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vk.com/club152685050Rigid-Body Dynamics and the| vkANSYS.com/id446425943-ADAMS Interface

Do not define interface points that lie next to each other and are connected by constraint equations or short beams. This type of connection would require too many eigenmodes and result in a model that is not well conditioned.

Figure 12.1: Connecting a Structure to an Interface Point (p. 318) shows three different ways that you may attempt to attach an interface point to a structure. The first two examples (a and b) demonstrate valid methods of attachment. The third example (c) demonstrates a poor method of attachment that should not be used.

Figure 12.1: Connecting a Structure to an Interface Point

Each method depicted in the figure is described below.

a) Constraint equations are connecting the interface point to the structure. This method is recommended because:

Force is distributed over an area.

A MASS21 element is used to define the six degrees of freedom of the interface point.

Moment loads are transmitted.

b) A spider web of beams is connecting the interface point to the structure. This method is recommended (and preferred) because:

Force is distributed over an area.

No MASS21 element is necessary (because the beams supply the six degrees of freedom).

Moment loads are transmitted.

• c) One beam is used to connect the interface point to the structure. This is not recommended because:

The force is applied to the structure at a single node.

Solid elements do not have rotational degrees of freedom. Therefore, moments will not be properly transmitted from the interface point to the structure (a spider web scheme should be used).

12.4. Exporting to ADAMS

After building the model in Mechanical APDL (including all interface points), the next step is to invoke the ANSYS-ADAMS Interface to create the modal neutral file, Jobname.MNF. Creation of this file is driven by a command macro called ADAMS.MAC.

To start the interface, select the following GUI path.

Main Menu> Solution> ADAMS Connection> Export to ADAMS

 

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Exporting to ADAMS

The Select Interface Points dialog box appears first. From this dialog box, you must select two or more interface points.

Do not select too many interface points, as one point gives rise to six degrees of freedom in ADAMS. Too many interface points may lead to huge files and models.

After you confirm your selection, the Export to ADAMS dialog box appears.

Figure 12.2: Export to ADAMS Dialog Box

Complete the following steps using this dialog box.

1.System of Model Units: The units used for the model is important to the ADAMS program, whereas Mechanical APDL only requires that you use a consistent set of units. The units chosen will be written to the .MNF file and can be recalled with the ADAMS/Flex module. If no units are specified, ADAMS assumes that the same units were used in Mechanical APDL as the ones chosen in the ADAMS model. See the /UNITS command for details. If you specify user defined units, a Define User Units dialog box will appear for you to input the conversion factors (for length, mass, force, and time) between SI units and

your chosen units. Below is an example of user defined units in which the component has been modeled using millimeter, tonne (metric ton), newton, and second.

Length Factor

1 meter/milli-

=

=

meter

1000

Mass Factor =

1 kilo-

=

 

gram/tonne

0.001

Force Factor

1 newton/new-

= 1

=

ton

 

Time Factor = 1 second/second

= 1

2.Number of Modes to Extract: Input the number of normal modes to compute. Normal modes are the eigenmodes of the component with all degrees of freedom of all interface points fixed. The number of normal modes depends on the frequency range of the excitation you will apply in your ADAMS model. You must select a sufficient number of modes to represent your structure in that frequency range. In

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ADAMS, if you have chosen too many normal modes, you are able to deactivate eigenmodes based on the frequency or an energy criterion.

3.Element Results: Specify whether or not the program should write stress and/or strain results. This option has no effect on the output for beam elements. If you want to output stress and strain for only a subset of nodes, you should create a node component named "STRESS" before running the ADAMS command macro.

4.Shell Element Result Output Control: Specify the shell element output location (top, middle, bottom). This option has no effect on the output for solid elements and beam elements.

5.Filename: Specify a filename for the modal neutral file. The default name is Jobname.MNF. If a file with the chosen name exists, it will be moved to a file named filename.MNFBAK.

6.Export to ADAMS: Select “Solve and create export file to ADAMS” to initiate the solution sequence. Static and normal modes are computed and all information required by ADAMS is written to the .MNF file specified above. Only the selected elements are considered. The current model is written to the database file Jobname.DBMNF.

Note

Note that the algorithm used to compute the .MNF file adds constraints to the interface points. If you create the .MNF file a second time using the same model in the same run, be sure to delete all constraints on the interface points (or resume the database file Jobname.DBMNF) before you run it again.

12.4.1. Exporting to ADAMS via Batch Mode

If you prefer to work in batch mode, you may choose to run the ADAMS command macro by command input. After building the model and defining interface points, use the following commands to compute the .MNF file.

/UNITS,Label

! Specify the units chosen for

modeling

NSEL,...

! Select two or more

interface

points

SAVE

! Save

the

model for

a possible

resume from

 

!

this

point

 

 

ADAMS,NMODES,...

!

Activate

ADAMS.MAC

to compute

the .MNF file

See the ADAMS command description for more information. When you use command input to compute the .MNF file, there is no option to change the file name. The default name of Jobname.MNF will be used.

12.4.2. Verifying the Results

It is a good practice to verify the correctness of the results after the .MNF file is created. Below are guidelines you can use to complete this task.

Check the number of orthonormalized eigenmodes in the Mechanical APDL output window. These eigenmodes are the result of an orthonormalization of the normal modes and the constraint modes. You should observe the following:

– The number of modes equals the number of normal modes plus the number of constraint modes.

 

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