring substantial benefits to DfAM.

International audience; In today's business environment, the trend towards more product variety and customization is unbroken. Due to this development, the need of agile and reconfigurable production systems emerged to cope with various products and product families. To design and optimize production systems as well as to choose the optimal product matches, product analysis methods are needed. Indeed, most of the known methods aim to analyze a product or one product family on the physical level. Different product families, however, may differ largely in terms of the number and nature of components. This fact impedes an efficient comparison and choice of appropriate product family combinations for the production system. A new methodology is proposed to analyze existing products in view of their functional and physical architecture. The aim is to cluster these products in new assembly oriented product families for the optimization of existing assembly lines and the creation of future reconfigurable assembly systems. Based on Datum Flow Chain, the physical structure of the products is analyzed. Functional subassemblies are identified, and a functional analysis is performed. Moreover, a hybrid functional and physical architecture graph (HyFPAG) is the output which depicts the similarity between product families by providing design support to both, production system planners and product designers. An illustrative example of a nail-clipper is used to explain the proposed methodology. An industrial case study on two product families of steering columns of thyssenkrupp Presta France is then carried out to give a first industrial evaluation of the proposed approach.

In the field of dimensional metrology of optical components, fitting algorithms play an essential role. Performing data fitting according to L∞-norm indicates about form errors since it directly minimizes peak-to-valley (PV). In parallel, reference data (softgauges) are used to assess the performance of the developed algorithms and must incorporate multiple parameters related to points distribution, manufacturing process and measurement errors, to be as close as possible to the real measured data. In this paper, a developed algorithm for surface fitting based on L∞-norm is presented and tested on simulated reference data of aspheres.

International audience; Variability is unavoidable in the realization of products. While design must specify ideal geometry, it shall also describe limits of variability (tolerances) that must be met in order to maintain proper product function. Although tolerancing is a mature field, new manufacturing processes and design methodologies are creating new avenues of research, and modelling standards must also evolve to support these processes. In addition, the study of uncertainty has produced widely-accepted methods of quantifying variability, and modern tolerancing tools should support these methods. The challenges introduced by new processes and design methodologies continue to make tolerancing research a fertile and productive area.

International audience; During the last decades, many researches and applications have been conducted on machining features modeling and classification. Althoughseveral approaches have been used in the domain of Computer-Aided Process Planning (CAPP), the consideration of freeform shapes has notbeen well explored at present. This paper aims to provide an up-to-date review of machining features and its conformance with STEP-NC (STEPcompliant Numerical Control). A new method based on discrete shape representation, differential geometry and graph theory is developed forfreeform machining feature characterization and classification. The effectiveness of the proposed method is demonstrated through an example ofa blade part.

Mechanical deviations induced by working conditions are always ignored in assembly analysis due to the rigid body hypothesis of parts. This paper presents a study of the deviations resulting from the actual working conditions. Finite element analysis (FEA) is conducted to simulate the deviations occurring in assemblies, in which the point-cloud geometry representation of deformed parts is adopted to enable the calculation of small displacements by means of relative positioning. The simulated deviations are incorporated in manufacturing deviations to determine the functional requirements based on Jacobian-torsor model. The approach is finally illustrated by a practical example.

ISO 2692 core concepts: Maximum Material Requirement (MMR) and Least Material Requirement (LMR) are well established for functional tolerancing and are used to guarantee assemblability, extremal values of clearance and minimum wall thickness. The LMR has not been extensively investigated in research and still not enough considered in industrial practices. In this paper, LMR’s capability to devise requirements on assemblies with clearance is shown, and the conformity assessment, in dimensional metrology, based on Least Material Virtual Size is presented. A case study demonstrates the effectiveness of this work. New insights for virtual size representation within tolerance zone are also discussed.

Geometric deviations arising from multiple variation sources in Additive Manufacturing (AM) process chain have been a major issue regarding product quality. Therefore, effective modeling of the geometric deviations is becoming critical for AM. In this paper, in-plane geometric deviations are investigated based on the assumption that typical deviation sources in AM can be mapped to different transformations (scaling, rotation, translation) on the nominal shape. By defining transformation parameters, analytical deviation models of regular shapes can be derived in the deviation space, and calibrated based on measurement data. A case study is presented to show the effectiveness of the proposed approach.

