Simcenter Testing Solutions What is an Operational Deflection Shape (ODS)?

Direct YouTube link: https://youtu.be/djGWnvP1kIM

 

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.

Contents of this article:
1. What is an Operational Deflection Shape?
2. What can be animated? What is required?
3. Simcenter Testlab Operational Deflection Shapes
   3a Geometry
   3b Measurement
   3c Autopower versus Spectrum
   3d Phase Reference Channel
   3e Analysis
   3f CAD and FEA Geometry
4. Not Just Vibration

1. What is an Operational Deflection Shape?

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.

Figure 1: Operational deflection shape of pickup truck shows axle mode is cause of vibration problem
 

In the animation of Figure 1, an axle/suspension resonance excited by the fundamental rotation of the tires is the root cause of the vibration in the pickup truck. This is easily discernible by looking at the animation. Viewing vibration measurements (1^st order wheel vibration) alone, as shown in Figure 2, does not readily lead to the same conclusion.

Figure 2: Vibration measurements do not show the axle tramp mode creating the vibration issue

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 shows why balancing alone was not entirely effective 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”!

2. What can be animated? What is required?

To perform an operational deflection shape analysis, three steps are required:

• Geometry: Create a geometry of the test object
• Measurement: Acquire data with consistent phasing
• Analysis: Create animation utilizing the geometry and measurement data

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:

• Measure all channels simultaneously – Good for animating all types of measurement data, including time data. May required a high number of channels.
• Phase reference channel – Allows multiple low channel count measurements to be pieced together for a complete animation. A single reference accelerometer is kept in a fixed location during each measurement rove. This technique is only useful for frequency/order domain measurements, not time domain.

3. Simcenter Testlab Operational Deflection Shapes

Here are instructions for performing an Operational Deflection Shape analysis in Simcenter Testlab (formerly called LMS Test.Lab):

3a Geometry

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.

Figure 3: Left – Geometrical representation of test nodes, Right – Drawing of test object

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 Simcenter Testlab, the following will need to be defined:

• Components – Test objects can have different components. For example, a truck could have components corresponding to body, wheels, seats, hood, roof, etc.
• Measurement nodes or locations – Nodes are the exact locations where accelerometers are placed on the test object.
• Connections – User defines connections between nodes. These are for visual interpretation only, and do not create any physical restraints that modify the physically measured motion.

To make a geometry, select “Tools -> Add-ins” from the main menu as shown in Figure 4.

Figure 4: Tools -> Add-ins -> Geometry

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.

Figure 5: Geometry Worksheet is added to workflow at bottom of screen

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.

Figure 6: Geometry sub-worksheets build a geometry be moving left to right

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.

Figure 7: Component sub-worksheet in Geometry

The steps for creating components are:

1. Click on ‘Components’ sub worksheet.
2. Type the desired component names in the Component column. Each component can be assigned unique colors or co-ordinate systems.
3. Press ‘Accept Table’ in the upper right when finished.

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.

Figure 8: Nodes sub-worksheet in Geometry

To create nodes on the components:

1. Select the ‘Nodes’ sub-worksheet
2. On the left side, highlight the component to add nodes (ie, accelerometer measurement locations). This can be repeated for each component as needed.
3. After highlighting the desired component, enter the point number under the ‘Name’ column and X, Y, and Z dimensions (expected in meters by default). These dimensions should be entered according to a ‘right hand rule’ convention.
4. Press the ‘Accept Table’ button in the upper right when finished.

To add connections between the nodes, press the ‘Lines’ worksheet as shown in Figure 9.

Figure 9: Lines sub-worksheet in Geometry

Lines can be added between points/nodes by:

1. Click on ‘Lines’ sub-worksheet.
2. In lower geometry display, hover mouse over first point to be connected. When the node ‘highlights’ click on it. Move the mouse to the node to connect. Click on it when it highlights to complete the connection. Repeat as needed.
3. To stop adding connections, either press ‘ESC’ key or click on node again.

If desired, the ‘Surfaces’ sub-worksheet can be used to create surfaces between points. With the geometry complete, now the measurements can be acquired.

3b Measurement

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 Simcenter Testlab Signature, Simcenter Testlab Spectral, and Simcenter Testlab Vibration Control. Select ‘Use Geometry’ from the pulldown in the upper right as shown in Figure 10.

Figure 10: ‘Use Geometry’ in the upper right of Channel Setup

The geometry node and measurement point identification must be spelled exactly the same (case sensitive) to be associated as shown in Figure 11.

Figure 11: ‘Use Geometry’ in the upper right of Channel SetupTo create the connection between geometry and the accelerometer measurements properly:

1. After selecting ‘Use Geometry’, press the ‘Refresh’ button to view the geometry
2. Select the point to be measured by either highlighting the row with the ‘Node Name’, or click on the node directly in the geometry. If selecting directly from the geometry, the corresponding row in the node list will be highlighted.
3. In the 'Channel Setup' worksheet, highlight the channel to be associated with the node/point id by clicking on the associated row number.
4. Press the ‘<<<INSERT’ button to copy the node to the channel identification.
5. Be sure to fill in the direction in the Channel identification: +X, -X, +Y, -Y, +Z, or -Z.

