An investigation of constraint-based component-modeling for knowledge representation in computer-aided conceptual design

Abstract

The earliest computer programs used for engineering design focused on detailed geometric design. Subsequently, computer programs for algorithmically performing the preliminary design of specific well-defined classes of objects became commonplace. However, due to the need for extreme flexibility, it appears unlikely that conventional programming techniques will prove fruitful in developing computer aids for engineering conceptual design. The use of symbolic processing techniques, such as object-oriented programming and constraint propagation, facilitates such flexibility. Object-oriented programming allows programs to be organized around the objects and behavior to be simulated, rather than around fixed sequences of function- and subroutine-calls. Constraint propagation allows declarative statements to be understood as designating multi-directional mathematical relationships among all the variables of an equation, rather than as uni-directional assignment to the variable on the left-hand side of the equation, as in conventional computer programs. The research presented here has concentrated on applying object-oriented programming and constraint propagation to the development of a general-purpose computer aid for engineering conceptual design. Object-oriented programming techniques are utilized to implement a user-extensible database of design components. The mathematical relationships which model both the geometry and physics of these components are managed via constraint propagation. In addition to this component-based hierarchy, special-purpose data structures are provided for describing component interactions and supporting state- dependent parameters. In order to investigate the utility of this approach to conceptual design, three sample design problems from the field of aerospace engineering have been implemented using the prototype design tool, Rubber Airplane: a long-endurance surveillance aircraft, a subsonical transport aircraft, and a small-payload launch vehicle. The additional level of organizational structure obtained by representing design knowledge in terms of components is observed to provide greater convenience to the program user, and to result in a database of engineering information which is easier both to maintain and to extend.