Partitioning is one of the four major operations used in the international standards issued by ISO/TC 213 on geometrical product specifications and verifications (GPS, for short). The ISO GPS standards define both tolerancing (for specification) and metrology (for verification) practices that are of critical importance to manufacturing industry. Collectively, these emerging ISO GPS standards have the potential to be some of the key enablers of digitization of manufacturing (also known as smart manufacturing, cyber-physical manufacturing, cyber-physical production systems, and Industrie 4.0). In simple terms, partitioning operations divide a surface (ideal or measured) into subsets that are then subjected to further GPS operations. This paper addresses the classification problem of partitioning operations to provide a science-based solution for the development of ISO GPS partitioning standards. Such a classification is considered to be one of the first tasks that must be completed before detailed standards on partitioning can be developed.

The assemblability, which is the feasibility and difficulty of assembly, will greatly affect the efficiency and quality of assembly process, so assemblability analysis plays an important role during the design stage of products. Cylindrical parts assembly is a common structure with wide applications, whose assemblability can be affected by surface deviations, especially in some precision assemblies. In this paper, a novel method is proposed for the assemblability evaluation of cylindrical parts assembly considering surface deviations. Firstly, based on the surface deviations modeling of cylindrical parts and assembly clearance calculation, the assembly posture adjustment process is defined and simulated to obtain the assemblable postures without interference between parts. Then, in the proposed six dimensional posture space, a search algorithm is established to obtain the boundary points of assemblable posture space. Afterwards, in order to represent this irregular hyperspace, the largest inscribed generalized cone of which is put forward. Finally, based on the characteristics of generalized cone, two indicators are defined to reflect the adjustable range of assembly posture, which can realize the quantitative evaluation of assemblability. The validity of the proposed method is verified by case studies of cylindrical parts assemblies with different surface deviations.

International audience; The digitalization of manufacturing fuels the application of sophisticated virtual product models, which are referred to as digital twins, throughout all stages of product realization. Particularly, more realistic virtual models of manufactured products are essential to bridge the gap between design and manufacturing and to mirror the real and virtual worlds. In this paper, we propose a comprehensive reference model based on the concept of Skin Model Shapes, which serves as a digital twin of the physical product in design and manufacturing. In this regard, model conceptualization, representation, and implementation as well as applications along the product life-cycle are addressed. In today's highly competitive markets, the ambitions for shortening the time to market and for increasing the product development performance fuel the application of sophisticated virtual product models, which are frequently referred to as digital twins. Enabled by the digitalization of manufacturing, cyber-physical production systems, model-based system engineering, and a growing endeavour for data gathering and processing, these models are increasingly enriched with production and operation data. Moreover, they allow the efficient prediction of the effects of product and process development as well as operating and servicing decisions on the product behaviour without the need for costly and time-expensive physical mock-ups [1-3]. Particularly in design, such realistic product models are essential to allow the early and efficient assessment of the consequences of design decisions on the quality and function of mechanical products. However, current approaches to the implementation of digital twins lack of a conceptual basis, which hinders the applicability of the digital twin vision to various activities in design and production engineering. Motivated by this need, this paper proposes a comprehensive reference model, which serves as a digital twin of the physical product in design and production engineering. In this regard, important model properties, such as scalability, interoperability, expansibility, and fidelity, as well as different operations on this reference model along the product life-cycle, such as composition, decomposition, conversion, and evaluation are addressed. Moreover, the application of this reference model to geometrical variations management is highlighted. The paper is structured as follows. In the next section, the vision of the digital twin and its evolution is reviewed. After that, a comprehensive reference model for the digital twin is introduced, which is then applied to geometrical variations management. Finally, a conclusion and an outlook are given.

Mesh models have been widely employed in current CAD/CAM systems, where the workpiece is considered as made up of a number of features limited by natural boundaries. The natural boundaries among features are in most cases edges where an abrupt change of point differential geometry properties occurs. However, such features and underlying surface portions could also be connected smoothly without an abrupt change at the natural boundaries. In the context of ISO GPS (Geometrical Product Specifications and Verification) Standards, partitioning is a fundamental operation that decomposes a mechanical part into independent surface portions for a functional specification purpose. In this paper, an enhanced mesh partitioning method is proposed to enable a feature-based decomposition considering kinematic invariance classes. The proposed two-step method includes an initial partitioning based on sharp edge detection and an enhanced partitioning process based on non-sharp edge rectification. The partitioning criteria rely on two surface descriptors derived from shapes’ principal curvatures: Curvedness and Shape Index. To refine the boundary points on non-sharp edges, conformal geometry is exploited during the enhanced partitioning process by mapping the 3D surface onto a 2D unit disk. Connecting regions without abrupt changes at their natural boundaries are well partitioned after the boundary rectification process. A statistical evaluation process is used to address invariance class identification for each partitioned surface portion on the part. Experiments and results on different mesh models are presented to demonstrate the effectiveness of invariance class partitioning for ISO GPS.

