Modelling constraints in part structures

Data modelling plays a pivotal role in Computer Integrated Manufacturing (CIM) as a vehicle for integration among the various component systems and users. Standardization in the representation of product information is receiving growing attention in recent years. In data modelling, the concepts of generalization, aggregation and classification are well known. They form the foundation of many, if not most, semantic and object-oriented models. Class hierarchy and the attendant inheritance semantics provide powerful modelling mechanisms whereby a more effective schema can be constructed in which common structural and behavioural specifications are shared. Aggregation has mainly been treated as an information structuring concept in most models, with focus on how complex-valued properties can be accommodated. In the application of computers in such areas as CAD/CAM and Geographic Information Systems where the structure of objects is of primary importance, the current aggregation semantics are inadequate. In this paper, we examine the aggregation concept from a number of perspectives. We discuss a proposed model of attribute semantics which can more effectively support the definition of constraints and derived data in an aggregation hierarchy. In the model, object attributes are categorized according to their meronymic type. The attribute categories serve as expansion scope for generating attribute references on a composite object. In the latter part of the paper we illustrate how part-of semantics can be incorporated into the current international standard for manufacturing data exchange of STEP/EXPRESS.