Computers in Industry 65 (2014) 136–147
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Computers in Industry journal homepage: www.elsevier.com/locate/compind
Explicit reference modeling methodology in parametric CAD system Yannick Bodein a, Bertrand Rose b,*, Emmanuel Caillaud b a b
Tata Technologies Europe, France ICUBE, University of Strasbourg, UMR CNRS 7357, France
A R T I C L E I N F O
A B S T R A C T
Article history: Received 27 April 2012 Received in revised form 3 June 2013 Accepted 21 August 2013 Available online 17 September 2013
Today parametric associative CAD systems must help companies to create more efficient virtual development processes. While dealing with complex parts (e.g. the number of surfaces of the solid) no CAD modeling methodology is existing. Based on the analysis of industrial designers’ practices as well as student practices on CAD, we identified key factors that lead to better performance. Our objective in this article is to propose a practical method for complex parts modeling in parametric CAD system. An illustration of the performances and the results obtained by this method are presented comparing the traditional method with the proposed one while using an academic case and then an industrial case. ß 2013 Elsevier B.V. All rights reserved.
Keywords: Parametric design Modeling methodology Functional modeling Complex parts CAD Education
1. Introduction The strong competition in today’s market increases the level of requirement in terms of functionality and quality of products. At the same time, the complexity of the design process is increasing, whereas product development time is decreasing. Such constraints on design activities require efficient CAD systems and adapted CAD methodologies. Contemporary CAD systems, based on parametric associative technology, facilitate the creation of fully parameterized and adaptive products. By parametric associative CAD models we mean geometrical representation of products where certain characteristics are controlled by non-geometric features called parameters. The term parametric design associated with parametric systems is defined by Shah [1] as "a process of designing with parametric models in a virtual surrounding where geometrical and parameter variation are natural". Parametric CAD models, if well parameterized, allow for the definition of new configurations of products just by changing the values of some parameters. The high potential of this technology for routine design has been fully recognized and highlighted in the research community for more than a decade by Anderl and Mendgen [2] and Hoffman [3,4].
* Corresponding author at: University of Strasbourg-UFR Physique et Ing,nierieP"le Meinau, 15-17, rue du Mar,chal LefSbvre, 67100 Strasbourg, France. Tel.: +33 3 68 85 49 50; fax: +33 3 68 85 49 72. E-mail address:
[email protected] (B. Rose). 0166-3615/$ – see front matter ß 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.compind.2013.08.004
However, to obtain a valid and functional CAD model, the preliminary task is to identify the right functional parameters from the requirements and to build the appropriate parametric structure. Several research studies propose practical or integrated methods for the identification of functional parameters [5] and for structuring information and entities in parametric CAD models [6,7]. Existing methods for parametric CAD systems embedded in CAD methodologies and guidelines mainly work at assembly level to ensure a product-centric development (top-down design approach) rather than a part-centric development (bottom-up approach). A review of general 3D product modeling techniques and methods is given in VDI2209 [8] and an example of a top-down approach is proposed by Aleixos [9]. Nevertheless, efficient modeling practices imply adequate modeling strategies for parts as well. The aim of the product development process is to ensure that the product and its components meet the specifications. Optimized design methods should facilitate the adaptation of a part to new requirements on the product. The main problem is that there are so many possibilities for designing with CAD systems that not all CAD models today make it possible to obtain the benefits promised by initial research results on the use of constraint-based and parametric CAD systems. The modeling method and procedure is very important for parametric design because it is possible to design adaptive products only with good modeling methods. Therefore, many companies propose uniform modeling methods and describe these methods in CAD guidelines. The most efficient guidelines also integrate knowledge of the product development
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process; in this case they are oriented not exclusively to CAD aspects but also to the design process. Such guidelines specific to the design of a product are frequently called "product-oriented methodology" in the industry. Product methodology is mainly focused on the whole design process, providing an efficient transition of information and knowledge between design phases. Aleixos et al. [9] pinpoint the need of "a generic modeling guide [which] is necessary to fix good practices and reinforce cooperation". Salehi and McMahon [7] complete the previous study and present the results of the descriptive study accomplished to identify the challenges, problems and weaknesses involved in the use of CAD systems in the automotive design process. Several methods and methodologies are available for CAD modeling and solid modeling in today’s CAD systems. However, according to our observations no company designers or students in academic projects creates CAD models in the same way and with the same modeling strategy. This situation decreases the efficiency of CAD systems during product development because it decreases the reusability of models as well as the ability of designers to work on the same models (collaboration). Besides, not all methods provide the same flexibility and robustness on CAD models. For the design of complex parts, the previous circumstances are accentuated and make the choice of modeling methods critical. Hence, the complexity of the design task in CAD is related to the complexity not so much of the product but rather of the design or product development process itself. The change of requirements during the development phase (conceptual, preliminary or detailed design phase) is one the most difficult problems that subsidiary companies in the automotive industry have to deal with. It requires them to define a precise associative CAD modeling strategy and methodologies. Research from Aleixos et al. [9] gives some insights in this respect. According to Rynne and Gaughran [10], "It is generally agreed that despite today’s computers and CAD software having become extremely powerful, they are of limited use to engineers and technologists who do not fully understand fundamental graphics principles and 3D modeling strategies". In fact, the set of modeling methodologies and guidelines does not reach the goal of uniform modeling strategy that would allow for additional CAD efficiency. Indeed, for the case of complex parts, guidelines and modeling methodologies are insufficient. This situation can easily be observed: the same part design by two separate designers will have a different construction history (sequence of features) and ability to adapt to design changes. The purpose of our study is to provide an effective modeling methodology and procedure for part modeling in constraint-based CAD tools. The main metrics that is used in order to show the effectiveness of the study is the modeling time. This study is part of a global research framework that aims to improve the efficiency of practices and methods on CAD systems related to product development processes. The first results of this research work argue for a strategy validated in the automotive industry and based on 4 levels which are: standardization (detailed in [11]), advanced methodologies, knowledge-based design, and expert rule checks. We focus here on Part methodology from the second level dedicated to advanced methodology. This practical strategy for parametric design CAD effectiveness is currently in application at Tata Technologies [12], a leading engineering consulting company. Amongst all methods and technical solutions, modeling a strategy for parts is one of the most complex tasks to achieve as it is strongly linked to human cognitive aspects during design. Hence, [10] indicates that more efficient use of Product Modeling systems are achieved if users have the capacity to generate cognitive models and the ability to decompose geometric elements, and cognitively assemble these in the context of achieving design intent. An advanced CAD methodology associated to the product
