Research On Fault Location Method Of Leaky Cable Based On LFM Pulse Compression Technology

With the rapid development of China's railway,urban subway and intelligent coal mine,the requirements of communication quality and coverage area are getting higher and higher.The stability of mobile communication signal is an important technical means to ensure the safe and stable operation of related facilities and systems.Leaky coaxial cable(hereinafter referred to as leaky cable)as a leaky waveguide,with the advantages of both transmitting signals and radiating signals,which makes the leaky cable wireless communication system has been more and more widely used.The outer conductor of the leaky cable has a narrow slot arranged in a regular manner.The slot of the leaky cable is intended to radiate the energy of the radio signal so as to achieve the purpose of transmitting and receiving the extraneous radio waves.The energy of the radio signal is guided by the length of the leaky cable,and its effective area is affected by both the inside and outside of the leaky cable so that the signal can be continuously received by the mobile communication unit.As a special antenna,the leaky cable can be radiated uniformly at each point when it is laid on a mine tunnel or a railway tunnel and the signal source is loaded at one end.However,at present,the traditional manual inspection method is still the main method of leakage detection and fault location,limited to the working environment and conditions of leakage cable,manual inspection is difficult to timely troubleshooting and timely maintenance.How to achieve the detection and fault location of leaky cable accurately and quickly,there has no relevant solution at home and abroad.Therefore,the research of the leaky cable fault detection and location method has important theoretical significance and practical value.In transmission line,traveling wave reflection method is a common fault location method.The principle of the traveling wave reflection method is to transmit narrow pulse voltage signal to the transmission line,and the pulse signal will produce echo signal through the reflection of the fault point.Then,the position of the fault point of the leaky cable is calculated by detecting the time difference between the echo pulse signal and the transmitted signal and calculating the speed of the transmission line under frequency characteristics.For long-range detection,it is necessary to transmit a signal with higher energy.When the amplitude is limited,it is necessary to increase the pulse width of the transmitted signal to improve the signal energy.Compared with coaxial cable,the coupling loss of the leaky cable will increase the attenuation of signal transmission in the leaky cable,so it is necessary to select high energy broadband signals in the detection and fault location methods of the leaky cable.As a common transmitting waveform of radar,linear frequency modulation signal has large time bandwidth product,and the wider it is,the farther the transmission distance is.Pulse compression technology is widely used in radar systems.In recent years,it has been applied in many kinds of signal systems,such as wireless communication,sonar,ultrasonic image and seismic exploration.Transmitting wide pulse can obtain high emission energy and can transmit farther distance.Increased pulse width will cause the decrease of echo resolution,while using pulse compression method obtaining narrow pulse at the receiving end,and the side lobe suppression method to improve the ratio of main lobe,to improve the range resolution.In this paper,the method of linear frequency modulation pulse compression is used to study the fault location method and related key technologies of the leaky cable.Based on lots of literatures,the transmission attenuation model of the leaky cable is established to study the law of the transmission of signals,and the fault location method of linear frequency modulation pulse compression of the leaky cable is proposed and its key technology of signal processing are deeply studied,the main research work is as follows:The theory of transmission line fault location is researched.Based on the equivalent distribution parameter model of the transmission line and the transmission line telegraph equation,the mathematical model of signal transmission is established,and the short circuit fault and the open circuit fault of the leaky cable are analysed,and the signal response characteristics of the fault are analysed.The transmission characteristic of the leaky cable for detecting signal is researched.Firstly,the structure of the leaky cable and the electrical characteristics of the leaky cable are analysed.The transmission attenuation model of the leaky cable is established,and the transmission attenuation of the leaky cable is divided into two modes: internal mode and external mode.In the internal model,similar to coaxial cable,based on the equivalent distribution parameter model,the impedance of the internal mode of the leaky cable is calculated according to the basic formula of the transmission line impedance.Then,based on the special structure of the leaky cable,the capacitance value of the unit length of the leaky cable