Research On Pulse Compression Technique For Electromagnetic Acoustic Thickness Measurement

Metal materials are widely used in the fields of aerospace,petrochemical,machinery manufacturing and so on.They are affected by high temperature,high pressure and corrosion during production,transportation,storage and using,which will lead to thickness thinning of materials and endangering safety.Therefore,it is necessary to detect its thickness.Electromagnetic acoustic thickness measurement technology is suitable for metal thickness measurement in various occasions because it does not require couplant and direct contact.However,the transducing efficiency of this technology is low,especially in the high temperature environment and using low voltage excitation,the signal-to-noise ratio of the signal is greatly attenuated.In order to improve the signal-tonoise ratio,pulse compression technology is applied to electromagnetic ultrasonic thickness measurement.However,current theoretical research on pulse compression technology in electromagnetic acoustic technology is still insufficient,such as the lack of pulse compression numerical model for electromagnetic acoustic thickness measurement,the lack of electromagnetic acoustic transducer design method for pulse compression technology,and the lack of corresponding experimental test platform.In response to the above questions,this paper focuses on the following work.Aiming at the problem of insufficient research on pulse compression theory for electromagnetic acoustic thickness measurement,a pulse compression model for electromagnetic acoustic thickness measurement is proposed and established.Based on the analysis of pulse compression algorithm and electromagnetic acoustic thickness measurement principle,a transmitting and receiving encoding method suitable for pulse compression for electromagnetic acoustic signals is designed.At the same time,in order to quantify the influence of noise on electromagnetic acoustic signals,the noise model of electromagnetic acoustic signals is established,and the processing algorithm based on Wiener filter is proposed,and the whole process is numerically modeled.Aiming at the lack of simulation research on the parameter design and transmittingreceiving signal model of thickness measuring transducer for pulse compression,the influence of transducer parameters on the transducing efficiency is simulated by finite element method.After determining the optimal parameters of the thickness measuring transducer,the transmitting and receiving circuits are modeled to obtain the echo signal.Then,taking the 13-bit Barker code transmitting signal as an example,the pulse compression performance of the echo signal is analyzed to guide the pulse compression coding design.Aiming at the lack of research on pulse compression coding design for electromagnetic ultrasonic thickness measurement,the joint model that combines the pulse compression model for thickness measurement with the finite element-circuit model of transducer.is established,the sub-pulse coding,transmitting sequence coding and receiving sequence coding of pulse compression are designed and simulated.According to this,the coding method of the excitation signal is obtained,which can effectively improve the pulse compression performance,and the signal-to-noise ratio can be improved by up to 30 dB in the simulation.Aiming at the current situation that there is almost no electromagnetic acoustic thickness measurement system based on pulse compression,a corresponding thickness measurement experiment system is designed.The experimental system mainly includes: a transmitting circuit that can realize any two-phase coded excitation and a controllable receiving circuit with high gain amplification.The experimental results show that the electromagnetic acoustic based on pulse compression technology can realize the metal thickness measurement with the 5 volts low voltage excitation,and can realize the metal thickness measurement in the high temperature environment of 450 ℃.