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development process, also named product-oriented methodology, is complex to define but often easy to formalize by rules and with a standard structure. The same does not apply to parts, where the formalization of modeling procedures is highly complex because the number of parameters that should be taken into account is very large and also includes the cognitive process of designers. Our research method is a field-based study that integrates data from firms in the automotive industry and from academic experiments with students. During the study we also used various data collection methods: CAD models analysis, interviews with designers, and a think-aloud protocol as a means to analyze the problem-solving process of designers in the creation of constraintbased CAD models. In a Think-aloud process, the subject undertakes a specific task (in the case of this research the production of a 3D-CAD model) while at the same time talking out aloud saying everything that they would otherwise say to themselves. Thinkaloud verbal protocols thus enable the researcher to gain information from the expert while they are in the process of performing the task [13]. The current problem can be discussed from several points of view: on training which emphases the fundamental role of the features sequence in a constraint-based increasing the level of guidance for designers on a specific design task with accurate modeling procedures, and/or focusing CAD (often called modeling strategy). In this article, we consider both of these approaches. Our proposition is a modeling method for an explicit management of relationships between features, using explicit references. We first present a review of the literature on existing procedures for complex parts modeling in constraint-based CAD tools. We also investigate the solid modeling procedures in CAD education and discover that no common methodology seems to be able to solve our problem. In Section 2 we go through the specific case of constraint-based CAD systems. In Section 3, our own methodology regarding the modeling of procedures for complex parts in constraint-based CAD tools is presented. The validation of the method in an academic context is then demonstrated in Section 4. A short description of the application of the modeling methodology within a general CAD methodology for the design of automotive radiators has been also proposed in Section 5. The Section 6 provides the limits of the proposed methodology. Conclusions and perspectives discussing CAD methodologies and CAD training are finally proposed. 2. Relevant literature To address the question of the definition of modeling procedures for complex parts in constraint-based CAD tools, a review was undertaken of the literature in which many facets of modeling, including feature-based, solid modeling, and CAD methods were examined. A synthesis of recent studies on constraint-based CAD in education was also carried out, focused on human cognitive aspects. 2.1. Constraint-based CAD in engineering: methods overview The foundations of modern parametric and constraint-based CAD tools, laid by Roller [14], Solano [15], and Shah [1] remain unchanged despite all the progress made on the technology by CAD editors for many years. Feature-based modeling techniques were initiated by Shah and promoted by Anderl and Mendgen [2] who demonstrate how constraint-based CAD tools allow for the addition of design semantics through the different phases of product design, in keeping with Pahl and Beitz’s point of view [16]. Hoffmann and Joan-Arinyo [3] explored, in particular, how semantics added to modeling operations can bring design closer to manufacturing. Our problem is however that all these studies
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consider only the technology and its benefits for the product development process, and not modeling procedures as a possible enabler of the effectiveness of these constraint-based CAD tools. Only Anderl [2] pinpoints the difficulty of the task while concluding with the necessity of design methodology: "the development of a design methodology for modeling with constraints, considering complexity as well as flexibility of constrained designs, is a strong requirement". On feature-based modeling, the research community has been quite prolific as this technology allows for the integration of information (semantic), which means that different types of features exist: functional features, assembly features, technological features, material features. A recent synthesis of feature-based modeling is proposed by Shahin [17]. Nevertheless, none of the studies on feature-based modeling consider modeling procedures for the design of parts. They focus only on the benefits of the feature’s technology for specific needs, such as semantic features proposed by Liu et al. [18]. Some other authors are more interested in the design and the parameterizing of a part from a product point of view and not from the modeling procedure in itself. Ma et al. [19] explore associative feature modeling for the integration that concerning the concurrent engineering. They demonstrate the benefits of associative (constraint-based) technology but do not consider the modeling strategy in terms of sequences of operations and choice of the references to be carried out by the designer during the design of a component. By considering the modeling methods, Aleixos et al. [9] propose an integrated modeling approach with a top-down that enables the addition of high semantic/pragmatic quality inside the CAD models (also known as design intent). They propose a generic methodology as a frame to help the subsidiary industries in the creation of modeling guide. However, they consider the information ways and their associative to be implemented inside the CAD model but do not take into account the modeling procedures that to build the components parts. McMahon and Salehi [6,7] highlight the difficulty of designers to structure the design information inside CAD models, and to find the relevant parameterization associated to the product development process. They did not consider the modeling strategy of designers during the design of parts, but their method is nevertheless ensure that can be more powerful to identify the references and the functional elements, which consider as an essential input in the modeling strategy definition. It is however obvious that research focused mainly on global product CAD methodology for the design of parts only is meaningless; the methodology needs to consider the product requirements. In other words, an efficient modeling strategy on a part must consider the sources of design changes and consequently the product. Regarding the specific case of solid modeling (small area of geometric modeling), Shapiro et al. [20] define a set of principles for mathematical and computer modeling of three-dimensional solids. They also propose main ideas and foundations of solid modeling in engineering and describe associated limitations of solid modeling such as "Persistent naming". Modern techniques, best practices and design methodologies associated to the product development process are detailed in the German norm VDI2209 [8]. According to this norm, during the design process with a parametric CAD system, a certain "thinking process" is necessary which includes a modeling approach for creating parametric models in a rigorous way. Keys and techniques for the definition of solid modeling procedures are however not described. To synthesize, the research community is mainly focused on how to integrate more information into the CAD system to help designers during the development of products, including all
