Structural semantic 3D modeling technology in the integrated design process

Introduction

Due to the development of information technology in the manufacturing industry (in particular, the product line of CAE/CAD/CAM systems), the position of electronic 3D models in the product life cycle (PLC) is consolidating [1]. It primarily applies to the research and development (R&D) stage, in which a 3D model fully represents the design solution [2].

Obviously, at the stage of engineering design it is the product design that contains the most important information, which is displayed by modern CAD systems as follows:

where Констр(Изд) – product design;
– product 3D model.

However, in the process of shaping product design in a CAD system, the user (design engineer) has to do exclusively with the abstract functionality of the CAE system being used, which leads to loss of the original design concept and, consequently, to limited ability of reusing and modifying the generated 3D model. Hence, capturing the design concept in a 3D model is a relevant task.

Electronic 3D model of a product

In the present-day CAE systems a 3D model is merely a consequence of performing basic operations (BO), i.e. the simplest design operations that are provided by the functionality of the CAD system used. The basic operations BO (БО) are captured in the so-called development tree of a 3D model, which is a linear sequence of interconnected BO. Then

where– an execution sequence of BO that shape the design route, i.e. a set of BO ensuring development of a 3D model of the product.

Within the framework of the Constructive Solid Geometry technology, implemented in virtually every contemporary CAE system, a 3D model can be represented in the form of a system

where КЭГ_k – structural element of geometry (SEG), defined in the paper [3] as an object with pre-determined behaviour and data structure specified during execution of a design procedure. An equivalent of SEG in the English-language literature is the term features.

Self-descriptiveness of a 3D model

The self-descriptiveness of a 3D model consists in its ability to convey information about a product relevant to the current stage of PLC [4]. Visualisation of product design is the main CAD system functionality, representing design solution in the form of a 3D model that features design completeness:

It is in the basic operations where all design data displayed by a 3D model are contained, these operations being hierarchically organised in a 3D model development tree:

where – 3D model development tree.

In study [5], it is pointed out that the development tree fully describes 3D geometry and is the main source of information about it.

The highest descriptiveness of a 3D model at the R&D stage is achieved when the engineering bill of materials (EBOM) of the product is displayed. A 3D model like that is a full-fledged component of a digital mockup of the product [6].

In the RF national standard GOST R 53394- 2017, engineering bill of materials is defined as a structure (system) consisting of structural and functional elements and links determining their mutual belonging [7]. This system-related definition of product structure corresponds to the modular principle first presented in study [8].

Engineering bill of materials of the product

According to the definition given above and the data of study [9], the following structural elements (SE) can be distinguished:
• operational elements (perform the regulated functions of the product);
• basic elements (coordinate one product with the others);
• connecting elements (connect the products materially in the mutual assembly process);
• process elements (implement the fabrication processes of the product manufacturing and its subsequent assembly).

The same SE can perform the functions of operational, basic and connecting elements. The most optimal option is combining operational elements with basic ones within a design, while reducing the number of connecting elements [9].

Each structural element has semantic integrity consisting in a structural and functional purpose, partially featured in its decimal number and description. Consequently, a SE has a sense in a given subject area, which is defined by a final product of the Assembly level (Assembly Unit). Based on the physical sense, an SE name is assigned and the attributes determining its informational 3D image are identified, whose values are the characteristics of a particular SE instance.

Considering structural and functional decomposition of the product design into SE, and composition of a 3D model from a sequence of BO (2), representation of a development tree of  view in a CAD system can be implemented by two methods, described in detail in the paper [10].

The principle of structural consistency

Lying in the base of structural consistency principle, when representing EBOM in a 3D model development tree, are three criteria, first identified in [10].

1. Fixing and representing the design concept within a 3D model.
2. Handling semantic units, relevant for a given subject area, during 3D model generation.
3. Ease and convenience of the process of development and re-use of a 3D model.

The principle of structural consistency proposed by the author of said paper is essentially a bijective representation of the engineering bill of materials of the product being designed (as a set of SE) in the product’s 3D model development tree:

where CMO – semantic macro-operation (SMO) of developing a 3D object corresponding to one SE;
n – number of SE in the design of the product under development.

The SMO is an ordered sequence of BO, shaping a resultant 3D object of a structural element and defined by the formula

A 3D model development tree of the product under development is shown in Fig. 1.

The main idea behind the structural correspondence principle lies in the information and semantic composition of CAD system basic operations according to formula (6) up to the level of a semantic macro-operation, under strict correspondence of the SMO → SE form. Thus, the engineering bill of materials of a product Стр (Изд) and the 3D model development tree  are equinumerous as the sets:

In this way, the advantages of approaches considered in [10] are implemented though mutually unambiguous correspondence between SMO and SE, facilitating fixation of the design concept in the process of 3D model development.

Shown in Fig. 2 is a proposed representation of the structure of a “Part” level product, exemplified by a housing from the set of a microstrip microwave module.