3c 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 Simcenter Testlab Signature, the measurement type can be changed from the default ‘Autopower Linear’ to ‘Spectrum’ as shown in Figure 12:

• The Autopower Linear measurement function does not contain phase
• The Spectrum does contain phase.

It is required to know the phase between measurements so the relative motion between points can be captured. Make sure to switch the measurement function in the ‘Vibration’ worksheet!

Figure 12: In ‘Online Processing’ set the function to ‘Spectrum’ instead of ‘Autopower Linear’.

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.

3d 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.

Figure 13: In ‘Online Processing’ turn on ‘Phase referenced spectra’ and define a reference channel that is not roved during the acquisition.

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.

Figure 14: Phase reference channel (red) measurement has zero degrees for phase in lower graph

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.

3e Analysis

With the measurement completed, the analysis can begin. Turn on ‘Tools -> Add-ins -> Operational Deflection Shapes & Time Animation’ as shown in Figure 15.

Figure 15: Tools -> Add-ins -> Operational Deflections Shapes

A new worksheet called ‘Animation’ is created as shown in Figure 16.

Figure 16: Animation worksheet

In the ‘Animation Worksheet’, animate the geometry with the measurement data as shown in Figure 17.

Figure 17: Animating the geometry with operational data

Animations are created by:

1. In upper left, press the ‘Refresh’ button. All measurements from the active section (even across multiple runs) will be made available for animation
2. Different measurements (example: 1^st order, 2^nd order, 3^rd order) are sorted into different columns. Highlight the column of interest for the animation.
3. Move the cursor to the desired frequency or rpm
4. Press the ‘Play’ button to animate. The cursor will scroll thru the data along the X-axis.
5. If scrolling is not desired, press the ‘Pause’ button.

The operational deflection shapes can be saved as shown in Figure 18:

Figure 18: Saving an operational deflection shape

To save the operational deflection shape:

1. Move cursor to desired position. Position can be rpm or frequency.
2. After reaching position, press the ‘Add Single’ button to record the position.
3. Move to other positions and press ‘Add Single’ if multiple shapes are of interest.
4. Positions are listed with semi-colons between them.
5. Enter an analysis name, and then press the ‘Calculate’ button to save the shape.

The analysis is stored in the Simcenter Testlab project. The animation can be retrieved and viewed in the Simcenter Testlab Navigator worksheet using the Geometry Display. 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 Simcenter Testlab Operational Deflection Shapes as shown in Figure 19:

Figure 19: The ‘Animation…’ button options

Clicking on the “Animation…” button, the ‘Fixed Animation Scale’ can be turned on and off:

• If turned OFF, the animation will always be at full scale.
• When turned ON, the animation is scaled relative to the maximum vibration level of the entire measurement.

3f CAD and FEA Geometry

If a finite element or Computer Aided Drafting (CAD) model is available, it can be used to create a fuller animation as shown in Figure 20.

Figure 20: A test grid of a few points used as the basis to animate a full CAD model.

A test model consists of only a few measurement locations and a wire grid.  When viewed by others who were not familiar with the test campaign, it can be difficult to understand.  In Simcenter Testlab, there is a "expansion" feature that can be used to use the limited test points to animate a full CAD model or finite element model.

More information in these knowledge articles:

• Import CAD Geometry into Simcenter Testlab Geometry
• Animate CAD Geometry: Simcenter Testlab Modal Analysis

4. 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 21:

Figure 21: The acoustic shape of an interior cavity of a vehicle

Measuring an acoustic shape is the same process as creating a vibration shape. Only the transducer is changed from an accelerometer to a microphone.

Torsional vibration can also be visualized as shown in Figure 22. An additional visualization component, called a rotational pointer is used to visualize the torsional rpm fluctuations.

Figure 22: Torsional shape with rotational pointers

Different kinds of data (sound, vibration, torsional, …) can be visualized simultaneously. The Simcenter Testlab software scales each type of data separately to allow the animation to take place.

It is not just products that can be tested. Fixtures used in performing a test, including shaker heads as shown in Figure 23, are also useful. Shaker resonances can cause difficulty in performing sine tests due to total harmonic distortion.

Figure 23: Operational deflection shape of shaker head

Enjoy operational deflection shape analysis!

 

Questions? Email peter.schaldenbrand@siemens.com or contact Siemens Support Center.

 

Related Links:

• Index of Testing Knowledge Articles
• Natural Frequency and Resonance
• What is Frequency Response Function (FRF)?
• How to calculate damping from a FRF?
• Modal Testing: A Guide
• Getting Started with Modal Curvefitting
• Modal Assurance Criterion
• Simcenter Testlab Modal Analysis: Modification Prediction
• Import CAD into Simcenter Testlab
• Animate CAD Geometry
• Alias Table: Mapping Test Data to Geometry
• Simcenter Testlab: Geometry Displays
• Geometry in Simcenter Testlab
• Simcenter Testlab: Multi-Run Modal Analysis
• Maximum Likelihood estimation of a Modal Model (MLMM)
• Difference Between Residues and Residuals?
• What is Operational Modal Analysis?
• Making a Video of a Time Animation