Characteristics Optimization And Application Research Of Piezoelectric Ultrasonic Transducers In The Long Distance Rail Detection

Ultrasonic guided waves(UGW)technique has the advantages of low detection frequency,long detection distance,strong anti-electromagnetic interference ability,and large coverage.Hence it has potential advantages in real-time detection of breakages in the rail.Based on the research background of UGW-based broken rail detection,this paper focuses on the characteristics optimization of piezoelectric ultrasonic transducers(PUTs)to improve the propagation efficiency of UGW in the rail.Due to the influence of energy attenuation,multimodal,dispersion,and on-site noise when the UGW propagates in the rail,the amplitude of the received UGW signal is low and the signal-to-noise ratio is poor.Therefore,this article mainly systematically studies the characteristics optimization of PUTs from the aspects of impedance matching,driving circuit optimization,and excitation signal optimization.The main work is as follows:1.To deeply study of the electromechanical characteristics of longitudinal vibration sandwich piezoelectric ultrasonic transducer(referred to as PUTs),the PSpice equivalent circuit models of the piezoelectric ultrasonic transducer and the PSpice equivalent circuit model of the pitch-catch setup are established based on one-dimensional wave and transmission line theory.The PSpice model of the PUT and the PSpice model of the pitch-catch setup are analyzed from the time and frequency domains,respectively,and the accuracy of the built PSpice models is verified through some experiments.It is shown that the PSpice model of the PUT established above is highly extensible and can be combined with amplifiers,driving circuits,diodes.2.To solve the problem of impedance mismatch between the piezoelectric ultrasonic transducer and the driving circuit and the rail surface,the effect of impedance matching on the electromechanical properties of the piezoelectric ultrasonic transducer was studied from the electrical and acoustic ends,respectively.At the electrical end,the effects of different electrical impedance matching networks on the electromechanical characteristics of PUTs are studied in both time and frequency domains.It is shown that in the two LC impedance matching networks,the matching network formed by the series inductance and parallel capacitance is better.At the acoustic end,an experimental method is used to study the effect of acoustic impedance matching on the transient characteristics of PUTs.It is concluded that when the epoxy resin is doped with 10%tungsten powder and the coating thickness is 8 mm,the acoustic impedance matching effect is better.3.Aiming at solving the problems of the existing driving circuits that the excitation voltage is not high enough,the extra high voltage DC voltage is required and the impedance matching is not considered,this paper proposes a high voltage pulse driving circuit based on the full-bridge topology.The driving circuit takes into account the suppression of overshoot and oscillation when the power MOSFET is turned off,and at the same time conducts the impedance matching and tailing absorption of the excitation signal for PUTs.The suppression of overshoot and oscillation adopts the RC snubber circuit,and the tailing absorption is accomplished by a bleeder resistor and a bidirectional thyristor.The correctness and effectiveness of the proposed high-voltage pulse driving circuit are verified through experiments.It was also found that the combined use of electrical impedance matching and absorption circuits can effectively improve the energy conversion efficiency of PUTs.4.To obtain the optimal performance of PUTs,the excitation signal of PUTs is optimized in terms of excitation signal frequency and excitation coding.First of all,to solve the problem of PUTs with having a resonance frequency shift after loading,this paper proposes an optimal excitation frequency tracking method based on a digital band-pass tracking filter.Then its correctness and stability are verified through some field experiments.Secondly,to improve the signal-to-noise ratio of the UGW signal,it is proposed to apply the Barker code excitation method to the broken rail detection,and use the pulse compression technique at the receiving end to realize the rapid recognition of the signal characteristics.Finally,for the case where the pulse-compressed signal produces undesirable peak sidelobes due to the effects of bandwidth,multipath,and noise,an adaptive peak detection algorithm based on a digital bandpass filter,triangle filter,and Hilbert transform.The accuracy and effectiveness of the above-mentioned Barker code excitation method and the adaptive peak detection algorithm are verified through experiments.The study in this paper provides a feasible solution for improving the propagation efficiency of UGW in the rails,and at the same time provides theoretical guidance for the large-scale application of the real-time broken rail detection system based on UGW.