Importing and Exporting Geometry

The following compares the different CAD file formats for importing geometry using Adams View.

Adams View imports standard geometry files from any software package that supports Parasolid file formats, and converts the data into a set of Adams View geometry elements. These elements correspond to standard Adams Solver GRAPHICS statements, polylines, or shells. You associate the geometry with any rigid body part of the Adams View model before or after you run an Adams View simulation.

Importing CAD Data reduces the need to recreate geometry primitives within Adams View and enhances your ability to realistically view the behavior of complicated mechanical systems. You can also use the imported geometry for model building in Adams View and for locating parts, constraints, and forces based on the imported geometric data.

For recommendations of the best ways to export data from the CAD packages so you can import it effectively into Adams, see Recommendations for Exporting CAD Data.

Adams View exports the geometric definition of an Adams model from Adams View to the geometry file format, Parasolid. The geometry file can then be read into CAD or CAE software.

When a CAD geometry with the density attribute is imported under a part and ‘Ignore CAD density’ is not selected, the behavior is as follows,

 

 

The Initial Graphics Exchange Specification (IGES) was originally developed in 1979, by the National Bureau of Standards (NBS), to facilitate the transfer of geometry and annotations from one CAD/CAM system to another. IGES is the predominant format used for the transfer of geometry information. The IGES specification defines a structured hierarchical file format and the representation of a variety of entities within the format. The entity set includes: geometric, annotation, structure, associativity, property, and attribute entities.

The next sections explain how Adams View works with IGES files. For a detailed explanation of the IGES entity types, please refer to The Initial Graphics Exchange Specification (IGES) Version 5.0 manual.

Adams View directly supports only a subset of the geometry encountered in the IGES file. Some of the unsupported IGES entities are approximated using a technique called linear approximation. Adams View translates the nonlinear entities (for example, surface, curve, and so on) it encounters in the IGES file to polylines. For IGES curves, surfaces, and annotation entities that are nonlinear in definition, linear approximation produces a piecewise linear string of points or polygons that approximates the geometry to within a specified tolerance. For more information, refer to the discussion of the tolerance option in Import - STEP, IGES.

The import process in Adams View automatically converts all coordinate data in the IGES file to the internal units (meters, kilograms, seconds) of Adams View.

The concept of assemblies within IGES only exists within the IGES CSG definition (Constructed Solid Geometry; entity type 184). Adams View explodes all of the instances of the assembly into individual Adams parts.

There are three methods of importing assemblies into Adams View from an IGES file.

The different method are explained in the next sections.

Defining levels in the IGES file is the best method to import an assembly into Adams View. When using this method, before importing the IGES file to Adams View, place each part in the assembly that corresponds to an Adams View part on a separate level. Refer to the CAD program that you are using to generate the IGES file for information on IGES levels.

After you generate the IGES file with the parts separated by levels, import the file into Adams View. Follow the instructions in Import - STEP, IGES. or use the IGES FILE READ command once for each part in the model using the PART_NAME parameter and the LEVEL parameter to ensure that the geometry from the appropriate level is placed on the correct part. You might find it easier to use the IGES FILE READ command than using the dialog box explained in the earlier section because you can create a command file that repeats the IGES FILE READ command as necessary.

For example, if you wanted to import an assembly of a pendulum, you first generate the IGES file with the ground part on level 1 and the pendulum on level 2. The complete geometric definition of the pendulum assembly exists in the IGES file but the levels identify the geometry on the ground part and the geometry on the pendulum part. Execute the following commands to read the IGES file and associate the desired geometry with the appropriate part:

FILE GEOM READ TYPE=IGES FILE= "pendulum.igs" PART_NAME= /pend/ground LEVEL= 1

FILE GEOM READ TYPE=IGES FILE= "pendulum.igs" PART_NAME= /pend/mass LEVEL= 2

The second method to import an assembly is to generate a separate IGES file for each part in the assembly. For the previous example, you would generate two files: pendulum_ground.igs and pendulum_mass.igs. You would then execute the following commands to read the IGES files and transfer the geometry to the appropriate part:

FILE GEOMETRY READ TYPE=IGES FILE= "pendulum_ground.igs" PART_NAME= /pend/ground

FILE GEOMETRY READ TYPE=IGES FILE= "pendulum_mass.igs" PART_NAME= /pend/mass

This method has the advantage of not requiring the existence of levels in the IGES file but does require that you create multiple IGES files.

The third method is to import the geometry for the entire assembly onto one part and then use Adams View commands to move the geometry to the appropriate part. This method is tedious if the IGES file contains a large number of parts or a large number of geometric entities.

Adams View generates a log file containing a list of any warnings or errors that may have occurred during the import into Adams View. Adams View creates the log file in the current directory and assigns it the same name as the IGES file with the extension .log.

Adams View does not support mass properties for IGES files if you import the geometry as a polygon or if the original shape is not solid.

Adams View does not support subfigures. Therefore, Adams View does not import any geometry defined as subfigures in an IGES file.

When Adams View exports geometry in IGES format, it places the geometry for each Adams View part on a separate level within the IGES file. This provides for easier handling of geometry for individual parts after the IGES file has been written.

