The Modal Correlation add-in of Simcenter Testlab Neo software compares dynamic vibration modes by calculating the Modal Assurance Criterion (MAC) between various combinations of simulation and physically measured modes (Figure 1).
Figure 1: Simcenter Testlab Neo Modal Correlation visually compares modes of vibration (left) and generates a Modal Assurance Criterion (MAC) table (right) to quantify their similarity.
Possible comparisons include:
Test Modes versus Test Modes: After performing a modal test, the identified modes can be compared to each other as a quality check. If the MAC table has high off-diagonal values, it suggests an insufficient number of measured points on the test object to uniquely define distinct modes. This is referred to as “spatial aliasing”.
Test Modes versus Finite Element Modes: Ideally, a perfect match between finite element and test modes would result in diagonal MAC matrix values of 100% indicating identical mode shapes between simulation and test. The frequencies of the modes should match as well.
Finite Element versus Finite Element Modes: When developing a dynamic finite modal model, engineers encounter numerous modeling choices (e.g., RBE2, RBE3, Glue connections). Simcenter Testlab Neo allows for the evaluation of how these different modeling decisions impact structural modes. This is useful for determining if a more computationally efficient modeling approach can still accurately represent the dynamic behavior.
This article covers Simcenter Testlab Neo Modal Correlation background and usage: 1. Modal Correlation Background 1.1 What is a MAC? (Modal Assurance Criterion) 1.2 Test Modes versus Test Modes 1.3 Test Modes versus Finite Element Modes 1.4 Finite Element Modes versus Finite Element Modes 2. Getting Started in Simcenter Testlab Neo 2.1 Getting Started 2.2 Importing Data 2.2.1 Experimental Test Mode Shapes and Geometry 2.2.2 Simulation Mode Shapes and Geometry 2.3 Working and Reference Geometry and ModeSets 3. Aligning Geometries 3.1 Aligning Geometry by Proximity 3.2 Aligning Geometry by Point ID 4. Results 4.1 Calculating MAC 4.2 Selecting MAC Calculation Type 4.3 MAC Table 4.4 Side-by-side animation
1. Modal Correlation Background
One of the primary objectives of Simcenter Testlab Neo Modal Correlation's primary objectives is to compare experimental modal results with other modes from the same structure or a simulated structure (Figure 2).
Figure 2: Comparison of mode shape results from a physical test on a real airplane (left) with its simulation model (right).
Comparing modal test results from a physical prototype with a simulation model helps ensure the simulation's accuracy.
A MAC evaluates the similarity between two mode shapes:
A MAC value of 100% means the shapes are identical.
A MAC of low values close to zero means the shapes are different.
When a mode shape is compared to itself, the Modal Assurance Criterion value is 100% (Figure 3):
Figure 3: Identical mode shapes (left and right) yield a 100% MAC value, indicating perfect correlation.
For modes with different shapes, the MAC is less than 100%. Very different shapes will have a value close to zero as shown in Figure 4.
Figure 4: Distinct mode shapes, such as bending (left) and torsion (right), result in a low MAC value (e.g., 1.5%).
The MAC value is calculated by performing a dot product of the modal vectors at each node between two mode shapes. This calculation quantifies the similarity between mode shapes from different models or test results.
A typical MAC analysis generates a table for all combinations between two different sets of modes. The MAC value for every mode pair combination is displayed numerically and with colors (Figure 5).
Figure 5: MAC Table comparing two mode sets. Each cell's color indicates the MAC value for a specific mode pair, visually representing their similarity.
In the MAC table, a set of modes is compared to another set of modes:
One set of modes (and the mode frequencies) is listed vertically on the left side.
The other mode set (with frequencies) is listed horizontally across the top.
The shape of each mode in one set is compared to all the mode shapes in the other set. The result is a colored cell indicating the similarity in shape between the different mode pairs.
