Research On Weak Fault Signal Enhancement Detection Method Based On Gradient Acoustic Metamaterials

Equipment condition monitoring as well as fault diagnosis is the key technology to ensure the safety and reliable operation of mechanical equipment.However,characteristic signals which indicate equipment fault at the early stage are often overwhelmed by background noise.How to detect these weak fault characteristic signals from strong background noise effectively is the research frontier in the field of equipment condition monitoring as well as fault diagnosis.Traditional time domain,frequency domain and time-frequency analysis methods are mainly based on linear theory,it means that the detection of weak fault signals is realized by filtering noise.Nevertheless,while filtering noise,the useful information in fault signals will also be lost to some extent.As a result,it greatly affects the detection effect of weak fault signals.Nonlinear methods,such as stochastic resonance method,can convert part of the energy of noise into signals,thereby enhancing the output of fault signals and realizing the detection of fault signals with lower signal-to-noise ratios(SNRs).However,the parameters of nonlinear systems are difficult to adjust.Acoustic metamaterials are artificial composite structures or composite materials with extraordinary physical properties that natural materials do not possess.Compared with periodic acoustic metamaterials,anisotropic gradient acoustic metamaterials,composed of spatially varying unit cells or media,can realize flexible control of sound waves and sound energy.It will provide a new way for weak fault signals detection if the sound energy at a specific frequency or frequency band can be enhanced based on the wave manipulation properties of gradient acoustic metamaterials.Supported by the National Natural Science Foundation of China(Grant No.52175087)and the Hunan Provincial Innovation Foundation for Postgraduate(Grant No.CX20190283).In this dissertation,weak fault signals enhancement detection method based on gradient acoustic metamaterials is proposed.This reserach focus on improving the detection effect of weak fault signals and expandng the detection range of weak fault signals.The main research work and innovations of the dissertation are listed as follows:(1)To overcome the difficulties to detect weak signals,a weak signal enhancement detection method based on the gradient acoustic grating metamaterial is proposed.The acoustic rainbow trapping effect is realized by using the resonance unit cells with different geometric parameters in the gradient acoustic grating metamaterial to trap and enhance sound waves.The results indicate that the acoustic rainbow trapping effect can not only separate broadband sound waves in the spatial domain,but also enhance the trapped sound waves,which can be applied to weak signal enhancement detection effectively.By setting the weak harmonic signals and periodic impulse signals with different characteristic frequencies and different noise intensities,the effectiveness of the gradient acoustic grating metamaterial in detecting weak harmonic signals and periodic shock signals is verified.The research results show that the SNRs of weak signals can be improved at least 10dB based on the gradient acoustic grating metamaterial,which provides an effective way for weak signals enhancement detection.(2)To further improve the acoustic enhancement effect and reduce the size of the gradient acoustic metamaterial,a weak signal enhancement detection method based on the gradient spacing-coiling acoustic metamaterial is proposed.The designed gradient spacing-coiling acoustic metamaterial can be viewed as the gradient acoustic grating metamaterial coupled with the spacing-coiling acoustic metamaterial.The method exploits the high-refractive-index properties and the effective slow-speed effect of spacing-coiling acoustic metamaterials,which further improves the acoustic enhancement performance and decreases the frequency of trapped sound waves.Compared with the gradient acoustic grating metamaterial without coupling the spacing-coiling acoustic metamaterial,the gradient spacing-coiling acoustic metamaterial has a better acoustic enhancement performance and the acoustic signal can be amplified about 80 times.Meanwhile,the gradient spacing-coiling acoustic metamaterial can decrease the trapped frequency of sound waves by approximately 43%at the same size.This property will greatly expand the application range of gradient spacing-coiling acoustic metamaterials in the field of weak signal enhancement detection.(3)There is a trade-off between the Q-factor and resonant response bandwidth of resonant gradient acoustic metamaterials,to obtain a response with the high Q-factor as well as broad response bandwidth,a weak signal enhancement detection method based on the gradient index acoustic metamaterial is proposed.The sound pressures can be enhanced by using the strong compression effect when the sound waves are propagating in the gradient index acoustic medium.The numerical and experimental results show that,based on the gradient index acoustic metamaterial,the local SNRs of weak harmonic signals can be improved about 8-9dB while the local SNR of weak periodic impulse signals can be improved at least 7dB.The enhancement property of the gradient index acoustic metamaterial enables weak harmonic signals and periodic impulse signals to be recovered from background noise successfully,which provides an effective way for the enhanced detection of weak signals.(4)The working frequency band will be fixed and untunable once the gradient index acoustic metamaterial is fabricated,to solve this problem,a gradient index acoustic metamaterial with tunable working frequency band is proposed.The tunable structure uses spring elements to connect the plates in series,and the air gap distance between two adjacent plates can be tuned by compressing the spring with different forces,so as to tune the geometric size of the gradient index acoustic metamaterial,and then tune the working frequency range.The numerical and experimental results show that the tunable gradient index acoustic metamaterial can broaden the working bandwidth more than 300 Hz.By taking the Gaussian pulse signal as an example,the effectiveness of the tunable gradient index acoustic metamaterial in the detection of weak acoustic signals is verified.(5)Due to the unknown resonance frequency band of bearing fault signals,complex background noise,and the difficulty in extracting fault features,a bearing fault signal enhancement detection method based on the gradient index acoustic metamaterial is proposed.Based on the frequency-selective enhancement property of the gradient index acoustic metamaterial,the resonance frequency band of the bearing fault signal can be searched by changing the measurement position in the gradient index acoustic metamaterial.The proposed method uses the gradient index acoustic metamaterial to enhance the useful signal rather than constructing filters to filter noise,as a result,all useful information of the bearing fault signal is well preserved.The results show that the bearing fault signal enhancement detection method based on the gradient index acoustic metamaterial can extract the bearing fault information effectively.Compared with the traditional spectral kurtosis method,this method can realize bearing fault signal detection with lower SNRs,and has a wide range of engineering application value.In this dissertation,the application of gradient acoustic metamaterials in the field of weak fault signals detection is deeply and systematically studied.At first,a weak signal enhancement detection method based on the gradient acoustic grating metamaterial is proposed to recover weak signals submerged in the background noise.Then a gradient spacing-coiling acoustic metamaterial is designed to improve the detection effect of weak signals.In addition,to obtain a response with the high Q-factor as well as broad response bandwidth,a weak signal enhancement detection method based on the gradient index acoustic metamaterial is proposed.After that,a gradient index acoustic metamaterial with tunable working frequency band is designed to broaden the working frequency band of the gradient index acoustic metamaterial.Lastly,based on the frequency-selective enhancement property of the gradient index acoustic metamaterial,the resonance frequency band of the bearing fault signal can be searched and the bearing fault information with a low SNRs can be extracted effectively with a low SNRs.All in all,weak fault signals enhancement method based on gradient acoustic metamaterials will provide a new approach for weak fault signals detection.