A driving point survey is an important part of any modal analysis test. By making several driving point measurements at various points around our structure we can “survey” the structure to help determine the optimal location for excitation. Consider the three driving point Frequency Response Functions (FRFs) in Figure 1 below.
Figure 1: Three different driving point FRFs from the same structure.
Which FRF in Figure 1 contains the most information about the natural frequencies of the structure? It is clear the blue FRF shows three peaks, while the red and the green FRFs each only show two peaks. By looking at the set of FRFs and comparing them, we can conclude that the location used to measure the blue FRF is the best driving point of the three, because the blue FRF contains the most peaks. This example highlights why performing a driving point survey is an important step in a modal test!
A driving point measurement is a dynamic measurement where the force input from the hammer (or shaker) and the response output from the accelerometer are measured at the same point on the structure, and in the same direction. (See Figure 2)
Figure 2: A driving point measurement on a cantilever beam. Both the force and response are being measured at the same location on the structure, and in the same direction (-Z).
However, sometimes it may not be physically possible to collocate the impact and the response measurement as shown in Figure 2, and a compromise must be made. For instance, imagine making a driving point measurement on a fuel tank, or other sealed vessel, where the interior surface is not accessible (see Figure 3). In this case, the accelerometer needs to be mounted on the same surface we will impact with the hammer, and should be located as close as possible to the impact location. It is important that the accelerometer be located where it will not interfere with the impact itself. Under no circumstances should the impact hammer be striking the accelerometer, or making any direct contact, as this will distort the FRF we are trying to measure (and likely overload the signal coming from the accel).
Figure 3 - When necessary, place accelerometer next to, and as close as possible, to the impact location.
While it is critical that the force and the response be measured at the same location and direction, the polarity of the two measurement directions can be different. For example, the hammer can be measuring in the -Z direction, while the accelerometer is measuring in the +Z direction. This is often the case in situations where the accel is mounted next to the impact location like is shown in Figure 3. As long as both hammer and accel signals are measuring along the same axis, the driving point measurement is valid.
Why is a driving point measurement important?
Very simply - a driving point FRF shows all of the modes of a structure that are excited by impacting at a particular location. If impacting at a point on a structure does not adequately excite a particular mode shape, the peak corresponding to that mode will be missing from the FRF. Why? To find out, let’s take a look at how the FRFs from Figure 1 were generated.
Consider the cantilever beam in Figure 4a below. By placing the accelerometer at point R and impacting at that same location, we get the red FRF shown in Figure 4b. Next, we move the accelerometer and impact at point G, which generates the green FRF. Lastly, we move the accelerometer and impact to location B, and measure the blue FRF. Again, we notice that there are three peaks in the blue FRF, and only two in the red and green FRFs.
Figure 4: (a) Cantilever beam with three driving points –“R”, “G”, and “B”; (b) driving point FRFs from each location
To understand why this is, let’s take a look at the first three modes of vibration for our cantilever beam in Figure 5 below.
Figure 5: The first three mode shapes of a cantilever beam. The higher mode shapes feature points that do not move called nodes.
In the first mode, the entire beam participates in the deflection pattern, all points are moving. In the 2nd and 3rd mode however, there are points that do not move at all. These points that do not participate (ie- move) in the deflection shape are called nodes, or nodal points.
Overlaying our cantilever beam with the first three mode shapes (Figure 6), it becomes clear that the Red impact location is on top of a node in the 3rd mode shape, and the Green impact location is at a node of the 2nd mode.
Figure 6: Nodal points in the 2nd and 3rd modes occur at Green and Red driving points respectively. These modes will not be excited by impacting at these locations!
Because nodal points do not participate in mode shapes, impacting our structure at the Red and Green locations will not adequately excite the mode shapes that have nodes at these locations. As a result, the peaks corresponding to the missing modes will not appear in the FRF.
This is the most important function of a driving point measurement: A driving point FRF shows us which mode shapes are being adequately excited by impacting our structure at that particular location. The modes that are excited by impacting at that location will create peaks in our driving point FRF, the modes that are not excited by that impact location will not.