is calculated by finite difference method,and then the attenuation constant of internal mode is calculated.In the external mode,according to the definition of the surface transfer impedance and the transmission inductance,the external leakage of electromagnetic waves is represented by using equivalent current and the equivalent voltage in the form.According to the effective approximate formula of weak coupling,the coupling coefficient is defined between internal and external mode,and then the transmission attenuation constant of the leaky cable is obtained.Several types of leaky cables are simulated and compared with the measured attenuation results of these leaky cables.The attenuation model is consistent with the measured attenuation.The transmission law and wave velocity characteristics of the detection signal in the leaky cable are further analysed.On the basis of the transmission attenuation model,the transmission variation of the detection signal in the leaky cable is analysed,and the relationship between the pulse amplitude and the transmission distance is obtained,and the transmission characteristics of the leaky cable are further understood.In the study of wave velocity characteristics,the general formula of wave velocity of the leaky cable is obtained by using the frequency correlation property of equivalent circuit.According to the approximate formula of the propagation constant of the RF coaxial cable,the expression of the wave velocity of the leaky cable is derived.It can be seen from the simulation results that the wave velocity characteristics of the leaky cable are close to the constant at high frequencies,and at low frequencies,the wave velocity will have a large nonlinear variation.The fault location of the leaky cable based on linear frequency modulation pulse compression is researched.Based on the attenuation model of the leaky cable and the characteristics of signal transmission,a linear frequency modulation pulse compression method is proposed to detect and locate the fault in the leaky cable.Firstly,the linear frequency modulation signal is analysed,and its signal characteristics and spectrum characteristics are analysed.This paper introduces the system composition of chirp compression and fault location,and focuses on the key technology of echo signal processing.Aiming at the denoising of echo signals,this paper studies the denoising algorithm based wavelet,analyzes the statistical properties of noise under wavelet decomposition,and analyzes the principle of the wavelet threshold denoising.According to the advantages and disadvantages of the hard threshold function and the soft threshold function of wavelet threshold denoising,an improved threshold function is proposed to make the values at the threshold point smooth transition to zero.And the threshold estimation is optimized,and the process of optimizing the algorithm is described.The simulation experiment of four sample signals in MATLAB is carried out.From the mean square error,the improved threshold function has good denoising effect.In order to realize the pulse compression of the echo signal,the working principle of the matched filter is analyzed,and the echo signal is compressed by the matched filter method.In order to improve the resolution of the fault point,side lobe suppression method for further study of the pulse compression signal,this paper analyzes the spectrum characteristics of commonly used window function,using convolution algorithm,combined with the rectangular window width of the main lobe and side lobe height has the advantages of minimum Hamming window,hybrid convolution window function is proposed in this paper.In order to improve the resolution of the fault point and further study the sidelobe suppression method of the pulse pressure signal,the spectral characteristics of the commonly used window function is analysed and a hybrid convolution window function is proposed based the convolution algorithm to combine the rectangular window with the narrowest width of the main lobe and the Hamming window with the minimum height of side lobe in this paper.The arrival time estimation algorithm of pulse pressure signal is researched.The accuracy of the LFM pulse compression fault location method depends on the arrival time estimation of the pulse signal.In order to correctly identify and detect the pulse compression echo signal,the discrete Fourier transform(DFFT)method is analyzed,and the two-step method based on energy detection and wavelet transform is studied.In order to evaluate the arrival time estimator,the basic model of the Cramér–Rao bound is analyzed,the Cramér–Rao bound of the arrival time is deduced,and the arrival time estimation of the pulse echo echo signal is analyzed and simulated.Simulation and experimental analysis are carried out.Firstly,the fault location method of LFM pulse compression is simulated by MATLAB,and the echo denoising method is simulated under different SNR conditions.The attenuation model of leaky cable is verified by simulation experiment platform and measured experiment,and the experimental results are analyzed.Then,the 30 meters and 60 meters leaky cable terminal disconnection fault is verified by physical experiment.