This chapter gives an overview of the research methodology and highlights the motivations behind this work. The chapter presents the definition of the problem that this thesis addresses and formalises the research approach. The hypothesis for this work has been formalised and described, also detailing an overview of the knowledge contributions of this work. Finally the validation strategy for this work has been presented.
53
4
Model for Agent-Based
Self-Configuration of
Modular Assembly
Systems
Figure 4.1 - Overview of Boundary Conditions of the Problem Domain Equipment Modules XML Module Description XML Module Description Equipment Module Description Syst e m R e q u ire me n ts Self- Configuration Methodology Process Requirements Apply
Glue Place Cure
XML System Requirements Business Informtion System Constraints Feeder Skills: Interfaces: Description:
Feeder unit equipped with part location device.
Bay Port (Male) Part Port (Male) Feeding XML Module Description System Configuration Equipment Configuration Base Table Glue Dispenser Transport Apply Glue Feed Pick Up
Place Cure Transport
Manipulator B
Conveyor Feeder
Assembly Process Configuration
54
4.1 Introduction
In this chapter it is proposed a model that encompasses the requirements description that enables an agent-based self-configuration of modular assembly system. The chapter covers all the different sources of requirements, highlighting the need for clear and their formal definition which will enable the self-configuration of modular assembly systems.
The concept of modularity is highly dependent on the general understanding of a module. A module is a building block with certain characteristics, both physical and logical, that enable it to be combined with other modules. The question that arises from this definition is what these characteristics are, and more importantly whether they can be generalised. It is clear that different modular systems can have different characteristics. The domain of modular assembly system is quite wide and complex, and there are several different types of solutions. The challenge is to find the common characteristics and establish a clear model for their descriptions. Despite the fact that this target domain is quite wide, what can be clearly generalized is that to establish any configuration methodology for a modular system, one requires a module description, which has to contain information on its capabilities and how the module can be combined with others.
Modular assembly systems (MAS) have existed for over two decades. They focus mainly on the advantages of fast physical integration of equipment. Nowadays, system builders often use the concepts of modularity on the physical side. Standardization is a complex and lengthy process and, in the case of assembly, quite impossible to tackle. However, the need for standards does not provide an obstacle for system configuration. If one equipment only plugs in to another of the same supplier, it is not ideal for the future of MAS, but it does not pose a problem for configuring a system, since there is a set possible solutions. The real need that arises from analysing the standardization issue is the need for a storage of terminology that should be used by different module suppliers, regardless of it being shared definitions or not. The intention of this chapter is to provide a model that is module
55 supplier independent, catering only for the aspects relevant to the configuration of modular systems.
In the assembly domain, we should consider two aspects of modules, namely their physical characteristics and their assembly process capabilities. Assembly modules exist to perform certain activities, which can be described as its capabilities or skills (EUPASS [4]). The capabilities of the module can themselves be described as a module, a different type of module but still a module. This is important to note because these modules also have their connection issues that are crucial for the definition of the assembly process sequence (Lohse [44]).
The context of system configuration has to be driven by a clear set of requirements. For the assembly system requirements an important aspect is to maintain certain common terminologies with the equipment modules descriptions. These have been identified as the assembly process capabilities and physical interfaces (EUPASS [4]). The physical interfaces allow for the definition of physical requirements and constraints. The assembly process capabilities allow for the identification of which modules can fulfil the capability requirements. The assembly processes descriptions should also follow a common taxonomy to enable the possibility of high level assembly process requirements, which will be complex compositions of assembly processes. This definition provides the basis for making configuration decisions based on clear hierarchical assembly process structure (Lohse [44]).
The choice of agent technology poses a few constraints on the configuration process since agents are required to communicate. Consequently, the information needs to be transparent for the agents to be able to exchange information and make decisions based on the semantic descriptions. The configuration of an assembly system is a process that involves different information types which must be modelled for the use of agents. It is important that the model is structured in a scalable manner, catering for possible modular assembly systems evolutions. Moreover, it is very likely that new assembly processes, new interfaces, and new equipment module types will be introduced over time.
The definition of clear assembly process and system requirements is crucial for the design of an agent system that will provide configuration solutions. If the
56 requirements are not defined there are no clear objectives to establish the configuration. The established requirements will define the information that can be inputted in the agent environment. This information also enables the decision making process of the agents, namely for establishing valid configuration solutions. In addition, it highlights the boundary condition of the problem this work intends to tackle. Figure 4.2 provides a high level overview of all the descriptions that are required by the configuration methodology. The proposed model is supported by three types of actors, which highlight the requirements of the agent environment. The system integrator provides the definition of the system requirements, namely what are the expected capabilities and constraints for the desired system. This description will follow a complex skeleton or template which imposes the required common terminologies. The next user is the configuration expert, which provides the required maintenance to the model in terms of common terminologies for both the interfaces and the assembly process. The final user is the module provider, who is responsible for populating the equipment module library (providing descriptions that follow the established terminology). It is also important to point out that the output of the configuration methodology will be provided in a structured manner to the system integrator for validation. The details of these models will be presented throughout this chapter.
The use of Extensible Markup Language (XML) format is proposed for the instantiation of the proposed models definitions, because it allows for a clear description that is transferable and usable across different systems. XML description is transparent and understandable format for both individuals and computer systems. The use of XML format provides a transparent description which is widely accepted for transfer of information, and is also able to cater for future extensions which enables the scalability of this approach. The wide use of XML also allows for a better acceptance of industry for the use of this approach.
The use of XML means that in order to encapsulate this information, the model will be provided in a XML Schema language known as XSD. This form of description is at its core hierarchical since it is based on the definition of nodes, with certain attributes, that contain other nodes making it hierarchical form of description. Some nodes and information might be optional in some cases, and mandatory in others. The full XSD model can be found in Appendix A.
57 Equipment Module Repository Assembly System Skeleton (XSD) Configuration methodology Assembly
Process Library Interface Library
Assembly Process Types Descriptions (XML) Assembly System Requirements Descriptions (XML) Equipment Module Description (XML) Equipment Module Skeleton (XSD) Interface Types (XML) Interface Types Skeleton (XSD) Assembly Process Types Skeleton (XSD) System Integrator Module Supplier Configuration Expert Assembly Process Types Descriptions (XML) Interface Types Descriptions (XML) Equipment Module Descriptions (XML) Assembly Process Skeleton (XSD) Assembly System Configuration Description (XML)
Figure 4.2 - Overview of Self-Configuration Requirements Model for Modular Assembly Systems