research on functional-behavior systems [21], so that this information can be used by the designer for decision making. Consequently, most of the work is targeted at design tasks and problem resolution rather than at CAD modeling methods and procedures. This aspect of CAD modeling procedures is mainly discussed in the CAD educational field. We focus on this field in the next section. 2.2. Solid modeling procedures in CAD education Concerning training on CAD systems, very limited attentions have been paid to students and how to create good models which can be easily understood, altered, and reused by others [22-24]. Lang et al. [25] make the distinction between knowledge of the CAD system functions (called procedural knowledge) and the knowledge needed to adapt a modeling strategy, called declarative knowledge, on which no training is given. The same concept is developed by Chester [26], who identifies two types of knowledge: command knowledge and strategic knowledge. The first type considers only the commands and functions of the CAD system, whereas the second type concerns how a specific task may be achieved and the process by which a choice may be made. According to the study, the strategic knowledge improvement is associated with the development of a meta-cognitive approach and with improvement in mental imagery processes. As an example of actions that enhance the meta-cognitive approach, he proposed that an expert teacher provide an explicit description of all the cognitive processes under way during a particular problem-solving episode. Rynne and Gaughran [10] complete these studies with a generic cognitive process model for developing 3D CAD expertise in part modeling. The common point of all these studies is to identify sketching ability, spatial visualization ability and model deconstruction ability as key features in the modeling strategy development process. Nevertheless, none of these studies consider and describe modeling procedures to be taught for efficient part modeling. But they do provide indications on how the possible limits of a designer’s capability should be taken into account by a modeling method and guidelines such as the decomposition of the model. Johnson et al. [23] discuss the complexity of features and the importance of feature selection and organization in CAD during teaching on Pro/Engineer and Solidworks CAD solutions. They highlight the benefits of using simple features in terms of CAD model comprehension and modification due to design changes, but still no indication on modeling procedures to be used during part modeling is given. Ma et al. [27], while taking into account the whole lifecycle of the product, revealed that checking product information validity is difficult for the current computer-aided systems, for example while using features, because engineering intent is at best partially represented in product models. Hamade et al. [28] in their study evaluate the learning process of a CAD student using Pro/Engineer solutions and correlate the level of the student’s performance with the time taken (by measuring time on exercise resolution) and with the counting of features (number of features-of-size used to build the test parts). Their study does not consider any modeling of best practices such as the complexity of features that have a direct relationship with the flexibility and adaptability of a CAD model. By the way, while some CAD modeling prescriptions to improve the communication of design intent have been proposed by [28], they are not backed up by empirical evidence [24]. To the best of the author’s knowledge, it seems that the only research that considers modeling strategies and procedures for the design of parts is by Hartman [29]. His research is based on an exploratory study examining the practices of 5 professionals
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Table 1 Common modeling procedure by Hartman. Common modeling methods 1. 2. 3. 4. 5. 6. 7. 8.
Determine sketching plane Sketch profile Add constraints/relations Add dimensions Apply feature form Repeat steps 1-5 to add major features Add material-removal features (holes, cuts, etc.) Add finishing features (round, fillets, etc.)
(experienced designers working on two different CAD systems). The procedures used for his study, like the "think-aloud modeling task", enabled him to examine the problem-solving process in the creation of constraint-based CAD models. In doing so, he attempted to find out the relationship between the expert’s mental model and the actions performed when modeling an object. The results of the think-aloud modeling task yielded five specific modeling procedures which were distilled into one common modeling procedure for the given object. This study highlights the main questions of designers during modeling tasks, as well as questions inherent to each step of the common modeling procedure (Table 1). The selection of the feature is critical to the modeling process as it influences the parent/child references established within the model. The definition of modeling procedures for complex parts has to integrate information that will impact the choice of the based features and the selection of references/supports during a common modeling procedure (shown in Table 1). The selection of the feature is considered as a critical element during the modeling process as it influences the parent/child references that have been established within the model. The definition of modeling procedures for complex parts has to integrate the information that will impact the choice of the based features and the selection step of references/supports while a common modeling procedure (shown in Table 1). Other questions raised by the previous studies concern the level of guidance which should be proposed to designers to ensure his ability to apply a modeling strategy and procedure for the design of a component. Most of the studies highlight the need of common methods to increase the CAD models reusability between designers without providing any solution to answer this problematic. The last common investigation is to evaluate the impact of modeling procedures on the design activity. Notice that the designer could have difficulties to handle the design task in sequentially while concentrating-over the modeling methods, notwithstanding the higher complexity to deal with. The purpose of our study is to propose an effective and practical modeling procedure for complex part modeling in constraintbased CAD tools. The next section presents the features relationships that can be found inside CAD systems.Features relationships inside CAD systems The main drawback of constraint-based CAD tools is that the sequence of modeling operations (history tree) displayed to the designer by the parametric CAD system is not necessarily representative of the relationship between the features, and that even if the update process is linear (standard behavior of parametric CAD system). This situation is illustrated in the next example with a solid created with separate bodies and then assembled with explicit Boolean operations. In the specification phase of the project modeling methods definition for a radiator tank (industrial application - see Section 5), we analyzed over 20 existing CAD models. In (Fig. 1) we present a structure of a radiator tank CAD model considered by the designer who created it to be well-parameterized. The distinction between the sequence of features (history tree) and the
Fig. 1. Structure of a part in CATIA V5 CAD system and relationships between features.
