Research On Harmonic Excitation Problem In Operational Modal Analysis

Harmonic excitation will occur in the running state of rotating machinery,the corresponding harmonic modes will seriously affect the accuracy of structural modal parameter and cause completely wrong results.In this paper,the problem of harmonic excitation in operational modal analysis is studied,the methods of harmonic modal detection and elimination are explored,the reliability of operational modal analysis using power spectral density transfer rate as initial data is verified.In harmonic modal detection,probability density curve,kurtosis value and spectral kurtosis detection methods are used.The characteristics of each method are discussed and verified by examples.The study shows that compared with the two methods in time domain,the spectral kurtosis method in frequency domain has the advantages of full-band detection,no filtering and strong anti-noise.In harmonic mode rejection,stopband filter is used,but limited by filter pollution.Then,before exploring the method of eliminating harmonic modes based on half power spectrum matrix,an example is given to verify the method of eliminating harmonic modes by harmonic-decomposition-reconstruction in experimental modal analysis.The study shows that compared with stopband filter,the method based on half power matrix eliminates harmonic modes without filtering and has shorter calculation time,and the result is more accurate.In practical engineering,the environmental excitation of the system is not an ideal type of white noise,so the above modal analysis method has some limitations.In this paper,the theoretical properties of power spectral density transfer rate are studied.The results show that the value of power spectral density of the same origin-reference degree of freedom combination is related to the degree of freedom of transmission output under the condition of multiple excitations and irrelevant excitations,but the transmission rate of power spectral density at the poles of the system tends to the same value,that is,the ratio of the degree of origin-freedom to the degree of reference-freedom,which is also the operation.The theoretical basis of modal analysis based on power spectral density transfer rate.Then the PSDTMSVD method is used to analyze the operational modal.Under the condition of non-white noise excitation and harmonic excitation,the obtained modal parameters results are still reliable.Finally,the validity and engineering feasibility of the above modal analysis method are verified by the modal vibration test of aluminium alloy rudder.