Research Issues in Assembly Knowledge Sharing

Assembly Design (AyD) and Assembly Process Planning (APP) are two important domains in manufacturing assembly, which require frequent collaboration for efficient product development. With the rapid development of Information and Communication Technologies (ICTs), various knowledge based systems have been developed over the years in order to store and reuse the product and process information. However most of the contemporary knowledge based systems lack the requirements of modern manufacturing industry (Fischer & Stokic, 2002). This is because; most of these kinds of

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knowledge based systems operate well in an isolated capacity (Cochrane, et al., 2005). However when subjected to knowledge sharing environment they fail to serve the purpose.

A potential hindrance in the way of knowledge sharing across different knowledge based systems (including assembly systems) is the incapability of such systems to acquire consensus on the semantics of knowledge content (Musen, 1992). These kinds of systems can be made semantically interoperable, if the semantics of the knowledge associated with such systems can potentially be exchanged without losing their meaning and intent (Chungoora, 2010). It implies that the systems should capture the semantics and the contexts of the knowledge in order to make it applicable for a range of domain systems. However technological support is required to fully capture the semantics of the knowledge and the choice of formal language is also an issue.

The concepts used to capture assembly knowledge may have different implications across the assembly design and assembly process planning domains. For example, the concepts; assembly feature, assembly component, Bill of Materials (BOM) and Product Family (PF), as shown in figure 3.1 are viewed from functional and design aspects during the assembly design stage and are associated with assembly processes and resources during the assembly process planning stage. This implies that these domains dictate the semantics of these concepts and knowledge sharing across these domains may be problematic without taking into account the context in which they are used.

Figure 3.1: Assembly Knowledge sharing problem

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So far we have considered the two assembly domains as two different databases where semantic conflicts exist due to the varying nature of these two domains. We term these issues as inter-domain assembly knowledge sharing issues. Another important issue is the intra-domain assembly knowledge sharing issue. Owing to the complexity involved in the manufacturing assembly environment, multiple viewpoints may also exist for the same domain e.g.

assembly design or assembly process planning. For example, if a designer using a particular CAD system wants to share information with another designer working on a different CAD system, semantic interoperability issues may arise.

Hence semantic conflicts also occur for intra-domain assembly systems as they are caused by multiple overlapping concepts and definitions and multiple representations of similar concepts (Chungoora & Young, 2011b). This problem may be further exacerbated for inter-domain assembly domains as the impact of overlapping concepts and multiple representations (contexts) may increase when we consider two different domains.

To understand the semantic conflicts in terms of manufacturing assembly we can, for example, say that the terms “assembly” and “product” are overlapping concepts in AyD and APP respectively. Similarly the terms: BOM, product family, assembly component and assembly feature are examples of multiple representations of similar concepts in AyD and APP. These multiple representation concepts consequently have different data structures for their respective domains. For example, the concept “BOM” may represent different lists of components for AyD system and APP systems and therefore the associated concepts for both the systems may be different causing the data structure to be different for both domains.

One way to solve the semantic mismatches problem is to use standards to induce interoperability for inter and intra-domain assembly knowledge sharing.

However it is important that the system participants should agree to use these standards. Although it may not be possible to have the same standards for all the assembly systems however even if we use standards as a recourse for

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interoperability, semantic conflicts could still result due to less rigorously defined concepts (Young, et al., 2007). Hence a mechanism is required to reconcile the semantics of multiple assembly systems in order to share assembly knowledge.

3.3 Requirements to Support Assembly Knowledge Sharing

The requirements to support knowledge sharing have been determined based on the analysis of literature especially Michel (2005), Young et al. (2007), Usman (2012), Chungoora et al. (2012) and Palmer et al. (2012), and from the understanding of the assembly knowledge sharing problem. From the analysis, three potential requirements have been identified and these are listed as follows:

1. There is a need to capture the semantics of multiple viewpoints of assembly information and the relationships between them (Young et al., 2007) in order to support assembly knowledge sharing.

