Identification of closely spaced modes using Ibrahim Time Domain method
A. Malekjafarian, R. Brincker, M. R. Ashory, M. M. Khatibi
Measuring the input loads to identify modal parameters of structures sometimes is difficult and even impossible in their operating condition. Therefore, experimental methods based on Operational Modal Analysis (OMA) are often preferred in these cases. In OMA only the measured responses of the system are considered. This approach identifies the modal properties of the system in the exact operational conditions and also real boundary conditions can be used that cannot be realized in the labs. The methods such as: Ibrahim time domain (ITD) method, the least-squares complex exponential (LSCE) method and the Eigen realization algorithm (ERA), are the well-known output-only methods in time domain that require free decay responses or Impulse Response Function (IRF) of the structures. It is often claimed that ITD does not work well in case of closely spaced modes because the method is SIMO, whereas the other mentioned techniques that are known to be MIMO are claimed to work well also in case of closely spaced modes. In this paper, the traditional ITD method is studied under the presence of closely spaced modes and is compared to a modified formulation of the ITD that can take advantage of multiple input loading (several free decays). A numerical case study with closely spaced modes in a plate is considered.
Proc. 4th International Operational Modal Analysis Conference (IOMAC), 2011, Turkey.