Insufficient Force: Force from a single shaker may not be adequate to properly excite a large test structure. On a large object, a single shaker may need a higher force level and create non-linear response. Using multiple shakers to excite the whole structure can keep the individual force levels low and in a linear response range (Figure 1).
Figure 1: One modal shaker (left) is enough to excite a small structure (middle), but multiple shakers would be needed to excite a large structure (right).
Directional: Excite multiple directions simultaneously to ensure modes with strong directional qualities are excited. For example, a test object might have both lateral and vertical modes. Exciting both directions simultaneously with two shakers can help ensure both modes are well identified in the data set (Figure 2).
Figure 2: Jet engine with two shakers, one applied vertically, one laterally.
Repeated Roots: Sometimes structures have multiple modes at the same frequency. This often happens in symmetric structures as shown in Figure 3. Multiple inputs are required in order to identify the repeated modes properly.
Figure 3: Symmetric structure (left) with two modes (middle and right) with nearly identical frequencies and same shapes at different orientations.
This article explains how to setup and use the Simcenter Testlab MIMO FRF testing acquisition workbook to perform a shaker test with a burst random source.
Table of Contents 1. Getting Started 2. Channel Setup Worksheet 3. Scope Worksheet 3.1 Source Setup 3.2 Autoranging 4. MIMO FRF Setup Worksheet 4.1 Time 4.2 Autopowers 4.3 Frequency Response Function (FRF) 4.4 Principal Component Analysis (PCA) 4.5 Channel Parameters 4.6 Acquisition Parameters 5. MIMO FRF Measure Worksheet 6. Validate Worksheet
1. Getting Started
Setup the shakers, amplifiers, and accelerometers according to Figure 4.
Figure 4: Setup for a multiple shaker modal test including computer with Simcenter Testlab software, SCADAS data acquisition hardware, amplifiers, shakers, accelerometers, and load cells.
Double click on the Simcenter Testlab folder, and the open the “Testlab Structures Acquisition” folder as shown in Figure 5.
Figure 5: Open the “Testlab Structures” folder to find the “MIMO FRF Testing” icon.
Ensure that the SCADAS frontend hardware is on and connected. Double click on the “MIMO FRF Testing” icon to get started. Simcenter Testlab MIMO FRF Testing requires 45 tokensto run.
A blank screen will open. Click on the white icon in the upper left corner to start a new project as shown in Figure 6.
Figure 6: After starting "MIMO FRF Testing" click on the white icon in the upper left corner to start a new project.
After pressing the icon, the computer and software will communicate with the Simcenter SCADAS hardware. The lights on the ethernet hardware connections should blink rapidly.
If successful, the software will open with a new empty project called “Project1.lms” as shown in Figure 7.
Figure 7: After Simcenter Testlab MIMO FRF Testing is fully started, a new project called "Project1" is created (top of menu). Workbooks along the bottom guide a user in performing a measurement by clicking on the different steps from left to right.
Under “File -> Save As”, store the project file to a desired name.
To set up the test, click on the “Channel Setup” worksheet. The channel setup page is used to define Point ID’s, Directions, Input mode and sensitivities of each channel. Additionally, select which channels will be used as references.
A geometry of the structure is not needed, although defining the Channel setup using the Geometry (Figure 8) reduces the risk of animation problems when visualizing operational deflection shapes or mode shapes.
Figure 8: Click on upper right corner of Channel Setup worksheet and select "Use Geometry" to select measurement points directly from the geometry.
A burst random signal is preferred over a continuous random signal to avoid the effects of leakage. More information about leakage here: "Windows and Spectral Leakage".
Additionally, the scope worksheet can be used to check and set the ranges of the sensors before beginning the test (Figure 11).
Figure 11: Sensor Autoranging Menu in Simcenter Testlab MIMO Testing is used to set optimal channel ranges for the test.
1. Look at the bar, LED or gauge displays for all of the channels. 2. First click on Start Ranging in the Autoranging panel. From this moment on, the software will keep the peak level on all channels. 3. Sweep through the range of conditions that will be encountered in the test. In case of a bar display, a horizontal line will follow the peak level. By clicking on Stop Ranging, the software holds the maximum peak level and the corresponding line in the bar display will remain at that level. Push the Stop Ranging button once you have run through all possible test conditions, and you do not want the software to detect for the peak level anymore. If the peak level encountered during the sweep is too high the bar, LED or gauge turns red. 4. Click on "Set Ranges" to set a suitable range for the level encountered on all channels. 5. Click on the More... button to see the detailed information for each channel. 6. If necessary, select particular channels and edit the values manually.