relationships between the features (displayed with arrows in Fig. 1) is immediate. With the CAD model proposed in this example, another designer will have much more difficulty to analyze the model before being able to modify it. And even with a representation of the parent/children link, the designer has to find which parameters (constraint, limit or dimension) are the sources of the link. As a result, a model in constraint-based CAD systems could be considered as a complex program without any comment inside the code, and that every programmer of the company should be able to debug. This is not an acceptable situation for a company and a CAD manager who wants to reduce design time. Consequently, we find that without restrictive CAD modeling procedures, both sequences between modeling operations and features’ relationships are by default not homogenous. The design intent of a designer can be determined from constraints and also from both the history tree and features’ relationships. The above distinction enables a quick understanding of CAD model designs by a foreign designer. A dramatic situation is quantified by Salehi and McMahon [6,7] in their studies within the context of the automotive industry: "Only 9% of the designers are able to identify and determine the relevant information and they agreed that it is quite difficult to change CAD parts and assemblies created by other designers". Parametric CAD publishers have to make considerable progress in this respect to show exactly the graph of constraints in relation to its history; even if constraints are oriented from the top down in the history tree (otherwise, explicit representation cannot be generated). Currently, it is possible to see constraints between features but not with a single process (Fig. 2). That means that designers have to select the feature in which they want to see the relationships, but there is no means to display all relationships between features in a single operation. The previous situation demonstrated that the reusability principle in constraint and feature-based parametric CAD systems is far from what is expected by manufacturers and presented by CAD software publishers. The power of a feature-based parametric system is not automatic, and is strongly dependant on the modeling method rather than on the technology itself. As seen in the literature review, no CAD educational program or research integrates this problem. We were then interested in developing our own modeling procedure for complex parts in
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Fig. 2. Example of the display of features’ relationships in CATIA V5.
constraint-based CAD tools. These issues are presented in the next section. 3. Our proposal regarding a modeling procedure for complex parts in constraint-based CAD tools In this section we propose our modeling procedure for complex parts in constraint-based CAD tools. We first present the inputs and questions that led us to build our procedure and then describe the explicit integrated modeling approach. We then look at the main events that create constraints during modeling activity, in order to integrate them into our explicit modeling approach. Depending of the categories of constraints (mandatory or possible), we finally propose two categories of specific modeling procedures. 3.1. Inputs for modeling procedure definition In order to complete the study proposed by Hartman [29] on modeling procedures, we held interviews with 20 designers of a supplier company in the automotive industry, to identify the main questions they asked themselves before deciding on which modeling strategy to adopt. The first questions asked by designers concern the level of requirements on the CAD models. The following is a synthesis of the main questions asked: What is the structure I need to choose: from which volume or feature do I need to start? What information do I need in order to decide which structure will be better? What is the level of parameterization and flexibility I want for my model? How much time do I have to design the model? Can the model be reused later to create another similar part/ product? How stable is the information I have concerning the model? What is the risk that the model will soon be modified? What are the possible functional changes required that could impact the part? What strategy should I adopt for the modeling of the associated geometry? Depending on the usage, the time and the state of the project (design phase) of the CAD model, the designer will adapt his modeling strategy. A simple example can illustrate this in the industrial world: the CAD models created during the Request for Quotation (RFQ) project phase and those created during the development phase are completely different, except if the modeling strategy for both phases is standardized within the company. For the designers working on conceptual and preliminary design phases, application of modeling methods can be considered as a loss of time, especially if the model will not be reused and altered later - a decision that is only taken at the end of RFQ process. In this case only the resulting geometry is important
(short-term vision). If a designer knows that he will reuse his model for another project in order to reduce time, he will integrate as a specification "the model must be able to be modified quickly". We use our methodology for routine design which allows us to know and control the main sources of design changes. The methodology is adapted to models that need to be reused and modified frequently. Once the level of requirements has been defined by the user, new questions appear for the designer when creating the model. These questions are focused more on the modeling strategy and the definition of the modeling procedures. Example of questions: Which sketching plane needs to be used? What is the level of complexity of my sketch? What are the references I need to use for constraints creation (geometric and dimensional constraints) inside sketches? What functional decomposition to use (Boolean operations)? When and where to create surface operations (following the definition of Lee [30]) in the specifications tree? These questions are of course not related to the knowledge of the functions of the CAD software but rather to the parametric relationships that would be created between the features. These questions are also not related to a particular CAD system. As shown in the literature review, and the various research focusing on particular methods to create parametric associative relationships, it is the answers to such questions that impact the performance of the parametric associative CAD model. We can compare the CAD design activities as a sequential approach where for each action, a couple of solutions are available and each choice impacts the choices available for the next step. That is why it is almost impossible to find two CAD models made with exactly the same construction method. Even if the CAD model is made by the same designer, if we ask him to redesign the same geometrical model he may adopt a different modeling strategy, taking into account difficulties encountered during the construction of the first model. There are thousands of different ways to create a geometrical model in CAD software (different combinations of semantic features can generate the same geometry), but can the correct sequence of features that will lead to a more robust geometrical model be found? 3.2. Explicit integrated modeling approach Our approach to mitigate this problem is to develop an explicit integrated modeling approach with the aim of minimizing the creation of constraints linked to the existing 3D geometry. We will also use "current shape" to indicate the 3D geometry. The concept seems roughly basic but profoundly transforms the conventional way of modeling in CAD and radically improves the capability of CAD models to carry out design changes. Our methodology consequently reduces the number of possibilities for the user when creating a model. If we consider the solution of possible part modeling procedures for a given part, in-field experiments enable us to estimate that our methodology reduces the scope for the modeling strategy by 80% (see Fig. 3). The effectiveness of the design can therefore be improved. 3.3. Identification of main events that create constraints during modeling activity The first step is to identify possible practices that increase the effects of "persistent naming". We propose the identification and
Y. Bodein et al. / Computers in Industry 65 (2014) 136–147 Solution Space (Possible part modeling procedures)
Modeling based on “best practices”
Modeling strategy based on explicit and functional references
Fig. 3. Solution space with "best practices" and "explicit references" filters.