As can be seen from Fig. 1, the 3D model development tree no longer represents a sequence of abstract BO, but has a fixed design concept, displaying the structure of the product being designed.

Semantic macro-operation

The mechanism for generalisation of the basic operations down to the semantic unit level was first described in the paper [10]. Its main purpose is to increase self-descriptiveness of the 3D model development process in a CAE system through transition from abstract basic operations to SMO that carry a fixed design and functional meaning in a given subject area.

The SMO structure is presented in Fig. 3. At the SMO output, a class of semantic structural elements of geometry КЭГ^Sem is generated, representing structural elements of a product as the component elements of a 3D model. The SMO is a composition of BO. Considering notation (6), it is defined as

where – parameters of the j-th СМО and q-th БO, respectively.

A semantic macro-operation inherits all the parameters of basic operations included in it. In this case, the user specifies a set of semantic parameters  , which describe the j-th SMO and possess a design concept linking all together by association. Of major importance in the SMO structure is the algorithm of its implementation Алг (СМО_i) . Depending on the input parameters, this algorithm regulates ordering of the basic operations into certain sequences (design routes), which ensure generation of a design solution instance. Such a sequence is written as:

Within the framework of a single SMO, generalisation of such sequences up to the level of a semantic similarity class is provided.

Semantic similarity

As a criterion for design solution class, semantic similarity is based on the design and functional properties of a real product of the “Assembly” level. In a general case, semantic similarity implies geometric (parametric) and structural (topological) variability of design solutions. Implementing both variability types simultaneously in the standard CAD system functionality does not seem feasible.

The design routes generalised in a single SMO and differing in the values of parameters, the number of BO and their composition, ensure structural and geometric variability of the output data at the level of a structural element of geometry. To ensure semantic variability (with preservation of the design concept), execution rules and conditions are integrated in the SMO algorithm. Dependent on the conditions is correctness of the input data in terms of their physical sense in a given subject area. The rules have a more complex structure, ensuring the tree-like nature of SMO. In the paper [11], they are denoted by the term “production rules” and represent an “IF – THEN” conventional conjunction.

A set of conditions and rules allows to implement in a single SMO a class of SE similar in their design and functional purpose. All design routes included in a given SE class are generalised to a uniform tree-like structure. Then an instance of a SE class is shaped as follows:

where m – number of a design route which defines the SE being shaped by a unique set of design and functional parameters and attributes.

Representation of a 3D model in the form of a SMO sequence ensures structural and geometric variability of the design solution. It is achieved by referring to SMO as to a self-contained module, linked to the rest by association, which enables to preserve semantic correctness of the design.

The 3D models of parts of the “Housing” type, as shown in Fig. 4, differ in both geometry and structure, while being generalised into the class “Housings of microstrip microwave modules” as per the attributes of semantic similarity.

Geometric variability of the housings shown in Fig. 4 is ensured by the different values of semantic parameters given in the table. All the mentioned parameters are set when constructing the design and functional base in the form of a compartment for a microstrip board.

The structural variability of the housings (see Fig. 4) is ensured by different sets and types of design and functional fragments: coaxial hole for connection, mounting lugs, etc.

The structural and geometric variability of 3D models of parts that make up an assembly unit allows to select a class of semantically similar assemblies. The rules set by the SMO algorithm may include installation of conjunctions within an assembled 3D model. It allows to link components not to abstract geometric elements, but to the structural elements of one another, in the task of both generation and modification of a 3D model of an assembly.

Fig. 5 shows semantically similar assemblies, namely, instances of the class “Housings of microstrip microwave modules” generated in the course of structural and geometric modification of the housings (parts) (see Fig. 4). The assemblies have the following semantic parameters:
Assembly 1
Assembly units...................................................9
Parts...............................................................12
Standard items.................................................12
Assembly 2
Assembly units.................................................14
Parts...............................................................17
Standard items.................................................16
Assembly 3
Assembly units...................................................8
Parts...............................................................14
Standard items.................................................18

Conclusion

The methodology of 3D model representation in a CAE system, proposed by the author, is aimed at increasing model self-descriptiveness, while at the same time reducing labour intensity and the time required for its construction. The distinctive features of the proposed approach are the preservation of the design concept in the form of a 3D model in the course of design solution generation and modification, as well as ensuring its semantic integrity. With this approach applied, the need for handling structural elements of geometry that are linked to a specific CAE system is eliminated, and the engineering bill of materials of the designed product is restored according to the semantic macro-operations. It is the SMO which is the principal design action allowing to withdraw from abstract geometric elements and handle real elements of product design when defining a 3D model.

The effectiveness of the proposed approach is proven by operating geometrically complex 3D models. Such models correspond to products that have a complex functional structure, e.g., the components of microwave microstrip modules considered as an example.

The next step is development of a library of SMO for a particular class of designed products, containing the functions for creating, storing, calling-up, editing, and deleting SMO. If from the user’s/designer’s standpoint SMO are integral (indivisible) items, this library will specify their internal structure and rules.