What is an Operational Deflection Shape (ODS)?
Operational Deflection Shapes (or ODS) analysis gives additional insight into noise or vibration problems that individual measurements alone do not.
An operational deflection shape is an animation of the vibration pattern in a structure. Both the amplitude and phase of vibration measurements are animated.
Figure 1 is an operational deflection shape of a vibration issue in a truck that occurs at cruising speeds. The vibration is felt in the steering wheel and seat by the truck occupant.
The vibration is measured at several different points or locations on the structure using accelerometers. In Figure 1, each blue cube represents a location where an accelerometer was used to measure vibration on a pickup truck.
In the animation of Figure 1, an axle resonance excited by the rotation of the tires is the root cause of the unwanted vibration in the pickup truck. This is obvious by looking at the animation. Viewing vibration measurements (1st order wheel vibration) as shown in Figure 2 does not readily lead to the same conclusion.
For example, the fact that the two wheels of the axle are out of phase (called “axle tramp mode”) cannot be determined from these two dimensional (2D) plots.
Previous to performing the operational deflection shape, the manufacturer had tried to balance the tires in hopes of eliminating the vibration. The operational deflection shape animation clearly shows why balancing would not be the most effective strategy in reducing the vibration experienced by the driver.
When it comes to diagnosing a vibration issue, the old adage “a picture is worth a thousand words” could be rewritten as “an operational deflection shape is worth viewing a thousand individual measurements”!
What can be animated? What is required?
To perform an operational deflection shape analysis, three steps are required:
Any type of measurement (orders, spectrums, time) can be animated. The key is that the phase relationship between all the channels is preserved during the measurement. To preserve the phase properly, the measurements can be performed via two methods:
LMS Test.Lab Operational Deflection Shapes
Here are instructions for performing an Operational Deflection Shape analysis in LMS Test.Lab:
Based on key vibration locations to be a measured, a geometrical representation of the test object should be created. The geometrical representation consists of test nodes, and connections between nodes, as shown in Figure 3.
It is also possible to import a CAD model as the basis for the geometry, rather than create one from scratch.
When creating a geometry in LMS Test.Lab, the following will need to be defined:
To make a geometry, select “Tools -> Add-ins” from the main menu as shown in Figure 4.
This creates a new worksheet called “Geometry” as shown in Figure 5. The geometry add-in is only required to create and build a geometry. Once the geometry is made, the add-in may be turned off to conserve tokens, even when performing an operational deflection shape analysis.
Click on the ‘Geometry’ worksheet. Across the top of the ‘Geometry’ worksheet are sub-worksheets as shown in Figure 6.
Moving thru the sub-worksheets from left to right goes through the steps needed to build a geometry. The sub-worksheets are in the order needed to create a geometry: Components, Nodes, and Lines.
In the first sub-worksheet called ‘Components’, enter the component names desired for the test object as shown in Figure 7. A single test geometry can consist of different components, for the example of the truck, the components include body, rails, axle, seat, steering wheel, etc.
The steps for creating components are:
Now select the next sub-worksheet called ‘Nodes’ to add measurement points to the components as shown in Figure 8. Each node corresponds to a point or location on the structure where an accelerometer will be mounted to measure vibration.
To create nodes on the components:
To add connections between the nodes, press the ‘Lines’ worksheet as shown in Figure 9.
Lines can be added between points/nodes by:
If desired, the ‘Surfaces’ sub-worksheet can be used to create surfaces between points. With the geometry complete, now the measurements can be acquired.
An important step when performing the measurement is to associate the measurements with the geometry of the test object. The software needs to know which physical measurement location corresponds to each point on the geometry.
This is easily done in the ‘Channel Setup’ worksheet of LMS Test.Lab Signature, LMS Test.Lab Spectral, and LMS Test.Lab Vibration Control. Select ‘Use Geometry’ from the pulldown in the upper right as shown in Figure 10.
The geometry node and measurement point identification must be spelled exactly the same (case sensitive) to be associated as shown in Figure 11.
To create the connection between geometry and the accelerometer measurements properly:
Autopower versus Spectrum
Next, the measurement must be setup to ensure the phase is properly accounted for between channels. If this is not done, the animation of the operational deflection shape will not be correct.
In the ‘Online Processing’ worksheet of LMS Test.Lab Signature, the measurement type can be changed from the default ‘Autopower Linear’ to ‘Spectrum’ as shown in Figure 12:
It is required to know the phase between measurements so the relateive motion between points can be captured. Make sure to switch the measurement function in the ‘Vibration’ worksheet!
A phase reference measurement channel is required to successfully preserve the phase while roving measurement groups. It is also a good practice when acquiring all channels simultaneously.
Phase Reference Channel
To keep consistent phase between roves, at least one accelerometer should be kept at the same location while the others are moved. This accelerometer will be the phase reference channel that is used to preserve the phase among the different measurement sets.
Turn on the ‘Phase referenced spectra’ check box as shown in Figure 13. Then press the ‘Define’ button and select a reference channel that is not to be moved during the acquisitions.
The reference channel should be on the test object and be fairly “active”, i.e. have vibration that is related to the other channels. For example, if testing a truck, it would not make sense to have the reference accelerometer on the floor of the test laboratory, where the floor vibration is not related to the operation of the truck.
The phase of the reference channel is subtracted from both itself and all the other channels.
The reference channel will have a phase value of zero at all frequencies after the measurement, but the phase of all other channels will be correct relative to the reference channel as shown in Figure 14.
This phase reference will work with all frequency based measurements: orders, spectrums, etc. The phase will be correct between the different measurement groups where the common channel was used.
With the measurement completed, the analysis can begin. Turn on ‘Tools -> Add-ins -> Operational Deflection Shapes & Time Animation’ as shown in Figure 15.
A new worksheet called ‘Animation’ is created as shown in Figure 16.
In the ‘Animation Worksheet’, animate the geometry with the measurement data as shown in Figure 17.
Animations are created by:
The operational deflection shapes can be saved as shown in Figure 18:
To save the operational deflection shape:
The analysis is stored in the LMS Test.Lab project. The animation can be retrieved and viewed in the LMS Test.Lab Navigator worksheet. When viewing the previously stored results of the analysis, the ‘Operational Deflection Shape and Time Animation’ add-on is not required.
There are a few software options that can be helpful when doing LMS Test.Lab Operational Deflection Shapes as shown in Figure 19:
Clicking on the “Animation…” button, the ‘Fixed Animation Scale’ can be turned on and off:
Not Just Vibration
Many other types of measurements can be visualized with operational deflections shapes, not just vibration.
For example, acoustic data can also be animated, to visualize an acoustic cavity mode as shown in Figure 20:
Measuring an acoustic shape is the same process as creating a vibration shape. Only the transducer is changed from an accelerometer to a microphone.
Different kinds of data (sound, vibration, torsional, …) can be visualized simultaneously. The LMS Test.Lab software scales each type of data separately to allow the animation to take place.
Enjoy operational deflection shape analysis!
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