A Computer Integrated Manufacturing (CIM) system supports the various activities and users involved in manufacturing systems. Such activities include computer aided design (CAD), computer aided manufacturing (CAM), production planning, parts list management (PLM) and document production. We take the view that a CIM system is primarily a form of cooperative working among component systems with the emphasis on coordination rather than integration. While there is some global control to ensure system-wide consistency, existing local applications are able to operate as before and, as far as possible, system coordination is performed "behind the scenes" in such a way that global consistency is achieved with a minimum loss of autonomy.

The general aims of our work are twofold. Firstly, we are investigating ways of adapting and combining state-of-the-art multidatabase technologies in real application systems and are using CIM as the driving force. Secondly, these technologies are "database technologies" and not all CIM component systems support database functionality. We therefore have to establish general principles for the augmentation of component systems by means of local agents such that they are enhanced with the required database functionality.

The general architecture of our demonstrator system, CIM/Z, is shown in the figure above. For simplification, we show only two component systems. We have a CAD system which has no database support and stores its data in files. On the other hand, we assume that a Parts List Management System (PLM) is implemented on top of a DBMS and further that its local agent shares its database.

An agent provides the coordination interface for a component system: It specifies which local objects are globally important, monitors the activities of the component systems and informs the global coordinator of any actions that may effect global consistency. An agent stores its information in a local database referred to as the agent repository (Rep).

The main task of the global coordinator is to ensure, with the help of the agents, the consistent state of the CIM system. The general coordination process is based on a conceptual model, referred to as the Coordination View. This model describes the component systems and the system-wide consistency constraints which are specified in terms of relationships between globally important objects of different component systems.

In addition to the representation of system-wide constraints, the coordinator must record the actions to be taken on the violations of such constraints. It is the responsibility of the local agents to react to actions delegated by the global coordinator to restore consistency. For example, assume a constraint that each part list of the PLM is associated with exactly one CAD assembly. It might be recorded that the violation of this constraint due to the deletion of a CAD assembly should result in a message being sent to the PLM agent to delete the associated part list objects. This would all be performed within a global transaction to ensure that either all of the local deletions are performed, or, the original deletion of the CAD assembly is undone. In the latter case, the user initiating the CAD delete operation would be informed of the failure of the delete and some compensating local action would be performed under the auspices of the local agent.

The issue of transaction management in CIM/Z is how to ensure the atomicity and durability of global transactions, specifically in the case where one or more component systems do not support transaction management. If a component system has no transaction support, the agent takes over the role and maintains a log of local operations which can be used to recover from any failures. In addition, we are exploiting multi-level transaction schemes to prevent conflict among concurrent transactions. Control over local transactions is gained through the local agents.

Funding

The project was funded by the KWF programme of the Swiss Federal Commission. Our partners are ABB (Asea Brown Boveri), Sulzer and Prof. Flemming of the Department of Mechanical Engineering at ETH Zurich.

contacts: Prof. H.-J. Schek