Measure and save ‘FRF’ to use for Modal Analysis curve fitting and measure and save ‘Coherence’ as shown in Figure 15.
Figure 15: Online Data Function – ‘FRF’ minor worksheet.
Frequency Response Functions (FRFs) are used to determine the modes of the structure via modal curvefitting.
The coherence acts as a check of the quality of the FRF functions. It is a statistical indicator which indicates whether or not a given input and output are correlated (i.e. linearly related) as shown in Figure 16.
Figure 16: The coherence (top, green) acts as a quality indicator for the Frequency Response Function (bottom, red).
Coherence has values between zero and one, with one indicating a perfectly linear and repeatable relationship between the input and the output, and zero indicating a complete lack of correlation.
Next, select "Measure & Save" for 'PCA on references and 1 response' to check that the exciter force signals are not correlated as shown in Figure 17.
Figure 17: Online Data Function – ‘PCA’ minor worksheet.
In a proper modal test setup, all the shaker input forces need to be uncorrelated. Additionally, the responses need to be completely caused by the measured input forces.
When two or more force signals are correlated, it means one of the force signals is a linear combination of the others, and it is not possible to calculate FRF’s.
Principal Component Analysis (PCA) determines how many uncorrelated input signals there are and if there are other unknown forces acting on the system.
Principal components are based on a singular value decomposition of crosspower matrix containing all references and one user selected response.
The rank of the crosspower matrix is equal to the number of uncorrelated signals, hence for a good setup this rank is equal to the number of force input signals.
This singular value decomposition is performed at every frequency line, and the resulting eigenvalues are also displayed as function of frequency, thus creating the principal component functions.
The results of a Principal Component Analysis for a two shaker modal test are shown in Figure 18.
Figure 18: For a two modal shaker test, using the “PCA on references and one response” shows two independent sources (red and green lines). There are no additional sources acting on the system (blue line).
For a two shaker modal test, there should be two high principal component analysis functions.
4.5 Channel Parameters
The ‘Channel Parameters’ minor worksheet defines abort levels for the active channels as shown in Figure 19.
Figure 19: Channel Parameters menu from the MIMO FRF Setup workbook.
This optional safety feature avoids continued exposure of the structure to excitation levels that are not acceptable. As soon as an overload is detected, the measurement will act as specified in the advanced 'Averaging Parameters' settings.
Use the ‘Acquisitions Parameters’ to define the required bandwidth and frequency resolution (Figure 20) for the measurement and the averaging parameters.
Figure 20: Acquisition parameters setup in MIMO FRF setup workbook.
Note that the adaptable fields offered to the user depend on the selected source mode. For example, the periodic average parameter is only visible when the acquisition mode is periodic.
The ‘Acquisition Control’ method must be set according to the selected input signal type (i.e Burst random requires Burst control). The software will not allow a measurement unless this is selected appropriately.
Lastly, when using a non-periodic input signal type a window function needs to be selected (note: uniform = no window).
Ready to measure? This worksheet is where the measurement is executed (Figure 21).
Figure 21: The MIMO FRF Measure worksheet is used to execute the measurement.
Do the following to perform the measurement: 1. Click on the "MIMO FRF Measure" tab in the workflow bar at the bottom of the project window. 2. You can change the run name directly in the Run-name field. 3. Press on the Arm button to arm the measurement. The system is now ready for an immediate start. 4. Press on the Start button to start the test. 5. Select which channels you would like to view online in the lower ‘Responses’ display panel. 6. When the run is complete, the test status shows 'Normal end'.
The overview pane is used review data (FRF, Coherence, AutoPowers, etc) from the current or previous runs. Additionally, the Geometry display can be used to verify that all the FRF’s have been collected and there are not missing points as shown in Figure 22.
Figure 22: Validate worksheet of MIMO FRF Testing.
After validating the quality of the FRF measurements the data can be used with Modal Analysis add-in to perform curve fitting to obtain modal frequencies, damping and mode shapes.