Extraction Of Modal Parameters From The Response Power Cepstrum

✍ Scribed by R.B. Randall; Y. Gao

• Publisher: Elsevier Science
• Year: 1994
• Tongue: English
• Weight: 447 KB
• Volume: 176
• Category: Article
• ISSN: 0022-460X
• DOI: 10.1006/jsvi.1994.1367

✦ Synopsis

There are many situations in which it would be advantageous to be able to separate a measured response signal into its forcing function and transmission path components. This paper shows how the cepstrum can be used to extract the poles and zeros of the frequency response function (FRF) from a measured response autospectrum, by curve-fitting analytical expressions to selected regions of the response power cepstrum which are known to be dominated by the FRF. For driving point measurements, this determines the FRF to within a scaling constant, while with transfer measurements it is necessary to add "phantom zeros", determined in an earlier measurement, to correct for the effects of out-of-band modes. The latter may not be necessary in situations in which it is required only to detect changes in resonance and antiresonance frequencies. It is shown how it would often be possible to achieve an approximate scaling by using zero frequency values obtained by simple measurement or calculation. The technique is demonstrated by using the response of a free-free beam to impact excitation, but it is hoped to extend it to machines with a periodic impulsive excitation, such as gearboxes and diesel engines. With this in mind, the analysis was also applied where a double bounce in the excitation gave a similar effect in the cepstrum as the forcing function of these machines.