Research On EMD Method For Improving End Effect And Suppressing Modal Mixing

Empirical Mode Decomposition(EMD)is a time-frequency analysis method,which was proposed by NE Huang in 1998 for processing non-stationary nonlinear signals.EMD can decompose the signal into several intrinsic mode functions according to the signal local characteristics.In the last twenty years,EMD algorithm gradually showed its strong vitality in the field of non-stationary signal processing.Now,EMD has been widely used in signal filtering and de-noising,blasting signal processing,mechanical fault diagnosis,structural modal identification and medical image analysis.Based on EMD algorithm theory,the end effect and mode mixing in the algorithm were discussed,and new methods to solve the problem were proposed.From the perspective of application,the improved EMD algorithm was extended to structural modal parameter identification,then improved HHT method was obtained which can effectively identify dense modal's parameters.The main contents of this study conclude the following four parts:(1)For the end effect problems,the mechanism of end effect was analyzed and the evaluation index of end effect was summarized.Based on Euclidean distance and Pearson correlation coefficient,a comprehensive index of waveform matching degree was established,which can simultaneously consider signal similarities in both shape and amplitude.Moreover,with energy difference as the target,the optimal continuation wave searching method was proposed,and the smooth extension at the signal end point was realized.(2)For the mode mixing problems,the reasons were summarized as intensive mode interaction and discontinuous event interference.For mode mixing caused by intensive mode interaction,the relationships between mode screening criteria,screening iteration times,frequency modulation,decorrelation operations and EMD mode decomposition ability were respectively discussed.Two improved dense mode decomposition methods were proposed.(3)The measured signal contains both discontinuous events and intensive modes.Only frequency modulation or CEEMD decomposition can not effectively suppress the mode mixing.Therefore,a combined method(called as FM-CEEMD)based on the complementary ensemble empirical mode decomposition(CEEMD)and signal frequency modulation(FM)methods was proposed.The effectiveness of the combined decomposition method was verified by simulation experiment analysis.(4)EMD is the basis of HHT structural modal parameter identification.The quality of IMF decomposition directly affects the precision of modal parameter identification,while mode mixing greatly reduces the decomposition quality of IMF.Therefore,FM-CEEMD was applied to HHT instead of traditional method EMD,and an improved HHT structural parameter identification method was obtained,which can suppress mode mixing significantly.The identified results from both simulation experiment and actual arch dam showed that the improved HHT method can avoid the loss of modal information and improve parameter identification accuracy.