International audience; Multiple measurements using various data acquisition systems are generally required to substancially enhance measurement accuracy , reliability and holisticity of freeform shapes. The obtained multiple measurement data of the shape are transformed and fused into a common coordinate system within a registration technique involving coarse and fine alignments. Standardized methods have been established for fine registration such as Iterative Closest Points (ICP) and its variants. For coarse registration , no conventional method has been adopted yet despite a significant number of techniques which have been developed in the literature to supply an automatic rough matching between data sets. The work presented in this paper proposes an improvement of registration techniques by the consideration of new discrete curvature parameters. Two main issues are addressed in this paper : the coarse registration and the fine registration. For coarse registration , two novel automated methods based on the exploitation of discrete curvatures are presented : an enhanced Hough Transformation (HT) and an improved Ransac Transformation. The use of curvature features in both methods aims to reduce computational cost. For fine registration , a new variant of ICP method is proposed in order to reduce registration error using curvature parameters. A specific distance considering the curvature similarity is combined with Euclidean distance to define the distance criterion used for correspondences searching. Additionally , the objective function is improved by combining the point-to-point (P-P) minimization and the point-to-plane (P-Pl) minimization with automatic weights. The algorithms are applied on simulated and real data performed by a computed tomography (CT) system. The obtained results reveal the benefit of the proposed improved curvature-based registration methods .-2

Geometric deviations have huge influences on the functional behavior of product, which should be analyzed and properly controlled. Tolerance analysis, as a way to evaluate geometric deviations, is an essential part of product development. Current Computer Aided Tolerancing systems provide solutions for tolerance analysis but have limitations in the consideration of form deviations. The Skin Model theory, as a new research topic, represents part with non-ideal model that comprises geometric deviations, thus developing into a new computer aided tolerancing approach. In this paper, the related work with respect to the generation of Skin Model Shapes and its application in assembly simulation and tolerance analysis is briefly introduced. In order to overcome its shortcomings in the tolerance analysis employing SMSs, this paper proposes an improved method by taking advantage of the method adopted in a CAT system. The proposed method supports the analysis of position and orientation tolerances and has been proved to be valid through a case study.

International audience; Reverse engineering and shape reconstruction play an important role in design and manufacturing through the increased use of shape acquisition and processing technologies in the product development process. The application of shape theories to geometric modelling and variability characterization are paving the way to shape engineering and more generic methods for reverse engineering.This paper investigates the fundamentals of shape representation, shape processing and mining at a conceptual, geometric and computational level to address geometric reverse engineering issues in mechanical design. New developed concepts based on discrete curvatures and their applications are presented. Challenges and future researches are also highlighted.

International audience; The integration of more physical properties into the Skin model is fundamental for extending the tolerancing process to the different phases of the product lifecycle. This paper presents a study of the deformation effects on the Skin model provoked by the thermal and working environment of the workpiece. The proposed methodology departs from the Skin model at room temperature, and generates Skin Model Shapes by performing a Finite Element Analysis (FEA). The simulation tool has been successfully tested in the study of a practical industrial application, a gas turbine blade, which combines many of the nowadays challenges of CAD, FEA and CAT.

Assembly modeling is a very important activity for computerized analysis of product assembly, and it has been a subject of intense research over the past years. However, most of the assembly models focus on the assembly connective relationships between parts rather than assembly spatial relationships. Variation analysis is becoming a critical issue when specifying and verifying the geometric and dimensional requirements of assembly. In order to support dimensional variation analysis, a mathematical assembly model representation considering assembly spatial relationships is proposed in this paper. Four types of coordinate systems are defined. The nominal and variational spatial relationships among assembly elements are represented as homogeneous transformation matrix among different coordinate systems. The variations of assembly elements, mating interface and locating interface are represented as variation transformation matrix among corresponding coordinate systems. The variation of the final assembly is modeled as stack up of the differential motion vectors of coordinate systems of all the parts with respect to global coordinate system, and it can be calculated by homogeneous transformation and variation transformation. The different situations for two types of assemblies are distinguished in this novel assembly model representation.

