Geometric and Feature modelling
Constraint-Based Feature Validation
Summary
Geometric modeling tools are now commonly used in various
phases of product development, for example to generate product images or
NC-code. Many applications, however, require functional information that is not
contained in geometric models.
A feature in a product model combines geometric information with functional
information, such as information about its function for the user in a design
application, or its manufacturing process in a manufacturing application. An
important issue in feature modeling is feature validation, which concerns the
meaning of a feature, given by its information content. A feature modeling
system should ensure that product modifications by a user are in accordance with
the meaning of the features.
In an ideal product development environment, multiple views of a product
coexist, each one representing the product in terms of features corresponding to
a specific application or engineering discipline. In concurrent product
development, modifications made to the product in any view should be reflected
in the other views, while the feature models in all views remain valid.
A constraint-based feature validation scheme has been
developed that provides the means to specify and maintain the validity of
features. Constraints are used to specify such feature validity conditions;
constraint satisfaction techniques are applied to maintain feature validity
under product modifications from multiple views.
For feature validity specification, various constraint types of different
domains have been identified: geometric, algebraic and topologic. Constraints of
these domains are combined in feature class specifications.
A scheme for persistent feature validity maintenance has been developed, which
is based on constraint management, integrated with management of a cellular
geometry representation. The constraint management is separated into two levels:
constraint storage and constraint satisfaction. Firstly, the constraint
management module maintains in a constraint graph the set of constraints that
result from modeling actions. Secondly, it ensures satisfaction of all
constraints by deploying several constraint solvers that are each specialized
for a particular domain.
The constraint management module maintains a coupling between the two levels,
and it propagates solving results between the constraint solvers. The advantages
of the two-level separation are that the interdependence of the constraints is
handled by the constraint management module, while the constraints are satisfied
efficiently by the solvers.
A geometric constraint solver has been developed based on extended 3D degrees of
freedom analysis. Newly developed extensions include degrees of freedom
ignorance for locus intersections, and restructuring of constraint loops. With
these extensions, powerful 3D geometric constraint solving is achieved. For the
analysis of over- and underconstrained geometric models, the concept of
dependency graph has been developed, to identify the set of involved
constraints.
Algebraic constraint solving is done with the SkyBlue solver; its constraint
priority scheme is used to propagate feature parameter changes in such a way
that the model is minimally disturbed.
Topologic constraints are checked by querying the cellular geometry
representation; it allows detection of violations of topologic feature
properties caused by feature interactions.
Product modifications are propagated between the views by link constraints that
provide a generic coupling of the feature models of the views. A view priority
scheme is used to handle conflicts between constraints of different views.
The multiple-view feature validation scheme has been
implemented in the prototype modeling system SPIFF. Feature and constraint
classes are specified declaratively in an interpreted object-oriented
programming language. SPIFF has a generic graphical user interface and provides
new facilities for feature visualization.
Constraint-based feature validation enables flexible and
expressive feature validity specifications, combined with powerful feature
validity maintenance. The validation scheme ensures that only meaningful
products can be specified, and that their validity is maintained under product
modifications from different views.
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full thesis
Dohmen M (1998), Constraint-Based
Feature Validation, PhD Thesis, Delft University of Technology, ISBN 90-9011285-5.