Software for Operational Modal Analysis
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What is Operational Modal Analysis?

You might wonder what Operational Modal Analysis, OMA, is and how it differs from the traditional experimental modal analysis that has been around for the last decades.

 Operational Modal Analysis is also called output-only modal analysis, ambient response analysis, ambient modal analysis, in operation modal analysis, and natural input modal analysis. No matter which name that is used the idea is the same: To do modal analisys without knowing and/or controlling the input excitation. This new modal technology is capable of estimating the same modal paramters as the traditional known techniques. The modal parameters are the mode shape, the natural frequency and the damping ratio. Some thinks that Operational Modal Analysis just is another name for Operating Deflection Shapes, ODS. This is not the case. Operational Modal Analysis separates noise and input and returns the modal information only.

There are a number of benefits in using the Operational Modal Analysis compared to the more traditional techniques.

Multiple Input Multiple Output Modal Technology

The Operational Modal Analysis are Multiple Input Multiple Output, MIMO, techniques. This means that the techniques are capable of estimating closely space modes and even repeated modes with a high degree of accuracy. Traditional modal anlysis techniques are typically Single Input Multiple Output, SIMO, or Multiple Input Single Output, MISO, or in the worst case even Single Input Single Output, SISO. Such testing procedures will not be able to find repeated poles due to the lack of mode seperation. Easier Laboratory Modal Testing There is no need for vibration shaker or impact hammer anymore. If you are in your lab doing modal testing in a test rig on some structural component, just do some random tapping on the structure while you are measuring the vibration response in muliple locations. The tapping must be random in time but also spatially. The excitation produced in this way will be a good approximation of a multivariate white noise stochastic process. Winning Technology in In-situ Modal Testing Vibration shakers and impact hammers are impossible as excitation sources when it comes to insitu testing of structures, such as buildings or rotating machinery. In cases like this the traditional modalanalyse fails, because there are a number of unknown input acting on the structures. What is a problem for traditional modalanalysis is a strength for Operational Modal Analysis. The more random input sources there are the better the modal results gets. Since the real strength of the technology really lies in the in-situ testing it is no wonder why the technilogy is called Operational Modal Analysis. Another important features that comes for free are that the estimated modes are based on true boundary conditions, and the actual ambient excitation sources.

Application of Operational Modal Analysis

There are many applications where Operational Modal Analysis is the natural choice of technology for supplying structural information.

Nondestructive Testing

In non-destructive testing, NDT, the objective typically is to monitor the health of a structure over time. For this reason it is also known as Structural Health Monitoring, SHM. Since the structure is observed during service no other modal tool can provide modal information in such a case.

Damage Detection

For many years Operational Moda Analysis has been the preferred tool in damage detection of large structures. Mode shapes has been used to identify local damages that caused curvature changes and e.g. the modal frequencies has been used to identify global damages. Research is still extensive in this area combining technologies such as modal analysis, neural networks or response surfaces, and stochastic decision theory.

Vibration Level Documentation

If the vibration level needs to be documented in locations where no measurements can be made, Operational Modal Analysis can do it if you have a Finte Element Model, FEM, available. From the modal test you will obtain the modal coordinates or modal response at some measurable locations. These modal responses will then be extrapolated to other unmeasured locations through the mode shapes of the FE model, and by superposition the actual responses at the location is estimated. Even though the FE models only return normal modes this extrapolation will be good most structures.

Fatigue Analysis

The above mentioned vibration level documentation can also be extended to estimate the accumulated damage at unmeasured location such as underwater joints etc. In this way, ordinary inspections at e.g. offshore facilities can be optimized, since a few measurement points can give the engineers valuable fatigue estimates to help in the inspection planning.

Scaled Mode Shapes

The drawback of Operational Modal Analysis has been that the mode shape scaling has been arbitrary causing incorrect modal participation factors. This has caused problems in applications such as response simulation and structural modification. Recently, however, techniques to obtain the right scaling has been developed and they are now being tested on full scale structures. The results of these tests indicates that for even larger structures it is possible to obtain an accurate Frequency Response Function, FRF, from response measurements only.

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