The characterization of dynamic behavior of any physical system is mostly determined by its inertial mass, stiffness and damping properties. In its simplest way, a typical natural frequency testing would reveal the system response. An expanded, in detail testing methodology, considering the response of the entire system characterization is called modal testing or experimental modal analysis (EMA).


This classical controlled method of evaluating the system response is practiced in multiple industry and application domains both as a design and as well an investigation, validation tool. Since this is a closed loop measurement, meaning that, both the input forces and the response vibrations are known through the testing, establishing the system's eigen parameters become lot reliable and controlled.


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Various operational modes of a typical steel structural bridge

There are many cases of equipment, systems or structures in operation that are not practically feasible to be considered for a typical modal testing; these are the likes of large civil structures such as bridges, towers, cranes and many other. Investigating operational responses to assess their system characteristics is a challenge of sorts.


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Consider a case of a wind turbine standing at 90 to 140 meter above the ground and rotating at a maximum speed of 18 RPM. How would one establish if the entire tower mast, nacelle, turbine blades are all operating within their dynamic limits? how would one know if the mast is not oscillating excessively due to impending wind loads and changing directions? what if the tower's natural frequency is anywhere close to its resonance condition?

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Various operational deflections of a vertical cantilever structure

Extending the basic concepts of EMA, methods of operational modal analysis (OMA) and operational deflection shapes (ODS) have been developed with complex mathematics and linear time and frequency decomposition methods.


In the absence of quantifying the input forces, the OMA considers the naturally available excitation as the input and measures the output in a controlled transpose approach. Using complex solvers such as canonical variance (CV) and balance realization (BR) methods, results are obtained to build a system model that near replicates to derive system characteristics.


Some more detailing of the techniques are in this link where OMA are discussed; The use of cross powers, time and frequency domain decompositions are well explained to establish how these are a credible solution, it also looks at the limits of this methodology and many of the care to be taken to conduct the tests appropriately.


ODS is another simple and versatile tool that can bring to perspective how a large system is behaving both as a function of its resonance and non-resonance conditions; this is a good visualization tool helping the user to know how and when the system under investigation is tending towards an unstable condition. This approach uses the vibration magnitudes and relative phase between multiple vibration sensors configured at identified locations on the test object.


In practice, both the OMA and ODS come handy and useful where typical modal testing cannot be applied. Combining the knowledge of the system under test, conducting tests with boundary conditions that help in obtaining stable and repeatable data, will all bring good value proposition to either of these methods.


With years of applied experience and real-life task exposures, NV Dynamics has developed credible knowledge in both using these tools based on their merits of the situation and as well as considering the typical outputs to be derived. These are well complimented by the latest software and hardware tools that make noticeable differences to quality of results and their interpretations. Get in touch with us to know the variety of cases we have handled and how our expertise can help you solve your case.