classification of the main solid modeling operations inside parametric CAD systems for which constraints on current shape are very often created. We determine two categories of constraints, one for which the use of current shape is mandatory and the other for which it is not mandatory to use existing 3D geometry. The following scheme describes the main solid modeling operations and all possible constraints that can be associated to the current shape (Fig. 4). We used the research work from Bettig and Shah [31] who define a standard set of geometric constraints for parametric modeling. Constraints to current shape can be created when creating a sketch using a planar face as a support or when creating constraints on individual sketch elements. It is also possible to create constraints based on the current shape with almost all design features, and not all constraints are explicitly given but they may be inferred. This is why CAD users are so used to creating constraints on existing 3D geometry. It is the way CAD software editors propose to work with parametric systems in their training. From our classification, we can conclude that only for localized modification operations (a sub-type of design features in Fig. 4) the use of existing 3D geometry is mandatory. These operations require the selection of boundaries of the solid (B-Rep) to be resolved. As an example, edge filet features require the selection of an edge (or a face) of the 3D current shape, and it is not possible to replace the selection of a boundary by an explicit reference (this could however be a parametric element like a plane) from the specifications tree. Based on this analysis of practices, we then propose adapted modeling procedures. 3.4. Integrated approach for constraints management in parametric associative CAD For both categories of constraints (mandatory or possible), we identify and propose a specific modeling method.
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3.4.1. Category 1 - constraints to current shape are not mandatory When creating a constraint using current shape, the mechanism used is an access to the Boundary representation (B-Rep) of the solid. More explanations about representations and algorithms used by constraint-based CAD systems are proposed by Shapiro et al. in [20]. The method adopted for Category 1 is to avoid the creation of constraints on current shape and to replace the selection of B-rep by en explicit parametric reference (not a result of geometry, but a user-created parametric entity like a plan or a line feature). Reference entities can be created as explicit references using elementary parametric elements like points (instead of vertex), plane or surface (instead of face), etc. (Fig. 5). The aim of this approach is to structure and control constraints between entities (in fact relationships between features) that are explicitly defined by the user. We also define additional and specific approaches for sketches. A well-known best practice in this respect is to minimize the "persistent naming" effect by reducing the number of topological modifications on a single primitive, which necessarily means avoiding topological changes inside the originator of the primitive: the profile itself (i.o.w. the sketch). The problem is the complexity of the sketches: the more a sketch is complex, the more there are chances that the sketch needs topological modification (remove a sub-entity such as a line or an arc-circle of a filet). The first visible effect of the proposed modeling method is that it creates a more detailed specification tree and consequently may require a designer to review the construction tree which could no longer directly reflect their design intent. As demonstrated by Johnson [23] a positive effect of using simple features is that CAD models are easier to understand by other designers. 3.4.2. Category 2 - constraints to current shape are mandatory In this category, the use of BRep is mandatory: as it is not possible to apply the same method as for the previous category, the method consists in the optimization of features localized inside the implicit representation (specifications tree structure). The goal is to create the features as close as possible to their primitive, in order to reduce the degrees of feature dependency, as described by Wang and Nnaji [32] (Fig. 6). Consequently, all filet/shell/draft operations should be defined as close as possible to their original primitives in order to ensure that the ID of the geometry used is the most stable (less variability). Paradoxically, for designers not trained in this procedure, the risk of instability during explicit representation (3D geometry) regeneration is higher because the number of features following these modification features in the history tree is greater. In other words, for a designer who systematically used the current shape
Fig. 4. Modeling operations and associated r.
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Fig. 5. Modeling operations based on explicit management of features’ relations.
for the definition of new features, the deletion (or modification impacting the topology) of features from Category II will create the instability of the model as some feature’s will lose their reference. This case is shown in the next figure: if the user systematically creates relationships between features using the selection of explicit representation in the definition of new features, Case 1 will be more stable than Case 2. That is why some CAD training recommends the creation of a filet in the last position of modeling operations (at the end of the CAD model specifications tree). These cases are illustrated in Fig. 7. We state that the critical point of our global modeling approach is to apply simultaneously the two methods for the two defined categories. The application of the method for category 2 exclusively increases the instability of the model and leads to counterproductive effects. The next section suggests a simultaneous application of both methods.
4. Academic experimentation: application of the explicit reference management modeling methodology In this section we describe an application of our methodology undertaken within an academic framework with students on several parts with different levels of complexity. Taking the point of view of Garcia et al. [33] upon the fact that CAD is not easy to learn since it not only requires computer skills but also mental capacity, spatial vision and physical coordination, we undertook an experiment upon this methodology with students from a Master course. In order to determine the benefits of the solid modeling method according the complexity of the 3D geometry, we select 3 different parts with various levels of complexity as support for our experiments (Fig. 8 and Table 2). We defined the level of complexity of the part using the number of surfaces of the solid, which corresponds to a very basic method.
Fig. 6. Semantics simplification reduces the degrees of feature dependency, Wang and Nnaji [32].
Case1 : category II features as the last features
Case2 : category II features close as possible from their primives
Solid
Solid
Solid
Solid
Feature
Feature
Feature
Feature
Feature
Feature (fillet)
Feature
Feature
Feature
Feature
Feature
Feature
Feature (fillet)
Relaonships between features (constraints) Fig. 7. Order of modification features in CAD model specifications tree.
Feature
?
instability
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Fig. 8. Overview of geometric models.
