Operational Deflection Shapes (ODS) Analysis Provided Valuable Information on Vibration Characteristics and Helps Improving Test Methods
– by M+P International

Vibration testing to international and laboratory standards is important for product verification and approval. Typically, an electrodynamic shaker is used to apply basic sine, random, and shock excitations to the specimen. However, despite being readily discernible from the available measurements, helpful additional information such as operational deflection shapes (ODS) is frequently overlooked. m+p Analyzer’s ODS analysis toolbox enables engineers to easily interpret the dynamic behavior of the vibration test system, providing important insights for safe testing and reliable protection of shaker and specimen in various test scenarios.

Operational Deflection Shapes analysis of the specimen or of the fixture provides in-depth information about the structural dynamics and is recommended for these vibration tests:

• Large test specimens and head expanders (fixtures used to adapt larger specimens on the shaker)
• Tests that have failed in the past
• Unusual test setups

Operational deflection shapes assist in determining the optimal control points, the necessary number of control sensors and the appropriate control strategy. The analysis results are valuable feedback, enabling engineers to further identify the source of vibration-induced failures, improve test methods and optimize test fixtures.

When using m+p international’s established software, m+p VibControl for vibration data acquisition and m+p Analyzer for operational deflection shape calculations, the data types are compatible.

Seamless Data Transfer from Vibration Test to ODS Analysis

Vibration data is directly obtained from a sine sweep, a random signal or the self-check (pseudo random) before the controlled vibration test, eliminating the need for an additional data acquisition device. The self-check results are the preferred option for a quick analysis and initial ODS evaluation. They are directly available before the vibration test and supported for every excitation mode of m+p VibControl. However, to get high quality results, a sine sweep test is recommended.

To extract operational deflection shapes, first a comprehensive and representative set of phase-referenced spectra is key. Ideally, measurement positions evenly distributed across the test specimen should be chosen to ensure an accurate spatial resolution for the operational deflection shapes.

For many applications such as in the automotive industry this is already the case, as the vibration level of the specimen is monitored at many locations during a test. Thus, measurements at all relevant positions may be acquired during a single test run. This, however, requires a high number of measurement channels and sensors, which may introduce a mass-loading effect in the case of a light test specimen. In such instances or if only a few sensors are available, repeated test runs can be conducted. The m+p Analyzer provides an easy way of assigning the results to a geometry by relocating the sensors and performing one or several additional tests. The results create a finely resolved animation of the geometry. Additionally, calculated nodes based on interpolation between two measured nodes can be utilized to animate points on the geometry where no measurements were taken, thus further refining the representation.

For each test run, m+p VibControl generates a spectrum for each measurement point, which is phase-referenced to the control channel. Finally, the results are loaded into the m+p Analyzer ODS analysis software. Within the software, a geometry is created, and each measurement is assigned to a corresponding geometry node for mapping purposes. The operational deflection shapes for individual frequencies may now be reviewed and analyzed.

ODS analysis results provide crucial insights into the vibration test and thus valuable supplementary information to test reports. They provide real added value by optimizing the necessary control sensors, their placement, and control strategy.

The following example shows an ODS analysis for an octagonal head expander.

ODS Analysis of a Head Expander

Head expanders are designed to increase the mounting area of a shaker and allow users to test specimens larger than those that can be directly attached to the shaker platform. The following example illustrates the results of a test campaign, where operational deflection shapes of an octagonal head expander were acquired using a sine sweep test. Figure 2 shows the head expander without any additional test specimen, figure 3 the geometry for deflection shape analysis. At 1924 Hz the points on the outer diameter are moving in phase generating an umbrella shaped motion. This result, together with additional lower-frequency operational deflection shapes, verified the preliminary finite element calculation. It also confirmed the correct placement of the reference sensor in a gap between the head expander and the armature, as placing it directly on the head expander may result in over- or under-testing.

Figure 2: head expander with sensors

Figure 3: undeformed and deformed operational deflection shape extracted at 1924Hz

Using the data acquired from a sweep test, it is possible to create a model of a test specimen or fixture and easily visualize the operational deflection shapes encountered during a vibration test. These results offer engineers additional insights into vibration characteristics, aiding manufacturers in improving the dynamic behavior of their products.

Operational Deflection Shapes (ODS) Analysis for Comprehensive Insights in Vibration Testing

• m+p Analyzer’s ODS toolbox enables easy interpretation of structural dynamics for both the specimen and fixture
• Obtaining ODS analysis results enables optimizing control sensor positioning and control strategy
• Vibration data is directly obtained from sine sweep, random, or self-check tests, eliminating the need for extra data acquisition devices
• Geometry creation and ODS calculation can be conducted using m+p Analyzer
• ODS analysis enriches the vibration test report with meaningful additional information and visualizations
• Data acquisition and operational deflection shape calculation are integral parts of m+p VibControl and m+p Analyzer, ensuring seamless data transfer

For further details, please contact our authorized dealer ENMO Sound & Vibration Technology.