Hotter colors (e.g., red, orange) signify highly similar mode shapes, whereas cooler colors (e.g., blue, green) denote distinct or unique mode shapes. The numerical MAC value is also displayed within each cell.
In experimental modal analysis, there are two scenarios where MAC calculations can provide some insight:
Test Mode Set Compared to Itself: After performing a modal test, if the MAC table exhibits high off-diagonal values, it suggests that insufficient measurement points were used on the test object. This phenomenon, known as “spatial aliasing”, indicates that measuring at more locations on the structure would be beneficial to uniquely identify each mode shape.
Two Different Sets of Modal Test Results: The results of two different modal tests can be compared. This can be useful to see how modes changed after a structural modification was made to a test object. This might help indicate if a vibration issue due to a resonance in the original structure has been affected and mitigated.
1.3 Test Modes versus Finite Element Modes
Modes from a simulation modal model can be compared to test based mode shapes. Ideally, when comparing a test mode shape to its corresponding simulation mode shape, the MAC value should be high (typically above 90%).
However, a high MAC value alone does not guarantee full correlation between the simulation and test modals. It is equally important to compare the frequencies of the corresponding mode pairs. Ideally, the frequency values should also match closely. For instance, even if two modes show a 100% MAC value, a significant frequency difference (e.g., 30 Hz versus 101 Hz) necessitates further investigation.
A complete simulation modal solution comprises both the geometry and the modal solution set. Currently, Simcenter Testlab Neo supports simulation models for modal correlation from:
To enable Simcenter Testlab Neo to read these simulation files, the Simcenter 3D driver must be installed. Find more information about the Simcenter 3D driver and its functionality in this article: Simcenter 3D Driver for Simcenter Testlab.
1.4 Finite Element Modes versus Finite Element Modes
When creating a dynamic finite element modal model, there are many modeling choices to be made (RBE2, RBE3, Glue connections, etc). The effects on the structural modes due to different modeling choices can be evaluated.
For example, if two different modelling approaches yield similar mode shape results, then the more computationally efficient approach can be used.
2. Getting Started in Simcenter Testlab Neo
Modal correlation functionality is available in Simcenter Testlab Neo versions 2306 and higher. To perform a modal correlation, calculated mode shapes from a physical test or simulation model are needed.
2.1 Getting Started
To get started, open the Simcenter Testlab folder and then open the “Desktop Neo” application.
Next, go to “File -> Add-ins” and ensure that the “Modal Correlation” add-in is loaded as shown in Figure 6.
Figure 6: Under “File -> Add-ins” turn on “Modal Correlation”
Simulation mode shapes and geometry are accessed by navigating to the file on the local hard drive or mapped network drive (Figure 8):
Figure 8: Nastran OP2 file with mode set and geometry on local hard drive.
To utilize simulation modes in Simcenter Testlab, links to the simulation files (Ansys *.rst, Nastran *.op2, Abaqus *.odb, etc) can be made. This requires installing a separate “Simcenter 3D driver”. It is also important to identify the unit system (Example: mm-ton-seconds) of the simulation data.
Usually simulation models are outside the Simcenter Testlab project and simply linked/referenced from the Modal Correlation add-in to use them.
More knowledge articles on using simulation data in Simcenter Testlab:
This section covers how to set a geometry and mode set:
Simulation modes will be set as a “Reference Geometry” and as a “Reference ModeSet”.
Experimental mode set for comparison will be set as “Working Geometry” and “Working ModeSet”.
In the “Modal” task, within the “Align Geometries” subtask, browse to the data in the file tree. Right click on the simulation geometry and select “Set reference geometry” (Figure 9).
Figure 9: Setting the reference geometry from a Finite Element model.
In the file tree, find the geometry associated with the test data acquired. Right click on the geometry file and select “Set working Geometry” (Figure 10).
Figure 10: Setting the test data as the working geometry file.