Indeed, there are no common referential available from the research community as all previous studies which were driven with different CAD models to evaluate the learning process of the designers. In [29], the complexity of a CAD model is evaluated by using the number of features in the history tree, whereas the authors consider that the number of features is dependant of the designer level as well as the applied modeling strategy. Also, the authors considered to be more appropriate to analyze the models that used during the professional CAD trainings to define the level of complexity of a component to be completely designed with CAD software. To identify the limit between the various complexity, we analyzed exercises and components created by students during a 10 day CAD training (training material from Tata Technologies for 5 days CATIA V5 basis and 5 days CATIA V5 mechanical advanced users). Almost all components created during the basic training are composed by less than 60 surfaces, thus considered as low complex parts. For the limit between medium and high complexity, we asked CAD trainers to make the distinction between medium and complex parts between 10 parts. The results were that over 230 surfaces, the part is considered as complex. This approach could easily be improved by integrating additional criteria, like the ratio between the planar surface and non-planar surfaces (i.e. filet, shell) or by sorting the surfaces by size. We also took into account the fact that a model can have thousands of surfaces, whereas the construction method can be very basic because the model can contain repetitive topology (patterns) of symmetrical functions. This is not the case in the 3 models of this experimental study. We then asked ten Master’s students to reproduce the geometry of these three parts in a commercial CAD software. The CAD software selected for this experiment was CATIA V5 from Dassault Systemes [34], the most frequently used software in the automotive industry. Before the experiment, students had only a few skills on CATIA V5. In this context they first received training on solid modeling, based on e-learning courses (i GET IT commercial platform developed by Tata Technologies) [35]. Nevertheless they already got a good background in mechanical engineering. We completed their training with an 8-h in-class course to ensure that their level of skills on the software was acceptable and sufficient to create 3D models of parts with a medium to high level of complexity. We were able to detect immediately the 2 students who had a background on other CAD software (spatial reasoning more developed as well as decomposition ability) and consequently
more expertise in solid modeling compared to others who had only a few months of experience in an academic context. 4.1. Procedure of the explorative study Experimental approach used for our study: Step 1: Initial design: creation of the geometric model without any indication on the modeling strategy. Students have 3D models and drawings of the three parts without the history tree (dumb solid). They have to break the component down into sub-volumes (constructive modeling) or create a solid row from which they will split off some sub-volumes (modification/ destructive modeling). Students are free to decide the modeling procedure to apply, and they can explore as many construction methods as they want to. No limitation of time for this step was given. Step 2: Routine design without modeling rules: creation of same CAD models with the purpose of improving the modeling procedures to make these models more flexible and robust. This step can be considered as a routine design situation because students already know which decomposition and approach they will apply to build the geometry. However, we ask them to anticipate that their models will be used afterwards to perform modifications. Consequently, students have to propose an optimized modeling method based on the experience they acquired during Step 1. Indications on possible design changes are given to students, so that they can define a modeling strategy based on it. The time taken to build the geometrical models was measured. Before Step 3, we provided a new 4-h in-class training course to the students in order to present and explain the modeling methodology described above (Section 3.4). Instructions were given for each part about explicit references, related to possible design changes that students could make. Step 3: Routine design with advanced methodology: creation of the same geometric model with the instruction to apply the modeling methodology: use explicit references and avoid direct links to current shape as much as possible (B-rep access to 3D geometry). The time to create the geometric models was monitored.
Table 2 Geometric models description.
Name Nb surfaces (degree of complexity)
Model 1
Model 2
Model 3
Centering screw support 55 (low)
Plastic carter
Radiator tank
190 (medium)
369 (high)
Step 4: Design iteration on previous models: modification of CAD models designed for Step 2 and Step 3. Modification instructions are grouped together in a 3D model which also integrates the final geometry (history tree is deleted). Modifications concern dimensional and topological changes. The time to apply design changes on each model was measured.
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Table 3 Comparison of modeling time according to modeling method.
Table 5 Adaptation of foreign models measurement.
Model identification
Model 1
Model 2
Model 3
Model identification
Model 1
Model 2
Model 3
Step 2 - modeling time (min) Step 3 - modeling time (min) Ratio
45 44 2%
187 121 36%
270 95 65%
Modification of foreign component (min) Initial modeling time
50 45
129 187
327 270
5. Results Modeling time measured for Steps 2 and 3 are reported in Table 3. From the experiment, we can conclude that initial modeling time can be reduced by using a CAD methodology based on advanced rules for features link management. This is a really relevant result as our initial assumption was that the creation of geometry with an explicit reference modeling methodology would take more time, due to additional features creation. Besides, regarding repetition or memory, the faster design times may be at least partially due to participants now having familiarity with modeling the geometry. In order to invalidate this, we used a control group composed of 2 new students for these steps. Time monitored was in the same range than for the initial sample of students. The benefits can be explained by the fact that some features are difficult to create, depending on the point at which this takes place in the specifications (history tree). The main illustration concerns the creation of localized modification operations (i.e. filet/shell see Fig. 4) which are very difficult to create at the end of the specifications tree. We also know that the results obtained, especially for a complex model where the average reduction time is around 65%, reflect the students’ level. So much progress could not be obtained with experienced industrial designers and on parts they are used to designing. Time measured during Step 4, which corresponds to the time needed by the students to adapt their models to a new design configuration, is reported in Table 4. Each student had to modify 6 CAD models (3 parts designed with 2 different modeling methods during Steps 2 and 3). From Step 4 in the experiment, we can conclude that the modification time (time to apply design changes based on task instructions) can be reduced sharply with a CAD modeling methodology based on advanced rules for features link management. Either for the first modeling activity or for the modifications of the parametric associative CAD model, the benefits of using our proposed modeling methodology are important. In another experiment concerning the effect of features associativity on designers’ understanding, we asked the students to use the CAD models of other students ("foreign" components) and to perform the same modifications. CAD models used in this experiment were the same ones created in Step 2 of the previous experiment (without modeling methods) and we asked the students to integrate the modifications proposed in Step 4. The time taken to adapt the CAD models is reported in Table 5 under "Modification of foreign component". Table 4 Adaptation to design changes measurement. Model identification
