Translating ANSYS Data

ANSYS writes modal neutral files (MNFs) directly, without creating intermediate data files, using the ANSYS-developed Adams macro included with ANSYS 5.3 and higher. As of ANSYS 7.1, there is no support for modal loads or preloads. However, ANSYS 6.0 and higher does support stress/strain modes for Adams Durability.

Learn more about how to use the macro for ANSYS 5.5 and higher. For earlier versions, contact ANSYS Technical Support.

For example input files, see the directory install_dir/flex/examples/ANSYS.

Whether you invoke ANSYS in interactive or batch mode, generating modal neutral files (MNFs) are straightforward. Beginning with a well-defined structural finite-element model, you:

Because Adams can manipulate units, the MNF must contain the units you were implying during the ANSYS session. You identify the units using the /UNITS command in your ANSYS input file or in the input window during an interactive session. The command syntax is shown below where LABEL is either SI, CGS, BFT, BIN, or USER.

Units for force, mass, length, and time corresponding to LABEL values are shown in the table below.

To use a units system other than those listed above, use the USER label:

where L, M, T, and F indicate unit conversion factors from the implied units to SI units. For example, to register mm, Mg, N, and S units, enter:

After defining the units, select the attachment points using the NSEL command. The NSEL command is described in detail in the ANSYS documentation. However, for convenience, we've provided a simplified overview of its syntax:

NMIN, NMAX, and NINC are the minimum, maximum, and increment in the selected node range.

NSEL,S creates a new attachment point list, while NSEL, A adds to the current attachment point list. For example, to promote nodes 1, 21, 211, and 231 to attachment points, add the following commands to your ANSYS input file or enter them in the input window during an interactive session:

It is important to note that these commands should be made just before the ADAMS macro command, because the ADAMS macro takes the current selected nodes as the attachment points. If keypoints have been defined in the input file or during the interactive session, either the KSEL or NSLK commands can be used to select attachment points. For details concerning keypoints and the KSEL and NSLK commands, see the ANSYS documentation.

After defining units and attachment points, you can generate an MNF file using the ADAMS macro. The syntax for the ADAMS macro is:

where the required parameter NMODES is the number of normal modes to be compared. The Adams Flex Toolkit automatically orthogonalizes the normal and constraint modes, and writes the results to an MNF with the same name as your ANSYS input file or database file.

The last two arguments of the ADAMS macro are for modal stress recovery. KSTRESS is an integer flag that determines whether to include stress or strain modes. Its values are shown in the table below.

Additionally for shell models, use KSHELL to specify the location of the stress or strain results. Its values are shown in the table below.

For more information about modal stress recovery, see Adams Durability online help.

If you create the model geometry by reading in a jobname.cdb file, an error in the ADAMS macro can occur if the jobname.cdb file contains a MODOPT command. The MODOPT command prevents the spectrum analysis from correctly assembling the mass matrix and generating a correct MNF. This problem has been fixed for ANSYS 6.0. To work around the problem for ANSYS 5.6 and 5.7, you must issue the following commands after reading in the database file:

For more details, contact ANSYS Technical Support.

The ANSYS translator writes the results of the modal neutral file (MNF) export to the terminal window. Please review this data for any obvious problems. In particular, ensure that the:

Also, verify that the free body normal modes have a reasonable natural frequency. You should expect to see six rigid body modes, unless DOFs were fixed with zero displacement boundary conditions.