Post on 22-Jan-2016
description
transcript
June 29, 2007
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The difference between twofeature models
Matthijs Sypkens SmitWillem F. Bronsvoort
CAD ’07 Conference, Honolulu, Hawaii
Faculty of Electrical Engineering, Mathematics and Computer Science
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Outline
• Research motivation• Feature modelling• The feature difference• Modelling the feature difference• Application: efficient remeshing• Conclusion
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Research motivation 1
Efficient repeated processing of large models
In particular: remeshing for FEA after model modification
CAD model FEA mesh
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Research motivation 2
Model modification:
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Research motivation 3
Meshes for variants of model:
25000 points; 128,521 tets 25000 points; 128,751 tets
~1000 tets infeature
~1900 tets infeature
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Research motivation 4
1. Common practice:
Full mesh generation each time
2. Our goal:
Remeshing of previous mesh
meshing
meshing
modify model remeshing
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Feature modelling 1
• Current product modelling systems use feature models
• Products are represented with features: holes, slots, pockets, protrusions, etc.
• Features have a generic shape that is controlled through parameters
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Feature modelling 2
Modification of feature models:• Parameter values / constraints• Addition and removal of features
As a result: change in geometry
Our aim: a description of the difference that facilitates efficient remeshing
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Adapting a model: Deriving a new mesh:
Intuitive solution:Let features carry their geometry (and mesh) with them
The feature difference 1
?
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When feature geometry is preserved: mesh local to that feature can be copied
Complications for change in interaction/attachment: local changes to feature geometry:
The feature difference 2
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The feature difference 3
How to describe the geometric difference?
Look from point of view of the features
Natural choice:the variation of the model is through the features
For each feature the local change in geometry is recorded
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The feature difference 4
Copying parts of the mesh:Parts can only be copied when underlying geometry can be mapped between models
• Geometry that can be mapped is persistent• To find intuitive persistence we look at
the feature geometry ( ≠ BRep geometry )
Geometry that cannot be mapped is non-persistent
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• Features’ own geometry is persistent, unless reshaped or not present in both models
• Change in interaction non-persistent geometry
• Manifestation of persistent geometry can change
The feature difference 5
model 1
model 2
Looking from the point of view of a single feature:
persistent
non-persistentpersistence according to the baseblock:
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The feature difference 6
The difference for elements of each feature:
persistent (P) non-persistent (N)
manifestationidentical (Pi)
manifestationdifferent (Pd)
model 1 (N1)
“old”
model 2 (N2)
“new”
[geometry]
[manifestation = on bound./in volume]
( Pd1 / Pd2 )
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The feature difference -2D Example (1)
• Feature F1 has a change of interaction with attached feature F2
• For feature F2 all remains the same
Relocating a feature:
Pi identical
Pd1 bound. in 1
Pd2 bound. in 2
N1 only in 1
N2 only in 2
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The feature difference -2D Example (2)
• Feature F1 has a change of interaction due to new feature F3
• Feature F3 is completely new to the model
• For feature F2 all remains the same
Adding / removing a feature:
Pi identical
Pd1 bound. in 1
Pd2 bound. in 2
N1 only in 1
N2 only in 2
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The feature difference - reshapingHow to handle changing feature shape?
“Self-interaction”
Solution not unique!
Align on fixed reference point consistent, deterministic
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The feature difference -2D Example (3)
• Feature F1 has a change in interaction with F2
• Feature F2 has been scaled and translated
• For feature F3 the interaction with F2 changes
Combining translation, reshaping and negative nature:
Pi identical
Pd1 bound. in 1
Pd2 bound. in 2
N1 only in 1
N2 only in 2
Note:F3 is a hole
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Modelling the feature difference 1Two main steps:1. Non-regular union
merge of objects; all original entities are kept
For the complete geometry of corresponding features
Implementation on top of geometric modelling kernel (ACIS)
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Modelling the feature difference 2Two main steps:1. Non-regular union2. Categorisation of entities Pi, Pd, N1, N2
Start union: default N1/N2
On merge (Vertex-Vertex,E-E, F-F, C-C):comparison Pi /Pd
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Modelling the feature difference 3• The difference model is the set of all individual
feature differences• Complete explicit construction not necessary:
Many features will be 100% persistentNew or deleted features are 100% non-persistent a single attribute is sufficient for those featues
• Feature correspondence between models essential
difference model
input
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Application: efficient remeshing 1
Assumptions:• Model modifications influence geometry only
locally• Considerable degree of feature correspondence• Mesh generation optimisation based
time consuming construction
limited, local change
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Application: efficient remeshing 2
Original mesh:Variational tetrahedral meshing (Delaunay connectivity)
Sketch of a remeshing approach:1. Construct difference model2. Per feature, copy points based on persistent volume3. Mesh new geometry4. Mark points on/near non-persistent geometry5. Optimise marked points
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Conclusions 1
• Feature point of view leads to natural/intuitive difference
• Feature difference applies to feature aspects in general any attribute local to a feature can be compared
• Difference model and remeshing handle changes in topology!
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Conclusions 2
• Speed-up when remeshing similar models for FEA intended for quality meshes of large models
• Easier direct comparison of FEA result persistent regions with largely identical mesh
Open for investigation:• Practical investigation of remeshing (work in
progress)• Dealing with mesh sizing• Other meshing algorithms / mesh types
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Credits
Research supported by NWO(Netherlands Organisation for Scientific Research)
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