In experimental modal analysis, after collecting a set of Frequency Response Function (FRF) data on a structure, the next step is to extract a meaningful set of modes and their associated modal parameters including:
This process of extracting this information from the FRFs is referred to a modal curvefitting or modal parameter estimation (Figure 1).
This article is an introduction on how to use Simcenter Testlab for modal curvefitting. It has several parts:
Background and Considerations
Modal curvefitting measured FRFs is a tall task! There are several considerations (shown in Figure 2):
To help with modal curvefitting, software such as Simcenter Testlab Modal Analysis can assist in the process. Using a Stabilization Diagram as a guide, the best mathematical modal model (frequencies, damping, etc.) that describes the FRF data set can be determined.
Before detailing the Simcenter Testlab software, an analogy might help to better understand curvefitting.
XY Dataset Analogy
To illustrate the curvefitting process, this analogy uses a set of X and Y data as shown in Figure 3.
The goal is to calculate a curve that best describes the X and Y data.
Viewing the data points, they appear to have an underlying mathematical relationship, which looks to be a third order polynomial. But not all the data points fall exactly along the polynomial.
In fact, when the XY data was generated, the data points of a third order polynomial were perturbed to create some variation. This variation is like noise on acquired experimental data.
What process can be used to find the best mathematical representation of the data? One approach would be:
This process is animated for orders one through ten in Figure 4.
Consider the first order polynomial curve to the XY data shown in Figure 5.
The first order polynomial follows the general trend of the XY data, but does not make a perfect fit to the data points. The curve parameters (slope and offset) were calculated to minimize the difference between the curve and the data points.
To a trained observer, it is obvious that a higher order polynomial is needed to fit the data points better. Increasing the polynomial order to three, a better fit to the data is obtained as shown in Figure 6.
That is pretty good fit! Of course, there is some random variation in the XY data, making it impossible for the polynomial curve to pass through all the data points.
Even a fourth or fifth order polynomial will fit the data well, with low error, as shown in Figure 4. Without any prior knowledge, how would a user know if a 3rd, 4th, or 5th order polynomial is correct? Engineering judgement from the user may ultimately be required.
What happens if the order of the polynomial is increased even more? A 10th order polynomial fit is shown in Figure 7.
Using a tenth order polynomial, the small variations in data points cause the polynomial to over and under shoot in order to try and pass through each data point.
The calculated error, based on the distance between data points and the curve, of the tenth order might be better than lower orders: the curve passes through more of the data points. However, it is clear that it is not fitting the underlying data pattern well - there are some large excursions in the curve which deviate from the original third order polynomial shape.
What is there to conclude from this analogy about fitting different order polynomials to a set of XY data?:
Let’s expand this polynomial analogy to curvefitting of actual modal FRF data using Simcenter Testlab Modal Analysis.
Modal Curvefitting with Simcenter Testlab Modal Analysis
The process of estimating a good set of modal parameters from measured FRFs has similarities to the XY data analogy:
But there are additional considerations, as illustrated in Figure 8:
There are several unknowns to solve for (modes, etc) and many FRF data points to consider. The modal curvefitter software and its algorithms should:
In the end, the user will need to manually inspect the results and make sure that all of the above is done properly.
The steps in performing a modal curvefit in Simcenter Testlab Modal Analysis are shown in Figure 9.
This process may need to be repeated until a satisfactory curvefit is achieved. The first three steps in the process are all done in the Polymax or Time MDOF worksheet of Simcenter Testlab Modal Analysis. The validation steps have separate workbooks called Modal Synthesis and Modal Validation respectively.
Open the Simcenter Modal Analysis software (Tools -> Add-ins -> Modal Analysis and Polymax Modal Analysis) to get started. It is highly recommended to use the Polymax algorithm for the modal curvefitting process, see the upcoming section on Modal Assurance Criterion for an explanation.
Select FRFs and Frequency Range
The first step in doing a modal curvefit is to select the measured FRFs to be analyzed and set the frequency range of analysis as shown in Figure 10.
Some key features of the Bandwidth selection minor worksheet:
This sum FRF data shown in the display is for visualization to help select the bandwidth for analysis. All the FRFs will be used in the analysis. The complete list of FRFs being analyzed is shown on the left side of the screen.
Inside the Stabilization minor worksheet (the top right of the screen), the Simcenter Testlab software creates a visual guide, called a stabilization diagram, for selecting potential modes as shown in Figure 11.
The stabilization diagram has many rows, each containing letters:
The model size, in the lower left corner, is normally set two to three times higher than the number of modes expected in the data to allow for enough potential mode selections. The model size is the number of modes the Simcenter Testlab software algorithm fits to the FRF data.
Stabilization Diagram Rows
The number of letters, or potential modes, in each row will never exceed the row number (Figure 12). It can be less than the row number because the Simcenter Testlab will not display meaningless mode selections. For example, if a potential mode was identified by the curvefitting algorithm with negative frequency or negative damping, it would not be displayed.