Geometric deviations are inevitably observable on manufactured workpieces and have huge influences on the quality and function of mechanical products. Therefore, many activities in geometric variations management have to be performed to ensure the product function despite the presence of these deviations. Dimensional and Geometrical Product Specification and Verification (GPS) are standards for the description of workpieces. Their lately revision grounds on GeoSpelling, which is a univocal language for geometric product specification and verification and aims at providing a common understanding of geometric specifications in design, manufacturing, and inspection. The Skin Model concept is a basic concept within GeoSpelling and is an abstract model of the physical interface between a workpiece and its environment. In contrast to this understanding, established models for computer-aided modelling and engineering simulations make severe assumptions about the workpiece surface. Therefore, this paper deals with operationalizing the Skin Model concept in discrete geometry for the use in geometric variations management. For this purpose, Skin Model Shapes, which are particular Skin Model representatives from a simulation perspective, are generated. In this regard, a Skin Model Shape is a specific outcome of the conceptual Skin Model and comprises deviations from manufacturing and assembly. The process for generating Skin Model Shapes is split into a prediction and an observation stage with respect to the available information and knowledge about expected geometric deviations. Moreover, applications for these Skin Model Shapes in the context of mechanical engineering are given.

Product design requires the consideration of geometric models and representations that reflect shape deviations and support tolerance management issues. Computer-Aided Tolerancing (CAT) systems have been developed as simulation tools for modelling the effects of tolerances on digital product simulation. However, geometric variations cannot be addressed efficiently with regard to form deviations. This paper investigates the concepts of Skin Model Shapes, which provide a finite describability and the digital representation of the Skin Model concept, and their unified discrete geometry representation. New contributions to tolerance representation and analysis are presented. Applications and perspectives for CAT systems are highlighted as well.

The modelling of product shapes and dimensions is now largely supported by geometric modelling tools. However, the underlying geometrical variations cannot be addressed efficiently when covering the overall product life cycle. The fundamental concept of skin model has been developed as an alternative to the nominal model and covers geometric deviations that are expected, predicted or already observed in real manufacturing processes. This paper investigates the fundamentals of the skin model at a conceptual, geometric and computational level. Representation and simulation issues for product design are presented. Finally, applications and perspectives are highlighted.

International audience; This paper depicts a new CAT (Computer Aided Tolerancing) system called Quick GPS (Geometrical Product Specification), for assisting the designer when specifying the functional tolerances of a single part included in a mechanism, without any required complex function analysis. The mechanism assembly is first described through positioning table formalism. In order to create datum reference frames and to respect assembly requirements, an ISO-based 3D tolerancing scheme is then proposed thanks to a set of rules based on geometric patterns and TTRS (Technologically and Topologically Related Surfaces). Since it remains impossible to determine tolerance chains automatically, the designer must impose links between the frames. The CAT system we developed here proposes ISO based tolerance specifications to help ensure compliance with the designer's intentions saving on time and eliminating errors. This paper will detail both the set of tolerancing rules and the designer's approach. The Quick GPS system has been developed in a CATIA V5 environment using CATIA VBA and CATIA CAA procedures.

For car and aircraft industries, the management of geometrical variations has become an important issue in product design process and concurrent engineering. Indeed, designers need to manage dimensional and geometrical tolerances and to know information that contributed to their determination. The goal here is to put tolerancing in a concurrent engineering context. There are important questions that would need to be looked upon: How to integrate the tolerance synthesis in the design process? How to ensure the transition from function to geometrical specifications on parts? How to keep traceability of tolerances during the design process? Few answers exist today in academic works and there are few supports in CAD systems. Therefore, to build a coherent data model taken into account tolerances, we describe in this paper a multi-level approach that enables a tolerancing process integrated with conceptual design. The first level integrates information relating to functional aspects of an assembly. The second describes the structure of the assembly, and concerns the integration of functional needs and technological solutions. The last level translates functional requirements into geometrical requirements between/or on parts of the products, and provides the geometrical specifications on each part satisfying the geometrical requirements. This multi-level architecture is represented as an object oriented data model based on UML (Unified Modelling Language) that enable data management for functional tolerancing in design and keeping traces when querying about data.