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The modelling of information and knowledge structures in product design and manufacture has a direct influence on the capability to semantically interoperate. This is because, the degree of formality present in the structuring of information in a model is analogous to the semantic enrichment of the captured model. In PLM, two significant types of models have been pursued namely (1) product models (Molina et al, 1995; Anderl, 1997;

Balogun et al, 2004; Sudarsan et al, 2005) and (2) manufacturing models (Giachetti, 1999; Zhao et al, 1999; Al-Ashaab et al, 2003; Liu and Young, 2004).

2.6.1 Product Models

A product model may be defined as an information model, which stores information related to a specific product (Molina et al, 1995; Anderl, 1997).

Another analogous description of a product model has been provided by Balogun et al (2004), who specify that the model represents a complex product from the top product level to the tolerance detail of every feature characteristic.

Product models occupy a key role at the centre of the product lifecycle (Young et al, 2007) since they hold and share product information that are generated, used and maintained over the processes of design, manufacture, delivery, maintenance and disposal (Lee et al, 2006). Product models may be composed of a number of sub-models such as (1) the structure-oriented, (2) geometry-oriented, (3) feature-oriented and (4) the knowledge-oriented models, which when unified into integrated product models (Chin et al, 2002) enable decision support capability to be achieved.

The concept of product models continues to evolve with time. Sudarsan et al (2005), for example, have successfully exploited a particularly interesting product model, known as the Core Product Model (CPM) as shown in Figure 2-11. The main advantage of the CPM is that it favours product model

extensions while providing a common ground. The model proposed by Sudarsan et al (2005) also aims at capturing different engineering contexts that involve view-specific product considerations. The “Product Family Evolution Model” (PFEM), for instance, represents the evolution of product families and the rationale of the changes involved (Wang et al, 2003).

2.6.2 Manufacturing Models

The concept of manufacturing models initially took root from contributions made by Al-Ashaab (1994). Manufacturing models consist of common repositories of manufacturing capability information and the knowledge and constraints over the use of manufacturing processes (Al-Ashaab, 1994;

Balogun et al, 2004; Liu and Young, 2004). The information structures exploited for this purpose comprise of defined relationships between all manufacturing capability elements.

Similar to how product models can be decomposed into their constituent individual sub-models, manufacturing models also enfold different concepts like (1) the manufacturing resource capability model, which concentrates on representing information about functions and characteristics of manufacturing resources and their combination into manufacturing processes (Giachetti, 1999; Molina et al, 1995; Zhao et al, 1999), (2) the process plan model, used to describe the information about the process plan strategy of a manufacturing process (Feng and Song, 2003) and (3) the manufacturing cost model, used for driving the meaningful estimation of production costs incurred during design and manufacture.

Core Product Model

DesignAnalysisIntegration Model

OpenAssembly Model

ProductFamilyEvolution Model

Figure 2-11 Framework Components of the Core Product Model (Redrawn from Sudarsan et al (2005))

In their work, for example, Feng and Song (2003) have met the aim of developing a “Manufacturing Object Model” to enable the interoperability of preliminary design with process planning. Their implementation platform utilises the Unified Modelling Language (UML) Object-Oriented (OO) approach for constructing the information structures behind the manufacturing model. Current documentation on manufacturing models (Tam et al, 2000;

Liu, 2004; Gunendran and Young, 2006) further point to the fact that mostly an Object-Oriented slant has been given as far as information modelling of manufacturing models are concerned, i.e. exploited information structures have remained lightweight in nature.

2.6.3 Integrating Product and Manufacturing Models

Clear evidence is available which demonstrates that there is a need to integrate the product and manufacturing models. Feng and Song (2003), for instance, mention that both models have not been shown fully integrated with each other. The integration of product and manufacturing models is key towards reinforcing decision support capability and knowledge acquisition in the product development lifecycle.

The ability to capture and reuse design and manufacturing knowledge in a meaningful manner is dependent on the semantic interoperability of product and manufacturing models. Gunendran and Young (2006), for example, have documented an information and knowledge framework for capturing multi-perspective design and manufacture and have mentioned that the integration knowledge may contain several rules, equations and options to support the information integration of multiple views. However, multi-view modelling to acquire manufacturing knowledge has been developed into solutions based on the use of UML, and therefore use a lightweight ontological approach which is inappropriate for inter-system interoperability (Young et al, 2007).

Hence, it is clear that a progression to achieve this semantic integration remains to be addressed.

2.6.4 Features and Part Families

Feature-based engineering bridges the gap between Computer Aided Design (CAD) and Knowledge Based Engineering (KBE) systems (Shah, 1995; Otto, 2001). A useful definition for a feature has been provided by Brunetti and Golob (2000), who mention that a feature is an information unit (element) representing a region of interest within a product, and is described by an aggregation of properties of a product.

Several authors have documented the importance of features of various sorts as providing valuable integration links between design and manufacture, such as the “machining features” effort from STEP. Gu (1994), for example, have recognised the significance of feature-based representation, as part of a product models for supporting integrated manufacturing. The ongoing significance of feature-based modelling is well established and has been under consideration by several researchers at different periods of time such as Young and Bell (1993) and Aifaoui et al (2006).

One of the recent types of feature that has emerged, with the scope of representing any geometric and non-geometric relations in an assembly, involves associative assembly design features (Ma et al, 2007). In their approach, Ma et al (2007) firstly identify the requirements for satisfying assembly features by specifying, for example, (1) the need for independent representation of feature relations and (2) the representation of relationships between features and parts for the inclusion of both geometric and non-geometric information. However, it is to be noted that a lightweight ontological approach using UML modelling has been pursued.

Feature technology follows two main paradigms namely that of (1) feature recognition and (2) design by feature. In the former, intelligent algorithms are used to extract features from existing geometry. However, a major limitation is present on this approach and relates to the effectiveness of the exploited algorithms to recognise interacting features (Martino and Giannini, 1998). In the design by feature approach, which is nowadays favoured compared to

feature recognition, a product can be modelled from a library of available features. There is, however, a drawback to this approach in that the representation of features is dependent on the context, i.e. viewpoint, being taken (Martino and Giannini, 1998). Nevertheless, where features can be understood within a part family context, there is the potential for them to provide a significant route to sharing information between lifecycle activities (Gunendran and Young, 2008), i.e. the semantics of part families can help support interoperability in product design and manufacture.

The concept of part families, in which specific parts are grouped according to their manufacturing operation requirements, is particularly relevant to group technology and cellular manufacturing systems (Ang, 1998; Chan at al, 2006;

Yang and Yang, 2008). Categorisation of part families with respect to specific viewpoints arising in design and manufacture, as is the case with features, is also a fact, for example, design, manufacturing and assembly part families (Westkämper et al, 2000; Simpson, 2004; Jiao et al, 2007; Gunendran and Young, 2008).

It has been acknowledged by Li et al (2006), whose work is concerned with the representation and sharing of part feature information in Web-based parts library, that one of the requirements to achieve meaningful part family description is to have a comprehensive norm for capturing part family information. This, from a semantic interoperability perspective, additionally implies the importance of addressing semantic descriptions of features and part families, as well as the ability to wrap semantically-rich product and manufacturing models.