Handling CAD data in VDAFS format

VDA surface interface (VDAFS)

VDAFS is an interface to exchange data between different CAD systems or to exchange data between CAD systems and other applications, where the data exchange of free surfaces and bounded surfaces is especially important.

VDAFS is developed by the Association of Automobile Industry (VDA). For a detailed documentation of the interface see DIN 66301.

Following picture shows a CAD model of a container built out of 8 bounded surfaces (see demo file vdafs.vda) .

Using MAKROS-A following types of VDAFS elements can be read from a VDAFS input file and used to generate a finite element model:

POINT: (x,y,z)- coordinates of single points

PSET: sequence of single points

CIRCLE: arc of a circle

CURVE: curve in 3D space

CONS: curve on a free surface

SURF: free surface in 3D space

FACE: bounded surface

MAKROS-A especially provides methods to generate interactive macro elements on bounded surfaces (see demo file “vdafs.dem”).

Handling of different VDAFS element types with MAKROS-A

POINT, PSET, CIRCLE, CURVE

VDAFS elements of type POINT and PSET are handled in MAKROS-A as point elements (element type 1), they can be used to transfer coordinates of single points. VDAFS elements of type CIRCLE and CURVE are approximated by one or more macro elements of type 22 (arc element).

FACE and SURF

Bounded surfaces are defined in VDAFS as free surfaces (SURF element) wit one or more closed curves (CONS elements) on the surfaces. SURF elements and CONS elements mostly consist of more than one segment. Plotting SURF elements, inner edges of segments are shown as dotted lines. Following picture shows one SURF and CONS element together with automatically generated nodes at the corners of SURF segments and CONS segments:

The generation of macro elements on one bounded surface is done in MAKROS-A by the following steps:

1) Generate nodes on the surface. There are different means to do this (see dialog “Node generation”).

2) Define macro elements using these nodes (see dialog “Element definition”). It is important that macro elements of neighbouring surfaces use on the common edge nodes that have nearly identical coordinates. Generating the entire macro model, these nodes are merged into one single node. To identify nodes of macro elements of neighbouring surfaces, corner nodes of macro elements are marked by the symbol “E” and intermediate nodes on the edges of macro elements are marked by the symbol “Z”. Following picture shows the macro elements of two neighbouring surfaces where the defining nodes are marked “E” or “Z” respectively:

3) Generate a macro model (see dialog “Macro- / FE structure”). After all VDAFS surfaces have been approximated by macro elements, the entire macro model is generated using the macro elements of all surfaces, where the nodes on the edges of neighbouring surfaces are merged into one node, if they have equal coordinates within a given tolerance.

4) Subdivide the macro elements into finite elements. This is done as with an ordinary macro model (see chapter “Subdivision of macro elements into finite elements”).

5) Smooth the finite element model if necessary (see dialog “Macro- / FE structure”). The calculation of finite element nodes when subdividing the macro elements is done by C0 – Coons interpolation between opposite edges of the macro element. Depending on how large and strongly curved the macro elements are, the generated nodes will differ from the geometry of the originating SURF element. This difference may be eliminated by smoothing the finite element model. The following algorithm is used: For each macro element an optimal plane is determined, where the nodes of the finite elements belonging to the macro element are projected. Then, nodes on the originating SURF element are calculated, that have the same projection on the plane. These nodes on the SURF element replace the finite element nodes calculated by the Coons interpolation. To determine an optimal plane, no small macro elements must be used, also the macro elements must not be to strongly curved.

Following picture shows the generated macro model and final finite element model of the container shown above (see demo file “vdafs.dem”).

Dialogs to handle VDAFS data

The dialogs to handle VDAFS data are grouped together in a window using the following pages.

File	Read VDAFS input file, save data to or load from a binary file
Plot	Plot VDAFS elements
Node generation	Generate nodes on bounded surfaces
Element Definition	Define macro elements on SURF elements
Macro- / FE structure	Generate a macro and FE model, smooth the FE model
Select / Remove	Define a VDAFS element selection, remove VDAFS elements

The window is activated by the command “VDAFS data” in the menu group “Generation” and deactivated using button “Cancel”.

As long as the window is active all other commands to modify the macro or finite element model are disabled.

Attention: Before a new VDAFS input file is read or a VDAFS binary file is loaded, all data that is actually in memory (macro model, FE model, VDAFS data etc.) is removed and must eventually be saved to disk. If several VDAFS files shall be combined in one finite element model, first one macro and FE model must be generated for each VDAFS file. Then, these structures can be combined in one structure using command “Load structure”.