Identification Method Of Partially Coherent Source And Its Transfer Path Of Mechanical System

The vibration and noise of vehicles and mechanical equipment have an important impact on the comfort of riding and use.The identification of the vibration sources and excitation source that cause noise and vibration is crucial for fault diagnosis,noise control,and parameter identification.In actual situations,noise sources are often neither completely coherent nor completely incoherent,but partially coherent.At this time,how to identify partially coherent noise sources becomes a key issue.Through the establishment of a new transmissibility matrix identification method,the location of coherent sources can be determined quickly,accurately and reliably.This method combined with the operational transfer path analysis method is an effective means to solve the problem of partially coherent sound sources identification.It is an important method to identify partially coherent distributed excitation by equating the distributed excitation to the concentrated excitation with the same effect on the target output point.The identification of the noise source requires the process of identifying the radiation source and then identifying the vibration source.Through the two-stage identification model of the partially coherent noise source and its transfer path,the contribution of the noise source can be completely quantitatively analyzed,and realize the prediction of sound pressure level and sound quality.The method has been applied in the traceability and contribution analysis of interior noise and centrifugal fan distribution excitation.In this paper,theoretical and experimental researches were carried out on the identification methods of partially coherent sources of mechanical systems and their transfer paths.The main research work is as follows:(1)Aiming at the problem of identifying partially coherent sources,a new transmissibility matrix identification method was proposed.This method was based on the assumption of partially coherent sources.The positions corresponding to a group of sensor combinations with the smallest transmissibility matrix F norm were the positions of partially coherent sources.The positions of all partially coherent sources could be obtained by one calculation.Compared with the existing methods based on the independent source assumption,the transmissibility matrix method had higher calculation efficiency and stronger robustness.The method has been verified in the experiments of noise source tracing in a semi-anechoic room and in a car.The partially coherent output power spectrum method was used to analyze the contribution of partially coherent noise sources,and the concept of aliasing rate was proposed,which could judge the system noise source coupling.The partially coherent sound source identification and contribution analysis method was applied to the identification and contribution analysis of the partially coherent sound source noise of automobile exhaust system and centrifugal fan,and the contribution of the independent part and the coupled part of the partially coherent source to the noise was quantitatively calculated.The coupling effect of the sources may have a negative contribution to the system output.(2)Although centralized excitation sources are often partially coherent sources,in some cases they can also be identified and calculated based on independent sources,while continuous distributed excitation sources must be partially coherent.In order to solve the partially coherent distributed excitation identification problem,an equivalent excitation force identification method for partially coherent distributed excitation sources was proposed The so-called "equivalent" referred to the same vibration response to the target point.The equivalent excitation source combination was determined by the minimum cumulative error of the target point,and the Tikhonov regularization method is applied to reduce the identification error,to achieve accurate identification of the distributed excitation sources,and form a complete distributed excitation inversion calculation and analysis process.Taking the centrifugal fan as an example,the method was studied experimentally.The vibration of the fan volute and the mount of the fan was tested under rated conditions,and the transfer function from the candidate equivalent excitation points of the volute to each response point provided parameters for inversion of the equivalent excitation force of the volute.In order to understand the real pressure pulsation distribution inside the fan and provide verification data for the inversion results,the pressure pulsation on the surface of the volute under rated conditions was tested.Since the main excitation frequency of the fluid inside the volute was the blade passing frequency,identification and analysis were focused on this frequency.The identification errors of the equivalent excitation force of the main peak frequency were less than 3d B.Through the partially coherent distributed excitation source identification model based on equivalent excitation force,the internal distributed excitation of the centrifugal fan was successfully obtained.(3)The identification of noise sources is different from the identification of vibration sources.It is necessary to find the noise radiation source first and then find the vibration source according to the location of the radiation source.In this process,it is necessary to find and analyze the contribution of partially coherent noise sources and their transfer paths.Due to the narrow space,complex transfer path and serious coupling of the sound field in the car,it is difficult to accurately find the noise sources.Therefore,the method is studied for the noise source in the car.The transmissibility matrix identification method of partially coherent sound sources was applied in the analysis,and established a two-stage identification model from target noise to radiated sound source,radiated sound source to vibration source,Finally,the location of the main sound sources in the car were determined and a quantitative contribution analysis was carried out.The reduction of noise in the car ultimately served the occupants in the car,making it more comfortable to ride.A single sound pressure level target cannot fully reflect the impact on the passenger comfort.For this reason,the traceability and contribution of noise based on the sound quality target and the discomfort target were studied,and the influence of the radiated sound source variation on the sound pressure level and sound quality index was analyzed.According to the identification results targeting sound pressure level,sound quality and discomfort,the transfer path of the noise sources that contributed significantly to the interior of the car were determined.The vehicle's transmission system and suspension system were modeled and analyzed,and improvement measures were proposed and implemented.Finally,ideal optimization results were obtained.