Research On Key Technologies On Thickness Measurement Using Electromagnetic Ultrasonic Transducer

Thickness measurement using electromagnetic acoustic transducer has the advantage of noncontact and has broad prospects in high temperature,online,and non-stop testing.However,due to the low energy conversion efficiency and low signal-to-noise ratio of the electromagnetic ultrasonic transducer,its further engineering application is limited.In response to these problems,this paper studies the influence of transducer structure and excitation parameters on thickness measurement signals and develops signal processing methods to improve the energy conversion efficiency of the transducer,thickness measurement signal-to-noise ratio and thickness measurement accuracy.First,the electromagnetic ultrasonic thickness measurement transducer technology based on the principle of eddy current energy partition is proposed and an electromagnetic ultrasonic thickness measurement transducer with high energy conversion efficiency is developed.Based on the analysis of the Lorentz force transduction mechanism of electromagnetic acoustic transducer,the opposite law of eddy current and magnetic induction intensity changing with the same structural parameters of the transducer is obtained.On the basis that this law is preliminarily verified through simulation,finding the maximum value of the energy conversion efficiency under the condition of large lift-off through the experiment and fabricating an electromagnetic acoustic transducer with optimized structure according to the experimental results.Secondly,the influence of electromagnetic acoustic thickness measurement transducer excitation parameters on the detection signal of electromagnetic acoustic thickness measurement is studied to provides a reference for the selection of electromagnetic acoustic thickness measurement excitation parameters.The linear relationship that the amplitude of the electromagnetic acoustic thickness measurement signal increases with the increase of the excitation voltage is obtained through experiments;the influence of the capacitance in the excitation circuit on the excitation energy and the frequency of the excitation signal is studied.The capacitance increases,the excitation frequency decreases,and the signal amplitude increases;the influence of the attenuation time of the thickness measurement signal and the capacitor charging and discharging time on the excitation repetition frequency is analyzed.Experiments show that capacitor charging and discharging have a greater impact on the repetition frequency,which provides a basis for the selection of the repetition frequency of electromagnetic acoustic thickness measurement.Finally,the signal processing method based on comparator which is suitable for high signal-tonoise ratio(SNR)and spectrum analysis signal processing method which is suitable for low SNR are studied.On the basis of analyzing the principle and signal characteristics of electromagnetic acoustic thickness measurement,a simplified model of electromagnetic acoustic thickness measurement signal is obtained.According to the characteristics of the signal time domain,a signal processing method based on a comparator was developed.Through Verilog,the real-time output of the thickness measurement results from the comparator signal acquisition to the real-time thickness measurement result was realized on the FPGA,and the real-time thickness measurement of the high signal-to-noise ratio signal achieved good Measurement accuracy;based on the characteristics of the frequency domain,a thickness measurement method based on frequency domain analysis is developed to realize the sample of the signal,and the signal spectrum analysis is realized on Matlab.The experiment proves that the algorithm is stable in the signal amplitude fluctuations.