Operational Modal Analysis



Do you need to do modal analysis without controlling the input?

You might wonder what Operational Modal Analysis, OMA, is and how it differs from the traditional Experimental Modal Analysis (EMA) 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 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 rare cases 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 multiple 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 inputs 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 technology 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.

Please have a look at the Technical Review for a technical introduction OMA and its similarities and differences to EMA.

Application of Operational Modal Analysis

There are many applications where Operational Modal Analysis is the natural choice of technology for supplying structural information. Below a few examples are listed. For more examples please visit the case studies page.

Back in the 1980's civil engineers were among the first to adapt Operational Modal Analysis (OMA) for testing of large structures. The size of the civil engineering structures and the fact that ambient excitation typically is uncontrollable pushed the development of OMA techniques heavily in this area. Today OMA is a wide spread technology used in many engineering field but testing of civil engineering structures is still the most popular OMA application. Back in 1999, ARTeMIS software was originally developed as a spin-off of research made Aalborg University, Department of Civil Engineering, and even the ARTeMIS name refer to its civil engineering roots as it is the abbreviation of Ambient Response Testing and Modal Identification Software.

Below a list of typical civil engineering applications are presented.

Bridges

Bridges have been in focus for decades due to the slender structure that makes them dynamical sensitive. The most famous example of a bridge disaster caused by dynamic effects is probably the Tacoma Narrows Bridge disaster. In case a bridge collapses, it has enormous impacts both economically and for the society. Large bridges are therefore typically tested in the commissioning state for design verification and perhaps flutter analysis. A typical example is the ambient test of the Vasco da Gama Bridge in Portugal that is shown in the below picture.

The test of the Infante D. Henrique Bridge and the Guadiana Bridge are other examples from the Laboratory of Vibrations and Monitoring at University of Porto.

Smaller bridges like highway crossings have also been the subject for many years. Here the major concern typically is deterioration over time due to environmental effects. Combine this with the vast number of highway bridges typically present in a country is one of the reasons why this type of applications also attrack a lot of focus. Several research projects has concerned structural health monitoring of this type of structures. One of the recent projects are the IRIS project concerning the Austrian S101 highway bridge.

High Rise Buildings

High rise buildings are other types of dynamic sensitive structures, and specially in earthquake active regions, it is important to know the fundamental modes of these structures. Below a typical example is shown. This is the Heritage Court Tower located in Vancouver, British Columbia, Canada.

One of the challenges of analyzing building are the typical use of architectual symmetry. This causes closely spaced modes, i.e. modes have different mode shapes but natural frequencies of more or less same values. This is also the case in the above example, but by using the Stochastic Subspace Identification techniques of ARTeMIS Modal Pro it is possible to extract all the excited modes even in this case.

There are several reasons why ARTeMIS software for Operational Modal Analysis (OMA) is a popular tool in wind turbine applications.

Design Verification and Optimization

First of all, there is a growing need to know the dynamic behavior of wind turbines under the right operational conditions when the true environmetal forces are acting on the structure. This need is caused by the rapid development in the dimensions of the turbines. The larger the dimensions get the lower the dynamic behavior in frequency gets. This put the numerical models under pressure and there natural becomes a need for experimental verification of these. In some cases it is even required to follow an experimental study with an updating of the numerical models.

Secondly, sub-parts such as blades and tower are typically having well documented dynamic characteristics that are provided independently of each other. However, problems arises when these subparts are integrated with e.g. the nacelle and the foundation. The integration of course has a serious impact of the dynamic behaviour of the subparts. In order to study this impact OMA is the natural tool to make use of. See e.g. the case study of the FE model updating of the Fuhrländer AG 2.5 MW wind turbine. OMA will return results for the complete turbine behavior under the right environmental conditions. This makes it possible to update numerical models of the complete turbine to behave correctly under the operational conditions of the test.

A typical example of a full-scale test is shown in the below pictures and documented in the paper Artificial and Natural Excitation Testing of SWiFT Vestas V27 Wind Turbines.

Frequency Domain Decomposition analysis.
Crystal Clear Stochastic Subspace Identification analysis.

Easier Documentation of Sub Parts Dynamic Behaviour

Dynamic characterization has been applied in sub part testing in case of e.g. blades. A popular approach has been a free response testing scheme where each mode of the blade is determined in a single test by applying excitation in carefully selected locations of the blade. This is a very costly way of testing as it is time consuming, and there is a potential risk that multiple modes are excited at the same time which consequently will polute the results. In this case, OMA has made life easier for several test institutions. The reason is that all modes now accurately can be determined from a single test.

Crystal Clear Stochastic Subspace Identification modal analysis of a wind turbine blade.