By hovering the mouse over a letter in the stabilization diagram, the potential frequency and damping value is shown below. The letter (which represents a potential mode) can be selected for the curvefit analysis by clicking on it.
Stabilization Diagram Letters
The letters indicate the repeatability of a particular potential modal solution. This is done by comparing the potential solutions (frequencies, dampings, etc) from the current row to the potential solutions in the previous row as shown in Figure 13.
The letters indicate the stability of the solutions, with the letter s being the most repeatable.
The modal participation vector is related to the mode shape component of the driving point FRF measurements.
Ideally, columns of letter s indicate the presence of modes (Note: a very lightly damped mode at 50 or 60 Hz, even with a strong column of letter s, is most probably electrical interference and not a mode of the test object).
It is only necessary to pick one letter s from any column of letters since the solution is repeatable over the rows/model order.
Stabilization Diagram Tolerances
How repeatable? In the lower left of the stabilization diagram screen is a button labelled “Tolerances…”. By pressing on it, the tolerance percentages used for the letter designations are shown (Figure 14).
Tolerances can be adjusted. For example, if it is desired to know the frequency of the mode with less than 1% variability in the frequency, the tolerance could be tightened to 0.1%.
After selecting the best set of potential modes, the next step is to calculate mode shapes as shown in Figure 15. Click on the “Shapes” minor worksheet at the top left of the screen.
Press the “Calculate” button on the middle of the left side to determine the shapes. The modes with shapes will be listed in the lower left. View the modes and scroll through them using the arrows on the lower right side.
Modal Model Validation
With a complete set of modes and mode shapes, now the results can be double-checked through some validation steps. It is important that no modes were missed, and that not too many modes were included.
Two commonly used validation methods are: Modal Synthesis and Modal Assurance Criterion. These methods are available in two separate workbooks.
Modal Synthesis: Checking for Missed Modes
Using the calculated mode set, FRFs can be synthesized and compared against the originally measured FRFs.
In doing this, it is often possible to catch mistakes where modes were left out of the analysis as shown in Figure 16.
Each measured FRF can be checked in this manner. For each FRF, a correlation and error percentage are calculated and display. It is desirable to have the correlation percentage as high as possible, close to 100%. The error percentage should be low, as close to zero percent as possible.
If modes where missing in the original analysis, go back to the Stabilization step and add them.
Modal Assurance Criterion: Checking for Too Many Modes
It is possible to have a good modal synthesis but have selected too many modes. The Modal Assurance Criterion can help identify this situation.
The Modal Assurance Criterion (MAC) compares two mode shapes. If the modes are similar in shape, the value of the MAC will be close to 100%. Two shapes that are different will have a MAC close to 0%.
Ideally, each mode identified during modal parameter estimation should have its own unique mode shape (like no two snowflakes have the same shape).
How could too many modes be selected? Consider Figure 17, which shows the Stabilization Diagram for two different modal curvefitting algorithms: Time MDOF and Polymax.
It would be easy for a user to select a duplicate or extra mode on the Time MDOF stabilization diagram (left side of Figure 17). It is less likely that a duplicate mode would be selected from the stabilization diagram on the right side, which used the Polymax algorithm. The Polymax curvefitting algorithm does a better job of rejecting potential modes that are not possibly real.
When an extra or duplicate mode is selected, the shape of the modes will be very similar to modes nearby in frequency. Similar shapes are physically impossible: each mode shape should be unique. This mode-shape-similarity will be flagged as a high “off-diagonal” MAC term as shown in Figure 18.
The figure above shows that the 304 Hz mode and 311 Hz mode are likely the same mode. One of the modes should be deselected. Like the XY dataset analogy, the error between the modal model and FRF data might be smaller, but the underlying modal model is wrong. Extra modes should be removed.
See the ‘Modal Assurance Criterion Knowledge Base’ article for more information. In addition to selecting too many modes, the MAC has other uses including indicating if too few measurement points were used in the modal survey.
Repeated Roots: Two Modes at Same Frequency
In many structures, it is possible to have two modes occur at the same frequency. This happens often in symmetric structures (tubes, disks, etc).
To determine if a structure has a repeated root, use the Mode Indicator Function (MIF). The MIF will dip in the presence of a mode. If there are two modes at close frequencies, and there are two MIFs, each MIF will dip indicating the presence of two modes at the same frequency as shown in Figure 19.
The Mode Indicator Function “dips down” at frequencies where there are two modes. In Figure 19, both the primary mode indicator function (in green) and the secondary mode indicator function (in blue) dip at the same frequency. This “double dip” indicates the presence of two modes at these frequencies.
Mathematically, it is necessary to have two references to have both a primary and secondary mode indicator function.
As a final touch, consider using the Maximum Likelihood estimation of a Modal Model (MLMM). The MLMM algorithm makes small, automated adjustments to the frequencies and dampings for a better match to the FRF data.
Modal Data Acquisition
Modal Analysis and Operational Deflection Shapes