The next step is to find the mode sets and set those as reference and working mode sets. In the file tree, navigate to the simulation mode set, and select “Set reference Modeset”. Repeat this with the modes found from measuring in Simcenter Testlab and set these to “Set working Modeset” (Figure 11).
Figure 11: Setting the reference mode set from a finite element simulation (left) and selecting the working mode set (right).
The lower right corner of the "Align Geometries" tab indicates how many points found a match (Figure 12):
Figure 12: Lower right corner 17 out of 20 nodes found matches.
A node corresponds a measurement location or finite element node.
If the modal test and simulation were performed independently, it is common for not all nodes to match perfectly. The goal is to match as many nodes as possible. To increase the number of matched nodes, aligning the geometries is important.
Further insight into where there are alignment issues can be visualized using the "Highlight" button in the ribbon (Figure 13):
Figure 13: Pressing the "Highlight" button helps identify where nodes did not find a match. Blue nodes found a match.
There are tools in Simcenter Testlab Neo Correlation to align geometries that are not aligned in the same co-ordinate system. The tools available to perform the alignment are described in the next section.
Aligning geometry (Figure 14) in Simcenter Testlab Neo Modal Correlation can be done in multiple ways depending on how the CAE model was set up during the simulation.
Figure 14: Alignment of test and simulation geometries include scaling, translation, and rotation.
There are three ways to align the geometries in Simcenter Testlab Neo’s Modal Correlation Add-in.
3.1 Aligning Geometry by Proximity
The “3-point approximation” method aligns geometries by selecting at least three pairs of nodes between the simulation model (Reference) and the test model (Working) to align them.
3.1.1. “Align Geometries” tab
In Simcenter Testlab Neo, navigate to the “Modal” task and the “Align Geometries” subtask (Figure 15).
Figure 15: The “Align Geometries” tab is part of the Simcenter Testlab Neo “Modal” task.
In the “Align Geometries” tab (with the “Reference” and “Working” geometries defined as shown previously):
Click on the drop-down menu in the Home ribbon at the top of screen and switch from “By Point Id” to “By Proximity”.
Then click the “Geometry alignment” button (Arrow pointing to cube icon) located in the lower left of the state control bar at the bottom of the screen.
Begin by clicking a node on the working geometry, then click on a corresponding node on the reference geometry (Figure 16).
Figure 16: Click on two corresponding nodes from both geometries. Each node turns red to indicate it is selected.
When a node is selected, it will turn red. To add the pair of corresponding nodes, click the “Add node pair” button in the state control bar.
After selecting at least three pairs of nodes, press the “Stop” button. It is best to select node pairs at the extremes of the geometry to get a good overall mapping. Selecting node pairs concentrated close to each other on small area of the structure can make the transformation difficult.
There is no set number of nodes required, and not all nodes must be paired for correct mapping If nodes are matched, the node count in the bottom corner will show how many have been successfully aligned.
3.1.2 Overlay, Transformation Parameters, and Tolerance
To overlay the geometries, use the “Overlay Display” (Figure 17). If it is not showing, click on the tab of the same name on the right side of the screen.
Figure 17: Overlaying geometries using the “Overlay Geometry” display. All nodes are paired as indicated in the state control bar.
Right click in the overlay display and choose “Fit Model” to see the geometries.
The “Transformation Parameters” (Figure 18) shows the name of the paired nodes in list form, and the mathematical transformation (scaling, translation, and rotation) used to align both geometries.
Figure 18: The “Transformation Parameter” tab is located on the right side of the screen.
Listing the calculated transformation parameters is useful. For example, if the simulation geometry was in millimeters and the test geometry in meters, the calculated scaling factor should be 1000. If it deviates from 1000, the alignment may need to be redone for a better match.
Sometimes a simulation model may be made in different units or was not scaled the same as the test model. In this case, the tolerance can be adjusted to help map nodes to each other. Tolerance can be changed in the Home ribbon in the “Node Mapping” section (Figure 19).