2. There is a need to identify a set of reusable assembly reference concepts whose semantics are well defined, for multiple assembly systems to use these concepts in order to share assembly knowledge.

3. There is a need to use an appropriate formal language in order to capture the assembly semantics and to provide shared meanings for multiple assembly systems.

The first requirement is to capture the semantics of multiple viewpoints of assembly concepts which in turn facilitates assembly knowledge sharing. In sections 2.2.2.2 and 3.2, it has been described that the viewpoint or perspective is important for assembly systems to interoperate with each other. It captures the intent of a particular domain or a system which is a requirement for seamless exchange of knowledge. For example, in figure 2.1 assembly design and assembly process planning viewpoints of a bearing shaft assembly were

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shown and discussed. It was found that the assembly design perspective is more inclined towards finding out the requirements related to the function, material, tolerance and fits of assembly components/features, whereas the intent of the assembly process planning domain is to figure out the requirements related to the assembly process and assembly resources. Hence the context of assembly information is important in the sense that it captures the true intent of the assembly information.

The assembly viewpoints can be captured by identifying a set of reusable assembly reference concepts whose semantics are well defined and this forms the basis of the second requirement of this research. A knowledge base can be created by defining and relating the assembly concepts which can subsequently be used as a common base for multiple assembly domains e.g. assembly design and assembly process planning domains. However if knowledge bases for these assembly domains are developed independently, then there is a potential chance that semantic conflicts could result which may subsequently hinder the knowledge sharing process. Therefore a common reference ontology is required which can capture the meanings of concepts at various levels of specializations. This common reference ontology is comprised of assembly reference concepts that represent assembly information at various levels of specialization.

For example, the concepts of assembly feature, dimension, tolerance, tolerance type and shape attribute can be used to capture the assembly design perspective of bearing shaft assembly shown in figure 2.1. These concepts represent information at various levels of specializations. For instance, the concept shape attribute is a Generic Reference Concept (see figure 3.2 and 3.5) which represents the shape of an object. The Generic Reference Concepts are applicable to multiple domains including the product lifecycle domain and are most generic concepts in the ARO.

The concepts dimension and tolerance represent the design information and are applicable to both single piece part manufacturing and assembly. Therefore

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they have been included in the Design and Manufacturing Reference Layer of the ARO (see figures 3.2 and 3.5). The concept assembly feature is applicable to assembly therefore an Assembly Specific Layer has been included in the ARO. The Assembly Specific Layer provides the concepts which are applicable to both assembly design and assembly process planning. The concept tolerance type is Assembly Design Reference Concept and because the limits and fits standard BS (4500) was applied on bearing and shaft assembly therefore the concept tolerance type was used in the ARO at this level.

Similarly the concepts assembly process and assembly resource has been identified during the exploration of bearing and shaft assembly (shown in figure 2.1) to capture the assembly process planning viewpoint. As these concepts support the capture of assembly process planning information therefore they have been placed in the Assembly Process Planning Reference layer.

The different levels of specialized concepts within the ARO are required to capture the assembly design and assembly process planning knowledge. These reference concepts can then be specialized and linked with domain specific concepts to support knowledge sharing across assembly design and assembly process planning domains (as was explained in section 2.2.2 with the help of bearing shaft assembly).

The third requirement is related to the use of the most appropriate technological support for the representation and sharing of assembly knowledge. There are various formal languages as discussed in section 2.4.2 which can support the representation of assembly knowledge. However it is imperative to understand that the choice of formal language should be made by taking into account its expressive power and reasoning potential to deal with the complexity involved in assembly. This research work will use KFL as a formal language because it is more expressive and computationally powerful than its competitors such as OWL as explained in section 2.4.2. The KFL supports higher order relations and functions which can be used to capture the semantics of assembly concepts such as tolerance. The KFL also deploys axioms to

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constrain the semantics of the concepts and infer new knowledge which help to share assembly knowledge.

3.4 The Assembly Reference Ontology (ARO) Concept