Model 1
Model 2
Model 3
Modification time on model Step 2 (min) Modification time on model Step 3 (min) Ratio
37 19 49%
20 9 55%
59 11 81%
If we compare it with the initial modeling time measured in Step 2, we see that for 2 of the 3 models, it is faster for the student to recreate the whole geometry rather than to reuse the CAD model created by another student. This basic experiment justifies the need for homogenous modeling methods to enhance efficiency during the design of complex parts and to improve collaboration between designers. 5.1. Explicit reference modeling associated with functional breakdown of the components Our previous research results provide a real workaround of the main limitations of parametric systems: history tree structure is difficult to understand, also the geometry is difficult to be reused, and the geometric model is not compatible with the design iterations. Then, we have focused on the association of the engineering functional analysis methods (such as APTE) with the creation of the explicit references in CAD. We can say that the "functional modeling" is quite simple: use references with a high semantic which have been determined through a functional analysis. By taking into an account the literature, several research deal with functional reference identifications as well as the functional geometry [5], also that the arrangement of functional references inside CAD models [6,7]. However, there are no research that associates the functional reference identification with a specific solid modeling method and strategy. The aim of this section is not to propose an inventory of all possible methods for a functional CAD modeling. Therefore, authors describe only the main steps of the functional CAD modeling approach that associated with our proposed modeling methodology. The main steps for the creation of functional geometry are: 1 Determination of the functional area of a part ! identification of the link between all functional areas ! identification of interface between functional areas. 2 Creation of specific references for each functional area. If a link exists between some functional areas, this link must be created between reference elements (and not between solids or using BRep directly). 3 Creation of solids for each function independently and then association of solids using Boolean operations. An overview of the CAD structure in terms of features’ relationships associated to functional references, also called functional modeling, is described in Fig. 10. Each function is carried out in a reference framework, in which additional reference elements are created. The functional reference elements are then used to structure explicitly the relationships between solid features. All solid bodies associated with the functions of the part are independent of one another, so that operations on each function are enabled without interaction with other functions (except if required by the functional analysis). From a CAD point of view, functional geometry can be duplicated directly from one CAD model to another or stored as stand-alone geometry in a catalog for further re-use (Fig. 9). An application of this explicit modeling methodology associated to the functional analysis which has been presented on a radiator tank component in Section 5.
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Fig. 9. Structure and links between features for functional modeling. We then obtain a functional geometry which is a solid geometry dedicated to a design function and based on functional references (see Fig. 10) methodology.
6. Industrial application: the modeling methodology integrated into a CAD methodology for radiator tank The modeling methodology proposed in the previous section and tested with students was integrated for the creation of a more global CAD methodology for a radiator tank on CATIA V5. This was done for a 1st Tier supplier from the automotive industry. This design of this radiator tank following the proposed methodology was used by the designers as a part of a CAD training. Associated to this CAD methodology, a training course in hybrid learning mode (10 h e-learning sessions + 2 h interactive lesson) was set up, along with assessments in order to measure the learning progress of designers. A total of 20 questions were elaborated for the assessment, and we focused mainly on these questions on declarative or strategic knowledge as defined by Lang [25] and Chester [26] (75% of questions). The course and assessment were implemented on a commercial CAD e-learning platform called i GET IT [35] developed by Tata Technologies [12]. An overview of the tank model geometry and its decomposition in functional geometry is proposed in Fig. 11. Each functional
References for a funconal geometry
geometry (i.e. nozzle, rib, clip) can now be extracted from the specifications tree and reused in another tank model. The aim is also to transform all tank models as sets of functional geometries, which also support the identification and development of recurrent geometry. Furthermore, after having trained 10% of the designers, another positive effect observed is that these designers trained with the tank CAD methodology are able to export the functional modeling methods by themselves onto new complex parts. A new dynamic and new point of interest for designers regarding new CAD methodologies for other complex parts has been detected. Templates of the tank allowing the designers to quickly apply the method on new products are also under construction. After one month of practice, the monitoring of the design activities allows to obtain a modeling time improvement of 20% for the creation of a new tank. More precise indicators and measures will be required after the training of the remaining 90% of the designers inside the company (around 80 CAD designers). As each project is different, measurements have to be taken carefully. A real evaluation of benefits should be made with an experimental protocol - the same project with two separate design teams.
Solid body (sequence of solid features)
Funcon’s referenal
Funcon1 Funconal geometry Fig. 10. Structure of a functional geometry.
Fig. 11. Tank model (left) and tank geometry with a functional decomposition (right).
