Research On Key Techniques For Denoising Of Electromagnetic Ultrasonic Signals At High Temperature And Low Power

A large number of oil and gas pipelines and related equipment work in flammable,explosive and high temperature environment,so low power electromagnetic ultrasonic transducer(Electromagnetic Acoustic Transducer,abbreviated as EMAT)which can work in high temperature environment is needed for non-destructive testing.However,the conversion efficiency of EMAT itself is low,low power excitation will make the amplitude of the echo signal lower than the noise threshold,and high temperature will further reduce the conversion efficiency.Therefore,high temperature and low power EMAT signal is faced with the problem of low SNR.Based on this,this thesis proposed a denoising algorithm for low-power EMAT signal from the point of view of preprocessing and post-processing,and combines the two together,a denoising algorithm for high temperature and low power EMAT signal is proposed.The research content of this thesis mainly focuses on the following three aspects.The main contributions of this thesis are as follows:(1)Aiming at the problem of extremely low SNR of low-power EMAT signal,a denoising method based on deep stack denoising autoencoder(DSDAE)is proposed from the point of view of post-processing,which can effectively denoise by recovering the target echo signal from the original signal with strong background noise.Denoising experiments show that under the condition of less prior knowledge and the excitation voltage is gradually reduced to 25,DSDAE has a good denoising performance for low SNR A-Scan signals collected on stepped specimens with different thickness.For the original signal whose SNR is 0 or less,the highest SNR can reach 20 after DSDAE reconstruction.The experiment of thickness measurement under the laboratory condition of 25 proves the effectiveness of the algorithm.(2)Aiming at the problem of extremely low SNR of low-power EMAT signal,a denoising algorithm based on coding excitation is studied from the point of view of preprocessing,which can effectively denoise by suppressing the strong background noise in the original signal.The receiving interval is introduced into the transmitted signal to reduce the influence of aliasing distortion,the random coding sequence is used as the transceiver control signal to improve the generalization ability,and the Gibbs trailing phenomenon is introduced into the transfer function to simulate the RLC circuit.Denoising simulation results show that the algorithm can still extract effective echoes when the SNR is as low as-6.The simulation results of thickness measurement show that the algorithm is effective.(3)Aiming at the problem that the SNR of low-power EMAT signal is further reduced by high temperature,from the point of view of pre-and post-processing,a DSDAE denoising algorithm combined with pulse compression(MF-DSDAE for short)is proposed.,which can effectively denoise by restoring the effective reference signal and then suppressing the strong background noise.By studying the influence of high temperature on the energy exchange efficiency of EMAT and the sound velocity of shear wave in the specimen,the EMAT transfer function is constructed.The simulation results of denoising show that MF-DSDAE can denoise effectively at 600.The simulation experiments of thickness measurement at different temperatures prove the effectiveness of MF-DSDAE.