A method, called the Eigensystem Realization Algorithm (ERA), is developed for modal parameter identification and model reduction of dynamic systems from test data. A new approach is introduced in conjunction with the singular value decomposition technique to derive the basic formulation of minimum order realization which is an extended version of the Ho-Kalman algorithm. The basic formulation is then transformed into modal space for modal parameter identification. Two accuracy indicators are developed to quantitatively identify the system modes and noise modes. For illustration of the algorithm, examples are shown using simulation data and experimental data for a rectangular grid structure.

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An eigensystem realization algorithm for modal parameter identification and model reduction

Operational modal analysis deals with the estimation of modal parameters from vibration data obtained in operational rather than laboratory conditions. This paper extensively reviews operational modal analysis approaches and related system identification methods. First, the mathematical models employed in identification are related to the equations of motion, and their modal structure is revealed. Then, strategies that are common to the vast majority of identification algorithms are discussed before detailing some powerful algorithms. The extraction and validation of modal parameter estimates and their uncertainties from the identified system models is discussed as well. Finally, different modal analysis approaches and algorithms are compared in an extensive Monte Carlo simulation study.

System Identification Methods for (Operational) Modal Analysis: Review and Comparison

An algorithm is developed to obtain the free responses of a structure from its random responses, due to some unknown or known random input or inputs, using the random decrement technique without changing time correlation between signals. The algorithm is tested using random responses from a ' 'generalized pay load" model and from the "space shuttle" model. The resulting free responses are then used to identify the modal characteristics of the two systems. The method is limited to structures that are linear or have small nonlinearities. N general, the experimental identification of structural modes of vibration is carried out by measuring the input (or inputs) to the structure under test and the resulting responses due to this input. Some vibration testing techniques, in order to simplify the identification procedure, use the free responses of structures. In such cases, although the input excitation need not be measured, some initial excitation is applied to the structure, and free responses are measured immediately after the initial exciting force is removed. There are situations where controlled excitation or initial excitation cannot be used. For example, if the structure to be tested is in operation, applying any kind of external force may cause undesirable interruption. Another example is the case of in-flight response measurements, where a complete knowledge of the excitation is usually not available. In such cases, the use of the "random decrement signature" technique (a special averaging procedure that is used to determine the step and/or impulse response from the random response) to obtain the free responses is a promising technique. The random decrement signature technique1 has been used successfully for failure detection and damping measurement of structures in single-statio n, single-mode response cases. Application of the random decrement signature technique to a multiple of signals changes the time correlation between the individual signals. If the resulting responses are to be used to identify several modes of a structure, the random decrement signature technique must be modified to keep the time correlation between signals unchanged. In this paper, an algorithm is developed to obtain the free responses of a linear structure from its random responses, due to some unknown or known random input or inputs, using the random decrement technique without changing time correlation between signals. The algorithm is tested by ap- plying it to random responses obtained from two real structures. The first structure is a generalized payload model previously tested using sine sweep method and analyzed by NASA Structural Analysis (NASTRAN). The second structure is the V& -scale space shuttle model with modal parameters previously determined using sine sweep method and fast Fourier transform (FFT). Only responses from four stations on the solid rocket boosters (SRB's) were considered in the case of the space shuttle model. The filtered random responses from these two structures were recorded and

Random Decrement Technique for Modal Identification of Structures

https://repositorio-aberto.up.pt/bitstream/10216/67103/2/56957.pdf

E. C. Mikulcik

Papers

A method for the direct identification of vibration parameters from the free response