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However, such experimentation cannot be performed in an industrial context, for economical reasons. It might therefore be useful to validate these measures with students in an academic context.. 7. Limits of the functional modeling methodology All designers have to be trained in this methodology. During the creation of a solid or of a profile (sketch or 3D wireframe) used to create a solid, each constraint (feature-features relationships) created must be selected keeping in mind "How to avoid any link with other solids? If a link is necessary, can I create a reference? If not, how to select the minimum number of B-Rep accesses". Currently, there is no CAD training that explains such a methodology. Most CAD trainings are software oriented and consequently the aim is to learn how to use each function of the software rather than how to create robust CAD models. Even during advanced solid training, such concepts are not demonstrated. This is mainly due to students’ or trainees’ level. 7.1. Instantiation of the CAD modeling methodology on products Interviews with designers and CAD managers in the automotive industry show that it is very difficult for a designer during a design phase (modeling activity) to apply a generic modeling concept such as the one proposed in the previous section. Guidelines have to be developed by instantiating the modeling methodology directly on the product. Only if the guideline proposes instructions that can be reused as-is, without any mental effort (working memory load) for their adaptation, will the designer be able to reproduce the methodology and follow it precisely. And this is not related to a lack of knowledge such as that observed by authors during the radiator tank study using the results of assessments completed by designers. Our point of view is that during a design phase which implies a problem-solving process, the part of thinking-about modeling methods has to be reduced to its minimum. The mental and abstraction capacity needed to apply a generic method on a particular use case must therefore be as low as possible (even for an experimented designer). The methodology must therefore be created on the products design by the company so that designers just need to follow the document. CAD software should be as unobtrusive as possible; meaning that an engineer should concentrate on the actual design and not on the system interface or procedures. For the same reason, many companies define templates (start-up CAD models of products with or without advanced features). Of course this methodology is applicable by all designers, even it implies additional training and somehow a good level of knowledge. For less experimented designers in the usage of CAD software, the level of guidance and support need to be high and consequently required more documentation. What the authors have observed during the field study is that the design of component with high complexity is often carried out by experimented designers which help in the deployment of this methodology. Recently, a new and promising trend appears in CAD [36]. Direct modeling allows the user to select intuitively and manipulate in real time geometric entities regardless of the feature’s history. Direct modeling provides a "just do it" modeling strategy that gives the designer the power to quickly define and capture geometry. The focus is on creating geometry rather than building features, constraints, and design intent into the existing models. The direct modeling method is therefore targeted toward quickness and responsiveness to change. This can be useful for implementing collaboration among CAD designers on a same CAD model from which they do not know the features sequence. Direct
modeling will not replace feature-based CAD system in the future but rather bring another solution to the complexity of parametric CAD. For the knowledge integration, the parameterization, the geometry standardization, the definition of feature constraints, relations, and dependencies would remain the best solution, because it ensures that the design modifications could be realized in a predefined way. The big challenge for CAD editors is today to make both approaches compatible (and not convertible) so that the designer could take benefits from both technologies at the same time In order to get more feedback on our modeling methodology, we have undertaken to test it in an industrial experiment, in reallife conditions. 8. Conclusion In this article we proposed a modeling methodology for complex parts, based on explicit management of functional references. Experimentation on the methodology in an academic and industrial context was reported and the benefits discussed. The proposed modeling methodology is currently in use in a First Tier automotive company for the design of radiator tanks and plastic parts. The methodology is also applied by Tata Technologies for the creation of design procedures and methodology for their automotive customers when dealing with complex parts. This methodology is also one approach that tries to answer to the existing lack of CAD modeling methods for complex parts. The industrial example presented deals with automotive industry purpose; but the methodology can be applied in any domain required CAD modeling. Our research work also highlights some perspectives for the development of more efficient CAD training concerning learning for CAD expertise. The next step of our research on improving the efficiency of CAD tools during product development processes will be to define a general method for the definition of a part-oriented modeling methodology allowing designers to identify the best modeling strategy according to the design changes. If the modeling strategy could be identified automatically and directly by designers, it would radically improve CAD models reusability and possibilities for collaboration between designers on the same models. It would therefore leave more time for innovation. Another perspective is targeted toward CAD education: it will be to identify the best training mode for our method and the level of support required to ensure a sound understanding and its application. Experiments with designers in the industrial context give certainty to the high level of guidance needed when creating support for CAD methodologies in order to ensure consistent modeling strategies between the actors. We also need to assess the designers’ satisfaction in their use of the method, especially in the industrial context. The results should be positive because, with outsourcing, product development processes require more and more collaboration with all stakeholders of projects, including work on the same CAD models. Even if the method does not solve all designers’ problems in part modeling, it still meets the need for easy rules and methods applicable by all designers during parts modeling, especially when the task is complex. References [1] J.J. Shah, Assessment of features technology, Computer-Aided Design 23 (5) (1991) 331–343. [2] R. Anderl, R. Mendgen, Modelling with constraints: theoretical foundation and application, Computer-Aided Design 28 (3) (1996) 155–168. [3] C.M. Hoffmann, R. Joan-Arinyo, On user-defined features, Computer-Aided Design 30 (5) (1998) 321–332. [4] C.M. Hoffmann, K.J. Kim, Towards valid parametric CAD models, Computer-Aided Design 33 (1) (2001) 81–90.
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Yannick Bodein is consultant at Tata Technologies Europe since 2006 on Product Lifecycle Management (PLM) and Computer Aided Design (CAD). He received his engineer degree from the University of Technology of Belfort Montb,liard, France, in 2006 and his PhD in 2011. He is mainly interested in optimizing CAD process performance based on product-oriented methodology and by knowledge integration into CAD systems. He has several experiments of CAD project deployment within different Tier one suppliers of the automotive industry.
Bertrand Rose is Associate Professor in industrial engineering at the University of Strasbourg, France, since 2005. He received his PhD in 2004 at Nancy Research Centre for Automatic Control. He received his engineer degree from Ecole Sup,rieure d’Informatique et Applications de Lorraine (Nancy, France) since 2001 and a Master of Sciences degree in Management of Production from Chalmers University of Technology, G"thenburg, Sweden. He is interested in the performance of the collaborative design process and the Lean and Green product development.
Pr. Emmanuel Caillaud received his engineer degree from Ecole Nationale d’Ingenieurs de Tarbes, France, in 1990 and his Ph.D. from the University of Bordeaux, France, in 1995. He is professor in mechanical and industrial engineering at University of Strasbourg since 2002. He was involved in several European and French projects on engineering design and he is reviewer for international journals and expert for international research funds. He has published in, among others, International Journal of Production Research, Concurrent Engineering: Research and Applications, International Journal of Advanced Manufacturing Technology, Computers & Industrial Engineering. His research focuses on knowledge and performance in engineering design, concurrent engineering, collaborative design and eco-design.
