Document Type : Research Paper


1 Department of Aerospace Engineering, Shahid Sattari Air University, Tehran, Tehran

2 Department of Aerospace Engineering, Sharif University of Technology, Tehran, Tehran


In this research, the modal parameters of a beam in free-free condition are extracted by performing different experiments in laboratory. For this purpose, two different techniques are employed. The first methodology is considered as a time domain method in Operational Modal Analysis. While the other one is frequency domain impact hammer test which is categorized as an Experimental Modal Analysis method and can be regarded as the most common method in modal analysis. Checking the results obtained by the two methods, one can notice a distinct inconsistency in modal damping ratios extracted by each method. However, based on recent publications on the subject, it can be inferred that the time domain methods have better accuracy in identifying damping ratios of structures. In order to confirm the findings, the effect of excitation is examined for each method by altering the excitation tool. For the operational method, it is concluded that changing the excitation tool will not have a noticeable influence on the identified damping ratios, whilst for the Experimental Modal Analysis method changing the hammer tip leads to inconsistent results for damping ratios. This study exemplifies the deficiency of Experimental Modal Analysis methods in their dependency on excitation techniques.

Graphical Abstract

A comparison between modal damping ratios identified by NExT-ERA and frequency domain impact test


Main Subjects

[1]     A. Malekjafarian, R. Brincker, M. R. Ashory, M. M. Khatibi, “Identification of closely spaced modes using Ibrahim Time Domain method”, Proc. of 4th “International Operational Modal Analysis Conference”, Istanbul, Turkey, pp. 443-449, (2011).
[2]     R. M. Lin, “Development of a New and Effective Modal Identification Method - Mathematical Formulations and Numerical Simulations”, Journal of Vibration and Control, Vol. 17, No. 5, pp. 741-758, (2010).
[3]     J. He, & Z. F. Fu, Modal Analysis, Butterworth-Heinemann, Oxford, pp. 180, (2001).
 [4]    T. Wang , O. Celik, F. N. Catbas, L. M. Zhang, “A frequency and spatial domain decomposition method for operational strain modal analysis and its application”, Engineering Structures, Vol. 114, pp. 104-112, (2016).
[5]     J. -N. Juang, & R. Pappa, “An Eigensystem Realization Algorithm (ERA) for Modal Parameter Identification”, In NASA/JPL Workshop on Identification and Control of Flexible Space Structures, Pasadena, CA, USA, (1984).
[6]     T. P. Le, P. Paultre, “Modal identification based on the time–frequency domain decomposition of unknown-input dynamic tests”, International JournalofMechanicalSciences, Vol. 71, pp. 41-50, (2013).
[7]     S. D. Zhou, W. Heylen, P. Sas, L. Liu, “Maximum likelihood estimator of operational modal analysis for linear time-varying structures in time–frequency domain”, Journal of Sound and Vibration, Vol. 333, No. 11, pp. 2339-2358, (2014).
[8]     A. Barjic, R. Brincker, C. T. Georgakis, “Evaluation of Damping Using Time Domain OMA Techniques”, Proc. of 2014” Society Experimental Mechanics Fall Conference and International Symposium on Intensive Loading and Its Effects”, Beijing, China, pp. 176-181, (2014).
[9]     F. Gomaa, M. Tayel, K. Kandil, G. Hekal, “Validation Study Illustrates the Accuracy of Operational Modal Analysis Identification”, International Journal of Emerging Technology and Advanced Engineering, Vol. 2, No. 11, pp. 658-667, (2012).
[10]   F. N. Kudu, S. Ucak, G. Osmancikli, T. Türker, A. Bayraktar, “Estimation of damping ratios of steel structures by Operational Modal Analysis method”, Journal of Constructional Steel Research,  Vol. 112, pp. 61-68, (2015).
[11]   A. Bajric, R. Brincker, S. Thöns, “Evaluation of damping estimates in the presence of closely spaced modes using operational modal analysis techniques”, Proc. of the 6th “International Operational Modal Analysis Conference”, Gijon, Spain, (2015).
[12]   P. Mohanty, & D. J. Rixen, “A modified Ibrahim time domain algorithm for operational modal analysis including harmonic excitation”, Journal of Sound and Vibration, Vol. 275, No. 1-2, pp. 375-390, (2004).
[13]   F. Magalhaes, E. Caetano, A. Cunha, “Operational Modal Analysis of the Braga Sports Stadium Suspended Roof”, Proc. of the 24th “International Modal Analysis Conference”, Saint Louis, Missouri, United States of America, (2006).
[14]   P. Mohanty, & D. J. Rixen, “Modified ERA method for operational modal analysis in the presence of harmonic excitations”, Mechanical Systems and Signal Processing, Vol. 20, No. 1, pp. 114-130, (2006).
[15]   R. Pintelon, P. Guillaume, J. Schoukens, “Uncertainty calculation in (operational) modal analysis”, Mechanical Systems and Signal Processing, Vol. 21, No. 6, pp. 2359-2373, (2007).
[16]   J. M. Caicedo, “Practical Guidelines for the Natural Excitation Technique (NExT) and the Eigensystem Realization Algorithm (ERA) For Modal Identification Using Ambient Vibration”, Experimental Techniques, Vol. 35, No. 4, pp. 52-58, (2011).
[17]     H. Moncayo, J. Marulanda, and P. Thomson, “Identification and Monitoring of Modal Parameters in Aircraft Structures Using the Natural Excitation Technique (NExT) Combined with the Eigensystem Realization Algorithm (ERA)”, Journal of Areosapce Engineering, Vol. 23, No. 2, pp. 99-104, (2010).
[18]   C. Devriendt, G. D. Sitter, P. Guillaume, “An operational modal analysis approach based on parametrically identified multivariable transmissibilities”, Mechanical Systems and Signal Processing, Vol. 24, No. 5, pp. 1250-1259, (2010).
[19]   A. Agneni, L. B. Crema, G. Coppotelli, “Output-Only Analysis of Structures with Closely Spaced Poles”, Mechanical Systems and Signal Processing, Vol. 24, No. 5, pp. 1240-1249, (2010).
[20]   L. Zhang, T. Wang, Y. Tamura, “A frequency–spatial domain decomposition (FSDD) method for operational modal analysis”, Mechanical Systems and Signal Processing, Vol. 24, No. 5, pp. 1227-1239, (2010).
[21]   G. W. Chen, S. Beskhyroun, P. Omenzetter,       “A       comparison      of operational modal parameter identification methods for a multi-span concrete motorway bridge”, Proc. of “the New Zealand Society for Earthquake Engineering Annual Conference”, Auckland, New Zealand, No. 54, (2015).
[22]     F. Pioldi, E. Rizzi, “Assessment of Frequency versus Time Domain enhanced technique for response-only modal dynamic identification under seismic excitation”, Bulletin of Earthquake Engineering, (online).
 [23] S. A. H. kordkheili, S. Hajirezayi, S. H. Momeni. M, “A Survey on Time Domain MIMO Identification Techniques for Experimental and Operational Modal Analysis”, Modares Mechanical Engineering, Vol. 15, No. 12, pp. 254-264, (2015).
[24]   S. J. Hu, W. L. Yang, F. S. Liu, H. J. Li, “Fundamental comparison of time-domain experimental modal analysis methods based on high- and first-order matrix models”, Journal of Sound and Vibration, Vol. 333, No. 25, pp. 6869-6884, (2014).
[25]   N. M. M. Maia, Extraction of Valid Modal Properties from Measured Data in Structural Vibrations, PhD Thesis, Department of Mechanical Engineering, Imperial College, London, (1988).