Driving Point Survey
By making several driving point measurements at various locations around the structure and comparing the FRFs, we can tell which location will best excite the modes we are interested in. This is known as a “driving point survey.”
Performing a driving point survey helps to avoid using a nodal point for excitation. For a realistic structure with 2 or 3 critical dimensions, this becomes even more important because the nodal “points” from the simple cantilever beam example turn into nodal “lines” as shown in Figure 7 below.
Figure 7: Mode of a circular disc featuring two nodal lines. Points along these lines do not participate in the mode shape.
However, by performing a driving point survey we gather the information needed to avoid nodal lines, and ensure the modes of interest will be properly excited and measured.
Driving point measurements are important for other reasons as well. As we learned in this Modal Tips Article, in order to view mode shapes, it is necessary to measure either a complete row or complete column of our measurement matrix. The driving point measurements represent the diagonal of the test matrix, and will always be part of this comprehensive modal survey data set. Driving point measurements also allow us to properly scale mode shapes, and calculate modal mass and stiffness for a structure. Unless we are only interested in resonant frequencies of a structure, a driving point measurement is always required.
Using Simcenter Testlab Impact Setup: Driving Points
After completing the setup of the trigger, bandwidth, and windowing using the steps in the Impact Setup workflow, the next step is the driving point survey.
One tip that makes the Driving Points process a little simpler is to move the Geometry tab in front of the Channel Setup tab in the Testlab workflow as shown in Figure 8. If you’d like to learn how to customize the workflow in Testlab, please see this article in the Simcenter Testing Knowledge Base.
Figure 8: Move Geometry ahead of Channel Setup tab to make the Driving Point survey easier.
Setting up our test structure in Geometry is a good thing to do before we begin acquiring data, as we can use it to name the input channels and ensure our data Point IDs match our geometry Point IDs. This is one of the reasons moving Geometry before Channel Setup can be helpful.
For this example the test structure is a flat rectangular plate, made up of 15 nodes, as shown in Figure 9 below.
Figure 9: Test Geometry of a flat plate.
For this example let’s say we need to know the first 8 modes of this plate, their natural frequencies and mode shape description. Since I am unfamiliar with this plate, I select several points around the structure at which to make driving point measurements. The selected points are plate:2, plate:4, plate:7, plate:8, and plate:15, as shown below in Figure 10.
Figure 10: Point IDs selected to act as driving points.
Now that our test geometry is done and our driving point locations are identified, we can move to the Channel Setup tab and begin the driving point survey. At this point we should have 2 channels turned on – a Force channel (Input 1) and an Acceleration channel (Input 2). Channel Setup is shown in Figure 11 below.
Figure 11: Channel Setup for our impact test.
Next we will incorporate our geometry into Channel Setup by clicking on the down arrow next to “Channel Setup” in the blue bar near the top of the window as shown in Figure 12, and click on “Use Geometry”.
Figure 12: Select “Use Geometry” to incorporate Geometry Point IDs into Channel Setup.
This will open up a new window pane in Channel Setup for our geometry. Click on “Refresh” to show the structure (Figure 13).
Figure 13: “Use Geometry” in Channel Setup to aid in naming input channels.
Our first driving point will be at the point named “plate:2” as shown in Figure 14 below. I can select this point in the geometry by clicking on the node icon or by highlighting it in the point ID list by clicking on the row header.
Figure 14: Select the first test point in the geometry and insert the ID on input channels.
Next, select the destination channels on the left half of the screen, again by clicking on the row headers for Input1 & Input 2 while holding the [SHIFT] key. Clicking “Insert” at the bottom of the screen will move the selected Point ID (plate:2) over to our two channel Point IDs (Figure 15).
Figure 15: Channels 1 & 2 renamed to “plate:2” to match the geometry point ID!
With the measurement channels correctly labelled for the first driving point (plate:2), we can move to the Impact Setup tab in the Testlab workflow, and into the Driving Points page (Figure 16).
Figure 16: Driving Points is the last step in the Impact Setup worksheet.
The Driving Points worksheet (Figure 17) contains multiple displays, highlighting different aspects of our driving point measurement.
