Enhancement of Impact-Synchronous Modal analysis (ISMA) with number of averages.

Abstract

A new method, namely Impact-synchronous Modal Analysis (ISMA), utilizing the modal extraction technique commonly used in Experimental Modal Analysis performed in the presence of the ambient forces, is proposed. In ISMA, the extraction is performed while the machine is running, utilized Impact-synchronous Time Averaging prior to performing the Fast Fourier Transform. The number of averages had a very important effect when applying ISMA on structures with dominant periodic responses of cyclic loads and ambient excitation. With a sufficient number of impacts, all the unaccounted forces were diminished, leaving only the response due to the impacts. This study demonstrated the effectiveness of averages taken in the determination of dynamic characteristics of a machine while in different rotating speeds. At low operating speeds that coincided with the lower natural modes, ISMA with a high number of impacts determined the dynamic characteristics of the system successfully. Meanwhile, at operating speeds that were away from any natural modes, ISMA with a moderate number of averages taken was sufficient to extract the modal parameters. Finally for high-speed machines, ISMA with a high number of impacts taken has limitations in extracting natural modes close to the operating speed.