Research Of A Fault Feature Extraction Technique Based On Acoustical Holography

At some situations, the fault diagnosis technique based on vibration signals has its restrictions. As the result of vibration emission in air, machine sound signal carries affluent information about the working condition of machine. It processes the advantage of non-contacting measurement. It can partly take place of vibration signal based fault diagnosis and to be used for mechanical fault diagnosis. The fault diagnosis technique based on sound signal is refered to as acoustical diagnosis technique. To implement this technique, it should take into account of the characterisc of mechines'noise and make deep research into the technique of acoustical feature extraction. So, the extracted feature can describe machineries'condition well. Traditional acoustical feature extration method can only describe the law about fault features changing on time, frequency, but they can not reveal the changing information of fault features according to the locations of sound sources. For more efficiently perform fault diagnosis for machineries using sound signal, a fault feature extraction technique based on acoustical holography is presented in this work. This technique use an array consisted of a samll number of microphones to acquire sound pressure, and a hybrid acoustical holography technique can be easily applied to reconstruct the exterior sound field of a radiator. Acoustical holograms are obtained, which includes the information about the numbers of sound sources, and their locations, and intensities. Since the original acoustical holograms can not be dealt with computer directly, an amplitude range is set up based on the sound pressure level of the object machine at normal condition, and difference map element and difference holographic gray scaled map are defined. On the basis of these maps, the variation information about the sound source'feature at certain locations can be identified. Moreover, the particular fault can be determined with the feature frequency and some operating parameters of equipment at the position. The main contents of this dissertation can be summarized as follows.Firstly, the background of fault diagnosis will be introduced. The research history of machinery fault diagnosis technique and acoustical fault diagnoisi technique will be summerized. Then, the development of noise source identification and acousitcal holography technique will be reviewed. Among them, the near-field acoustical holography and equivalent sources method and beamforming are analyzed in specific. With the aim at extracting fault features for machineries in situation by using acoustical feature methods, a number of noise source identification methods are compared with advantages and disadvantages. Then, questions to be solved by this work are presented and a foundation of this work is constructed.After that, the basic theory and reason of sound radiation from vibrating structure of machaneries are analized. And also the mechanical sound radiation problems are described in numerical formulations. And the near-filed acoustical holography (NAH) algorithm is deduced. Some filters in wave number domain for NAH are also discussed. Then, the NAH are discritized. Through numerical simulation, it is shown that NAH can accurately identify sound sources certain cases. It also is proved that it causes wrap-round errors and windowing effects in the calculations. To overcome these shortcomings, the holography surface must be enlarged two times as source surface. As for large scale objects at high frequency, it will need a large number of microphones. The test and reconstruction calculation will cost lots of time. Also, the testing costs increase. These disadvantages hinder the broad applications of NAH in practice. Through theoratical analysis, a fundation is constructed for the acoustical holography based fault feature extraction technique.Wave superposition algorithm is a sound field reconstruction algorithm suits for full frquency band. Its principle is that: a serial of equivalent sound sources are collocated inside the radiator, and the original sound field can be subsitute with that radiated by these equivalent sound sources. The intensity of these sound sources can be evaluated with point collocation or least square method. Reconstruction can be performed with these intensities. The wave superposition formulation and the principle of sound field reconstruction based on wave superposition are deduced. The non-uniqueness of the solution is discussed. Then, the discritization of wave superposition algorithm is studied. Since the sound field reconstruction based on wave superposition is belong to the ill-problems in acoustic field, i.e. its solution is not stable, a regularization method is adopted to elimilate the impact of discritized ill-posed problems. At last, the following factors that will influence the reconstruction accuracy are chosen to be discussed: measurement plane, equivalent source plane, reconstruction plane, and measurement noise and ect. Then, some guidelines are found about improving the reconstruction accuracy. It will be helpful to direct the sound field reconstrution based on wave superposition algorithm.Near-field acoustical holography processes the disadvantages of needing of a large number of microphones and the strictness of measurement requirements, such as requiring radiators'surfaces being regular, requiring of near field measurement and requiring of large size microphone array. These disadvantages restrict the technique's widely applications in industry situations to some extent. A hybrid acoustic holography technique based on beamforming and wave superposition algorithm is presented, which overcomes the above disadvantages of NAH. The wave superposition algorithm can be applied to reconstruct the sound field accurately. However, it has a precondition that it shall be told where to collocate the equivalent sources. In concequence, beamforming is adopted to localize sound sources. After that, a serial of equivalent sources are collocated around these sound sources. Then, the exterior sound field can be accurately reconstructed with the wave superposition. Firstly, sound sources localization based on beamforming are studied. A sound source model is reconstructed. The principle of signal processing based on microphone-array are introduced. The disigning of array parameters are discussed. Considering the high noise in situation in plants, a modification is made by deleting the diagonal elements. Through that mean, the denoise characteristic of beafmorming is improved. Some specific problems needing consideration in implementation are disuccused. Then, the principle of the hybrid acoustical holography are discribed. At last, the efficiency and accuracy of the technique are proved by numerical simulation.The traditional acoustical fault feature extraction technique mainly on the basis of acoustic emission, sound power, frequency analysis, time-frequency analysis. These methods can describe the law of fault features changing on time, frequency, but they can not reveal the changing information of fault features according to the locations of sound sources. For more completely perform fault diagnosis for machineries by using sound signal, a more efficient fault feature extraction technique based on acoustical holography is needed. After a concise review of the research history of acoustical feature extraction technique, several most frequently used acoustical featrue extraction methods are introduced. After that, a hybrid acoustical holography technique is developed. At last, a sound model consists of several pulse ball is constructed to perform the numerical simulation. It is shown that the extracted fault feature can accurately reflect the changing of the sound field condition. So it is shown that the technique is accurate and feasible.After that, experiments are done to evaluate the feasibility and accuracy of the techniques present by this work: the hybrid acoustical holography and the acoutical fault feature extraction technique base on it. That will make a basis for its application in situation. The present hardware configuration of the vibration laboratory is intruduced. A set of microphone array and data acquizaiton system are disigned and implemented. The principle of the experiment are described. On basis of that, experiments are perfomed in a semi-anchoic chamber. The sound source model are made up of a sound box and a motor. Sound source identification and fault feature extraction are performed on the sound source model. After acquisition of the sound pressure data, the experimental results are analized. The efficiency and accuracy of the technique are proved by experimental results.Conclusions are given at last. The innovations are summarized. At the same time, some advices are given for the furture reaseach for the fault feature extraction based on acoustical holography. Also, remarks are given on some problems.