Identification of Closely Spaced Modes Using Operational Modal Testing

The closely spaced modes exist in symmetric structures such as circular plates, gears or disks. Theoretically, closely spaced modes are known as two separated modes with the same amount but, these modes are often detected as only one mode in the classical modal methods. In this article, at the first, some classical modal method is introduced and the most important of their difficulties is considered then, Operational Modal Analysis (OMA) is applied to abate these problems. The Stochastic Subspace Identification based on covariance (SSI-COV) driven is used for this purpose. Different conditions for closely spaced modes is considered and simulated on a free-free steel plate using Matlab software. In order to consider the SSI-COV method in identification of closely spaced modes experimentally, classical and operational modal testing are done on a steel ring. The numerical and experimental results from simulations demonstrate the effectiveness of OMA for identifying and separating the close modes.