The use of Impulse Response Function for Modal Parameter Estimation

This article discusses the errors that are introduced when calculating the impulse response function from the frequency response function using DFT/ FFT analysis technique. The impulse response function is used in many applications of system analysis where a description of the system characteristics is preferred in the time domain rather than in the frequency domain. The case where the system is a single degree of freedom system (SDOF), with viscous damping, is analysed. It is shown that the estimated impulse response function is not described by a single exponentially decaying sine wave but rather as a sum of exponentially decaying sine waves. It is important to apply this slightly more complex model when the measurements are to be used for mathematical modelling of systems containing lightly damped resonances and where measurements free of the influence of leakage are performed. This article represents an expansion of papers presented at the Modal Analysis Conference ISMA 15, held in Leuven, Belgium 1990 and included in the Proceedings of IMAC 9, Firenze, Italy 1991. Resume Cet article décrit les erreurs pouvant survenir lors du calcul de la réponse impulsionnelle à partir de la fonction de réponse en fréquence, dans le cadre d'une analyse DFT/FFT. La fonction de réponse impulsionnelle intervient lorsque, pour décrire les caractéristiques d'un système, les techniques d'analyse en fréquence dans le domaine temporel sont préférées à celles du domaine fréquentiel. Le cas d'un système à un degré de liberté (SDOF) avec amortisseurs à fluide est ici exa-