Operational Modal Analysis for Rotating Machines: Challenges and Solutions

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Operational Modal Analysis (OMA) is widely employed and became an industrial standard technique for identifying the modal parameters (i.e. resonance frequencies, damping ratios and mode shapes) of mechanical structures. Its main applications are in the automotive, aerospace, civil engineering domains and many others. The advantage, if compared with Experimental Modal Analysis (EMA), is that it is not necessary to stop the machine, but its modal characteristics can be estimated during its operating cycles. In other words, OMA does not rely on known and deterministic excitation, but it uses exclusively the natural vibrations of the structure. It is very useful in cases in which the forces cannot be measured or when it is very difficult to excite a structure and it is more convenient to exploit the natural ambient excitation. The input forces are unknown and the main concept behind OMA is that the structure can be identified from a dynamic perspective by analyzing only the measured output signals. Several hypothesis need to be verified in order to apply OMA. First of all the structure must be Linear Time Invariant (LTI), but this is not the case if several parts are moving with respect to each other. Secondly, the forces acting on the structure must be represented by white noise in the frequency range of interest. This means that all the frequencies must be uniformly excited. This is often the case for wind excitation, but it is not valid anymore if periodical loads due to rotating elements are introduced in the system. The main scope of this dissertation is to fully understand and propose solutions to the challenges and the limitations occurring when applying classical OMA techniques in case of rotating machines. Several simulation and test cases will be discussed in order to validate the proposed solutions.