Enhancement of coherence functions using time signals in Modal Analysis

Experimental Modal Analysis (EMA) and Operational Modal Analysis (OMA) are two widely used techniques in the identification of modal parameters. EMA is synonymous with a laboratory environment requiring complete system shutdown while OMA is implemented in a real environment where the ambient forces cannot be isolated. A new method, namely 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. Transfer functions, from where the modal parameters are extracted, are obtained from Fourier transform of cross and auto correlation functions. These functions are estimated quantities and their outcomes are dependable on the averaging techniques used. The coherence functions are commonly used to measure the acceptability of the estimations. Impact-Synchronous Time Averaging is compared against Spectral Averaging while performing Modal Analysis in a situation containing ambient and operating forces. Results showed that while the transfer functions obtained from both the averaging techniques were of similar quality, the Impact-Synchronous Time Averaging indicated better coherence than the Spectral Averaging. (C) 2011 Elsevier Ltd. All rights reserved.