Geometric and Feature modelling
Assembly Features in Modelling and Planning
Summary
Computer programs can nowadays hardly be left out from the
complete assembly process. Not only during modelling of assemblies, but also
during planning of the actual assembly, programs are indispensable. In fact,
modelling and planning can hardly be separated. To avoid problems in the
assembly process, models should be verified in an early stage with plans to
check whether they can actually be assembled.
Both the modelling of single-part components and assemblies, and the modelling
and planning phase must be integrated.
Current generation models are highly focused on geometry.
However, only geometric information is not enough in modelling and planning.
Besides geometric information, there is also a need for functional information
related to the geometry. In recent years, techniques have been developed for
modelling and planning in manufacturing that combine geometric and functional
information for single-part components. These models are called feature models,
where features contain both geometric and functional information.
This feature concept is not only useful in manufacturing, but can also be used
in assembly, as is shown in this thesis. Assembly features are subdivided into
handling and connection features. Handling features contain assembly-specific
information for handling components. Connection features contain
assembly-specific information for connections between components.
Both for modelling and planning of single-part components and
assemblies, an integrated object-oriented product model has been developed,
combining form features with handling and connection features.
A prototype modelling environment has been developed. A model can be manipulated
using a geometric or a graph-oriented user interface. The model itself provides
the possibilities for component-driven and relation-driven modelling. In the
first method, one starts with completely finished components, and by adding
relations to them the final assembly is created. In the second method, one
starts with the relations, and from these relations the components can be
created.
The product model has been verified within several analysis
and planning modules.
Assembly features can profitably be used within assembly analysis. Especially
the internal freedom of motion between components, stored in the connection
features, accelerates the translational stability analysis. Rotational stability
analysis can also be accelerated by using knowledge of possible rotation axes
stored in the connection features.
In grip planning, during the determination of finger domains on components, the
form features and the assembly features play a significant role. Although the
time-consuming EFS method is still needed, the feature models can in many cases
provide a considerable time reduction.
Further, feature models provide additional information that can profitably be
used in compliant motion planning. Within connection features, the type of
connection is known, which is important for compliant motions.
Also the complete search space to find the optimal assembly
sequence can be substantially reduced, resulting in faster generation of the
optimal assembly sequence. Some connection features already contain knowledge
about possible, or impossible, assembly sequences themselves.
It can be concluded that feature-based product models for
assembly can considerably help both in assembly modelling and planning, on the
one hand in integrating single-part and assembly modelling, and on the other
hand in integrating modelling and planning.
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full thesis
van Holland W (1997), Assembly
Features in Modelling and Planning, PhD Thesis, Delft University of
Technology, ISBN 90-9011056-9.