Impact-synchronous modal analysis (ISMA) - An attempt to find an alternative

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Authors: Rahman, A.G.A., Ismail, Z., Noroozi, S. and Chao, O.Z.

Journal: 5th International Operational Modal Analysis Conference, IOMAC 2013

Pages: 1-8

Modal frequencies, modal shapes and modal damping comprehensively define the dynamic characteristics of a structure. Prior to developing a mathematical model of the dynamic behaviour of any structure, these parameters need to be firstly determined. Currently, the two techniques used to extract these parameters are the classical Experimental Modal Analysis (EMA) and the Operational Modal Analysis (OMA). The fundamental difference in the two techniques lies in the manner by which the system is excited. A novel method called Impact-Synchronous Modal Analysis (ISMA) utilising the modal extraction techniques commonly used in EMA but performed in the presence of the ambient forces is proposed. The differences between EMA and ISMA lie in the averaging technique used. ISMA utilised Impact-Synchronous Time Averaging (ISTA) triggered by the impulse of the artificial force introduced by the impact hammer prior to performing the Fast Fourier Transform (FFT) instead of frequency domain averaging adopted by conventional EMA. Results showed that modal parameters were successfully determined in the presence of periodic responses of cyclic loads and ambient excitation using ISMA in both laboratory rig and industrial machinery. The well correlated results with classical EMA during static condition show the effectiveness of performing ISMA in the presence of the unaccounted forces.

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