Identification of Closely Spaced Modes from Operational Modal Analysis

The analytical models are not accurate enough in practical cases due to inaccuracy in modeling of the joints, numerical errors and other problems. Modal testing has been recognized as an experimental alternative for modeling the dynamic behavior of structures and obtaining the modal parameters including: natural frequencies, mode shapes and damping factors. 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. However from experimental point of view these modes are detected as only one mode. In this article Operational Modal Analysis (OMA) is applied to identify closely spaced modes. 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. The results from simulations demonstrate the effectiveness of OMA for identifying and separating the close modes.