Figure 19: The tolerance setting in the ribbon defines a spherical diameter around each node. The closest corresponding node that falls within the sphere gets mapped.
To see the list of nodes that are paired with the model, the “List” option in the ribbon will display mapped nodes. These can be manually edited in the “Transformation Parameters” menu, as long as a user is actively clicking to pair modes. To import a list of nodes from a file, the “Import” button in the ribbon supports *.JSON and *.XML files.
To save time when working with similar models, the 'Export' button exports paired nodes, which can then be imported for future tests.
3.2 Aligning Geometry by Point ID
If geometry was created to ensure that the Point ID (e.g., node names) matching the simulation file, aligning by “Point Id” is an option. Start by selecting “By Point ID” in the geometry mapping ribbon (Figure 20).
Figure 20: Choose “By PointId” in the ribbon if the two geometries have the same node names.
Here, geometries with identical Point ID names are automatically mapped by the software. For example, both geometries might contain the same namde “1873”, “2321”, etc.
To check the names, press the “List” button in the geometry mapping ribbon (Figure 21).
Figure 21: Using the Map list to show that the node names are an exact match (0 meters away).
A list of matched nodes based on name will populate. Nodes can be manually changed by double-clicking in the 'Paired Nodes' table in the ribbon and editing values.
4. Results
Once the geometric alignment and mapping are finished, the MAC table can be calculated.
4.1 Calculating MAC
Click on the “Modal Correlation” subtask. The MAC table should automatically generate (Figure 22).
Figure 22: A MAC table is calculated by moving to the “Modal Correlation” subtask.
The number of correlated DOFs (Degree of Freedom) is shown in the lower right, display as correlated versus the total:
Not all DOFs need to be mapped for correlation. Target as many DOFs as possible.
Low numbers of DOFs will result in an inaccurate MAC matrix, and animations shown may not be correct.
There is a difference between a “Node” and a “DOF Id”:
Node: A single location, such as an accelerometer location or finite element.
DOF Id: There are three DOF Ids per node. Modal vectors are in three directions (X, Y, Z) at a node.
The number of DOFs can be three times larger than the number of nodes. For example, if at a given node only one out of three possible directions was measured, than the number nodes would equal the number of DOFs for the measurement point.
4.2 Selecting MAC Calculation Type
The modes used for the MAC table calculation depend on the selection in the ribbon (Figure 23):
Figure 23: The Home ribbon offers three distinct MAC calculation options.
The options are:
MAC: Calculates a MAC table of working mode shapes compared to the reference mode shapes. This compares simulation modes in the reference set to test modes in the working set.
Working Auto MAC: Calculates a MAC table of working mode shapes compared to themselves. For example, if the working geometry and mode set are from test, this would be a useful post-test analysis.
Reference Auto MAC Calculates a MAC table of the reference mode shapes compared to themselves.
4.3 MAC Table
An example MAC table is shown in Figure 24.
Figure 24: MAC table compares the reference and working mode sets (listed vertically and horizontally) against each other.
As explained previously, values in the matrix closest to 100 (red, orange colors) indicate the most similar modes, while distinct modes have values closer to zero (blue, green colors).
4.4 Side-by-Side Animation
To animate mode shapes from the reference and working sets side-by-side, click a mode in the list or a box in the MAC matrix (Figure 25).
Figure 25: Animating a mode shape from the MAC table in Simcenter Testlab Neo Modal Correlation
Viewing the modes side by side can give insight into which physical areas of the structure could be modeled better or re-tested.
Animation settings can be changed in the animation menu at the bottom left corner of the screen.
With 'Linked Views' turned on in the display section of the ribbon, both geometry models move together in the same direction when either one is manipulated (Figure 26).
Figure 26: When views are “Linked” both the working geometry with the reference geometry will rotate together.
This is useful for viewing mode shapes at different angles. To manipulate models individually, click the 'Unlink Views' button in the Display menu.
