ODS analysis – a powerful diagnostic tool for understanding structural behaviour
ODS (Operating Deflection Shapes) analysis is one of the most popular tools within structural dynamics. It provides very useful information for understanding the dynamic behaviour of structures, the applied techniques are easy to understand as no heavy math is involved, it is easy to use and it can be performed by relatively simple means. With BK Connect, you can perform all three types of ODS analysis: Time ODS, Spectral ODS and Non-stationary ODS.
What is ODS analysis – Operating Deflection Shapes analysis?
Use scenarios for ODS analysis
- Validation of FEM predictions from forced response simulations by comparing them to measured ODS data
- Comparing measured noise and vibration values to reference values
- Ensuring that structural resonances are not being excited and critical speeds do not occur
- Identifying and separating rotational and structural noise and vibration phenomena originating from rotating and fixed parts, respectively
- Relating noise radiation to structural vibration
- Reducing excessive noise and vibration
- Investigating machine wear resulting in, for example, unbalance, shaft misalignment and rattling
- Analyzing transient phenomena such as door slams, explosions, launches and firing, crash and drop impacts
- Analyzing machinery running at constant or slightly varying speed
- Analyzing non-linear systems and performing frequency-variant analysis such as engine run-up/down
- Monitoring of civil engineering structures that are loaded by ambient forces, such as waves on offshore structures, wind load on buildings, and traffic load effects on bridges
Types of ODS analysis
ODS analysis is typically divided into three groups:
- Time ODS
Time ODS is used to investigate the vibration pattern of a structure as a function of time. Time ODS includes all frequencies in the analyzed frequency range and is very useful for showing an overall ODS at a given point in time for either stationary or non-stationary signals, such as transient signals.
- Spectral ODS
Spectral ODS is used to investigate the vibration pattern of a structure for specific frequency or order components. For frequency component investigations, FFT analysis is used and the conditions must be stationary. For order component investigations, order tracking is used, and the conditions can be slightly quasi-stationary, for example, a slightly varying engine speed. The ODS of different spectral components is subsequently extracted, shown in a shape table and animated.
- Non-stationary ODS
Non-stationary ODS is used to investigate the vibration pattern of a structure for specific frequency or order components as a function of rotational speed or time. Results are based on FFT analysis or order tracking. Non-stationary ODS is very useful for identifying which noise and vibration behaviour is related to rotational and which to fixed parts of running machinery. Run-up/down ODS, where an engine is run up and down is a frequently used variant of non-stationary ODS.
A time range can be selected and swept through while animating the vibration patterns. A decimation factor can be applied to skip samples. The vibration at the various DOFs can be saved at discrete time instances in a shape table.
Shapes are animated by selecting frequencies/orders in the spectra and the vibration patterns are documented in a shape table for easy retrieval and comparison. The vibration at the various DOFs can be shown as acceleration, velocity or displacement with peak, peak-to-peak or RMS scaling. SI or imperial units can be used.
Shapes are animated by selecting combinations of frequency/order and rpm/time in a contour plot. In this case, FFT-based Run-up ODS is used, where the oblique lines represent orders, and the vertical lines represent structural resonances. As for Spectral ODS, the vibration patterns can be documented in a shape table.
Instrumentation for ODS analysis
As only the output of the structure/machinery is measured, the instrumentation for ODS analysis is relatively simple and inexpensive. It consists of a data acquisition system with hardware, measurement and analysis software, response transducers (typically accelerometers), and potentially a tacho probe. The tacho probe is required if you perform ODS analysis on machines with slightly varying speeds, where order tracking must be performed to avoid frequency smearing. Or you might need a tacho probe to annotate run-up/down tests with RPM tags, even if order tracking is not performed.
Example of an ODS system consisting of accelerometers, tacho probe,
and data acquisition hardware and software.
Regarding accelerometers, many of the requirements for other structural tests such as modal testing apply here as well. Parameters to consider range from dynamic range, frequency range and sensitivity, to low weight to avoid mass loading and easy mounting using accessories such as clips and bases. Specific requirements such as high temperature range, hermetic sealing, robustness, etc., might apply as well.
HBK offers complete solutions for all three types of ODS analysis – from accelerometers and tacho probes, to data acquisition systems with hardware, and measurement and analysis software. Our data acquisition systems are based on our LAN-XI hardware, which can be configured as either a single front-end or as distributed systems, and on our user-centric BK Connect software.