A Function-Behaviour-Structure Machine Design Model and its use in Assembly Sequence Planning

By expanding some concepts behind the Edinburgh Designer System (EDS), the paper proposes a function-behaviour-structure-based representation scheme for mechanical systems and assemblies (MSAs) that possibly supports both conceptual design and design for manufacturing. A machine structure might be viewed as being composed of two basic elements: components and connectors. Components are concrete elements such as a step shaft or a gear. Connectors are abstract elements corresponding to fastening, joining, constraint, or other functions and behaviours which are placed between two components. At the different design stages, a connector could be a form feature, a physical component or even a sub-assembly. Functional and behavioural descriptions are embedded in the structural model as predicate logic. This 'integrated design object model' provides an insight into machine design. One of the applications of such a representation is to offer designers on-line help for evaluating product assemblability by automatically generating an assembly sequence. The paper describes the problem of an assembly sequence as the sequentially movable separability of a collection of objects. A three-step alogrithm was developed to reason about assembly sequence. In the first step, sub-assemblies are recognized from the design object model using knowledge-based rules. In the second step, the precedence between sub-assemblies is obtained using the depth-sort method. Then the whole machine structure is disassembled through two kinds of atom operations: sequential translation and rotation in one direction needed for disassembling. It is successfully applied to typical mechanical products in the automotive industry such as a pump or a differential gear box.