Figure 17: Various displays of the Driving Points worksheet in Impact Setup.
These areas are listed below:
A: Time history (upper) and Autopower PSD (lower) of force channel B: Time history (upper) and Autopower PSD (lower) of response channel C: Instantaneous (current) driving point FRF Magnitude (upper) and Phase (lower) D: Average of driving point FRFs Magnitude (upper) and Phase (lower) E: Coherence F: Control & Setup area for measurement
If we look in the area labeled “F” in Figure 17 we see that the “Input point” is already filled in as “plate:2”. This is because we selected it in Channel Setup. The “Response channel” is set to “2” to indicate that our accelerometer is plugged into Channel 2 on our frontend. We can select the number of averages we’d like to use for our driving point FRF, as well as whether we want to implicitly accept each average, or if we’d like to explicitly accept (via a popup window) after each average. These settings are shown in detail in Figure 18 below. Click on “Start” to acquire the driving point measurements for the first point.
Figure 18: Input point will automatically read the Point ID from Channel Setup for the driving point measurement.
After we have performed the 3 averages, the driving point measurement name will appear in the “Driving Points” box in area “F”, and we are ready to move the accelerometer to our next location, “plate:4”. To update the channel information, we repeat the steps shown in Figure 14: first select the node in the geometry, then insert the name into channels 1 & 2. Once the channel setup info is updated, return to the Driving Points worksheet, and it will appear as shown in Figure 19.
Figure 19: Use Channel Setup to update the naming of the next driving point measurement.
Repeat this process for the rest of the planned driving points. When finished there will be a total of 5 FRFs (Figure 20).
Figure 20: Select all driving point FRFs and click “Display” to compare the FRFs.
By selecting them all and clicking on “Display” we will see all 5 curves overlaid (see Figure 21 below).
Figure 21: Driving point measurements overlaid
The driving point measurements will also appear in our Project, in a folder called “Driving Points”, and can be plotted in Navigator. By comparing all five driving point FRFs we can quickly see some big differences between the excitation locations (Figure 22).
Figure 22: All 5 driving point FRFs in a multi-trace display. Some driving point locations did not excite all modes!
Each driving point location features a different number of peaks in the FRF. This indicates that only certain modes are being excited by impacting our structure at those locations. By making a table of the resonant peaks in each FRF, we can more easily see which driving point location will work best for this structure (Figure 23).
Figure 23: Driving point location comparison. Only plate:15 excited all 8 modes of interest.
The red boxes in Figure 23 indicate that there is no peak at that frequency in the FRF (orange indicates weak excitation). Once we count up the modes and resonant frequencies found in each of the 5 driving point FRFs, we quickly see that only the FRF measured at plate:15 excited all 8 modes of interest. Every other impact location missed at least 2 modes we are interested in. By looking at the mode shape descriptions in Figure 23 for each of the modes, it becomes clearer why certain modes are not excited by impacts at certain locations. For example, consider the torsion mode, Mode #2 in Figure 23. Only points that are not on the two center lines of the plate will participate in this mode shape. In this case, only plate:4 & plate:15 are not on a center-line (see Figure 24).
Figure 24: Points that are on a center-line of the plate will not participate in the torsion mode.
The first 8 mode shapes of the plate are shown below in Figures 25-32.
Figure 25: Mode 1 - Bending
Figure 26: Mode 2 - Torsion
Figure 28: Mode 4 - 2nd Bending
Figure 29: Mode 5 - Longitudinal Bending
Figure 30: Mode 6 - 3rd Torsion
Figure 31: Mode 7 - 2nd Longitudinal Torsion
Figure 32: Mode 8 - 3rd Bending
In general it is best to avoid centerlines of our structure, focusing on corners and impact locations at the extreme ends and edges of the structure. This will typically excite the most modes and avoid nodal lines of the first few modes, which are generally of primary interest. However, on a new or unfamiliar structure, it is always in the best interest of the tester to perform a driving point survey. This will help to ensure that the structure is adequately excited, and the critical mode shapes of interest are measured.
