Research On Electromagnetic Acoustic Thickness Measurement Based On Pulse Compression

Electromagnetic acoustic technology has great potential in the field of metal thickness measurement due to its non-contact characteristics.However,In severe conditions such as high temperature,high pressure,and high corrosion,the signal-tonoise ratio(SNR)is extremely reduced.It is difficult to improve the SNR using traditional methods.Pulse compression from the radar is an effective method to improve the SNR.It can improve the SNR of electromagnetic acoustic signals significantly.However,the types and waveform parameters of pulse compression excitation signals which are suitable for electromagnetic acoustic thickness measurement are unclear.And the influence of various excitation waveform and parameters on the SNR is still unclear.To solve the problems,this paper has done the following work:In order to design a pulse compression signals for electromagnetic acoustic thickness measurement,a pulse compression electromagnetic acoustic thickness measurement model is established.Based on the analysis of the principle of electromagnetic acoustic thickness measurement and pulse compression,the pulse compression electromagnetic acoustic thickness measurement plan is determined.Then,the transmitting circuit model,electromagnetic acoustic transducer(EMAT)model,receiving circuit model and pulse compression algorithm are established.The effect of SNR improvement is verified through simulation.Chirp and non-linear frequency modulation(NLFM)signals are designed.And the frequency modulation(FM)signal parameters for electromagnetic acoustic thickness measurement are determined.The nature of Chirp signal and its sidelobe suppression method are analyzed.And the influence of sidelobe suppression on the output SNR is analyzed.The NLFM signal is designed through the frequency band characteristics of the system.With the same frequency band and pulse duration,the simulation shows that NLFM signal is better than Chirp signal.Conventional two-phase coded signals and separate transceiver(T-R)two-phase coded signals are designed.And the two-phase code type and sub-waveform parameters are determined.Based on the design of two-phase codes and sub-pulse waveforms,a sidelobe suppression algorithm for conventional two-phase coded signals is determined.The T-R two-phase coded signals and its method of thickness measurement are proposed.The simulation shows that the conventional two-phase coded signal is not as good as the NLFM signal,and the T-R two-phase coded signal is suitable for low voltage excitation.The electromagnetic acoustic thickness measurement excited by FM signals and two-phase coded signals is studied by experiment.And the influence of excitation signal waveform and parameters on the SNR is obtained.Based on building an experimental platform for pulse compression electromagnetic acoustic thickness measurement,the SNR improvement effect of each excitation signal is verified.The pulse width and frequency band to the FM signal output SNR,the code length and waveform to two-phase coded signal are studied.The influence of the SNR of the signal output leads to the respective applicable occasions of the FM signal and the two-phase coded signal.