Adams View uses the Adams ID of the part as the integer ID identifying the IGES level. It writes dynamic geometry (graphic spring dampers and outlines that span multiple parts) to a separate level with an integer ID that is equal to 1 greater than the highest Adams part ID.

Adams View writes the colors of geometric entities to the IGES file. It supports any color defined within Adams View. The RGB (red, green, blue) values are stored as entity number 314, Color Definition, in the IGES file. Note that not all IGES vendors support RGB color values.

Adams View translates invisible geometric objects to the IGES file as blanked entities. The visibility of the object as it appears in Adams View determines how it is interpreted when written to the IGES file as shown in the table below.

A summary of the supported IGES entities is shown in the table below. Note that the three-dimensional graphic icons representing Adams modeling elements (joints, and so on) are not translated to the IGES file.

STEP is officially known as ISO 10303 - Exchange of Product Model Data. A summary of STEP is provided in the ISO document, STEP Part 1: overview and fundamental principles. The following is a paragraph from the ISO document describing STEP format.

The Standard for the Exchange of Product Model Data (STEP) is a neutral mechanism capable of completely representing product data throughout the life cycle of a product (Neutral in this context indicates independence from any particular CAx software system). The completeness of this representation makes it suitable not only for neutral file exchange, but also as a basis for implementing and sharing product databases and archiving. There is an undeniable need to transfer product data in computer-readable form from one site to another. These sites may have one of a number of relationships between them (contractor and subcontractor, customer and supplier); the information invariably needs to iterate between the sites, retaining both data completeness and functionality, until it is ultimately archived. The most cost effective manner to encapsulate such information is in a neutral format, independent of any Computer-Aided (CAx) software system.

Contact the U.S. Product Data Association (USPRO) for more information on STEP. Their address is P.O Box 3310, Gaithersberg, Maryland, 20885-3310. Their phone number is 301-975-4658, their FAX number is 310-926-8730, and their e-mail is uspro@scra.org.

Adams calculates mass properties for parts using STEP geometry by multiplying the volume of the STEP geometry by the material density defined in the Adams part. Mass properties that are explicitly defined in Adams will continue to be used.

Adams View supports STEP applications protocols AP 203 and AP 214 for import and AP 203 for export. A summary of the supported STEP entities is provided in the table below. Note that the three-dimensional graphic icons representing Adams modeling elements (joints, and so on) are not translated to the STEP file.

In addition, note that we do not support conformance class 5, which contains faceted B-rep shape representations. AP 214 is still in the committee-draft stages of its development and, therefore, we do not encourage its use.

The Wavefront file is unitless even though most of the ViewPoint graphics internally use one hardcoded unit system. You will need to scale the vertices to make them fit the units set in the Adams View model. For example, if the Viewpoint graphics were originally created using the length unit, mm, then, if your Adams View session units are in meters, you’ll use a scale factor of .001.

You may find that you need to experiment with different scale factors to create a model that contains practical scales. To experiment with the different scale factors, just undo the import if it does not work using the Undo command.

Before importing the graphics, create a dummy part to which you’ll attach the Wavefront graphics. Creating dummy parts is helpful because the orientation within the Viewpoint dataset is usually different than the orientation of your model. Dummy parts provide an easy method of moving and rotating the graphics to match your Adams model. To move the graphics, you just move the dummy part.

Here is a procedure for ensuring that the geometry is attached to the correct part:

A convenient way to read in an entire Viewpoint dataset and avoid some of the problems mentioned earlier is to reduce the number of group names in the dataset before importing it into Adams View.

Generally, groups in the Wavefront file become parts in Adams View. If a part already exists that has the same name as a group, however, the graphics simply attach to the existing part without creating a new part. If Adams View finds a group with no corresponding part, it creates a new part for the group.

If you have multiple groups with the same name in the Wavefront file, all groups attach to the same Adams View part but become separate geometric entities. This allows you to use different colors within the same Adams part to create more realistic images.

The only lines that are significant in the Wavefront file are the ones that begin with g (groups), f (facets), and v (vertex). Adams View ignores everything else.

To reduce the number of group names, open the dataset in a text editor and substitute names before importing the set into Adams View. For example, a car body dataset typically has the following group names:

Change them to the same name so they attach to a single part. For example:

The following sections provide recommendations for how you should export data from different CAD packages so you can best import them into Adams:

For those CAD systems based on the Parasolids kernel, there are many benefits to transferring geometry in Parasolid files. Adams View creates solid bodies from the Parasolids information that allows for further Boolean operations as well as the selection of geometric features such as the center of a circle. It is important that you export the version of the Parasolids file that Adams View supports.

In general, the simpler the representation of the geometric information the better. This is why we often recommend that you used Stereolithography over formats such as IGES and STEP. Other simple formats include render and shell files. Shell files are Adams-specific so you will not find them as part of the standard CAD export formats. Please remember that some of these formats do not have colors.

Successful transfer of IGES and STEP files from CAD systems to Adams View depends on both systems. Some CAD systems export information that is easily processed by Adams View while others CAD systems generate files that are difficult or impossible for Adams View to import. We have taken this into consideration in the Recommendations for Exporting CAD Data.

The following table provides recommendations for the file formats in which you should export data from a CAD program so you can import it effectively into Adams. Please note that the recommendations are in order of preference. For a comparison of the different file formats, see Comparison of CAD File Formats.