Research On Thickness Measurement And Defect Detection Technology Of Polyethylene Pipe Based On THz-TDS

In the process of manufacturing and application of polyethylene material pipelines,accurate measurement of pipeline thickness and pipeline defect detection are two important technical problems affecting pipeline quality and safety.Due to the characteristics of natural gas flammable and explosive,it is easy to cause safety hazards.Quality control and defect detection of natural gas pipelines is particularly important.The detection of non-metallic material thickness by THz-TDS(THz Time Domain Spectroscopy)technology has been applied in engineering practice.However,in the process of practice testing,challenges and difficulties have arisen in the existing terahertz thickness detection technology.First,there is no theoretical support in the process of refractive index determination,artificially selecting the effective range of refractive index,and increasing the error of refractive index determination.Secondly,due to the obvious dispersion phenomenon of the terahertz wave propagation in the polyethylene material,the high transmittance signal is weak,resulting in low signal-to-noise ratio,and the proportion of the interference signal is amplified during the signal extraction process.The signal is distorted and submerged,and the optical path time difference cannot be accurately extracted.Third,the terahertz defect detection point scanning method takes a long time and has low efficiency,which cannot meet the needs of on-site detection.For the first problem,the signal-to-noise ratio(SNR)is quantified by the Butterworth filter to obtain the effective refractive index interval.Aiming at the second problem,the Gaussian deconvolution algorithm based on the autocorrelation improved algorithm is used to extract the feature and improve the signal-to-noise ratio of the time domain signal,and to extract the signal from the interference and improve the signal-to-noise ratio.For the third problem,the CCD terahertz intensity imaging method was used to test the defect test block,and compared with the point scan imaging method,it was found that the CCD imaging method satisfies the performance requirements of the engineering site detection.In this paper,a series of systematic researches on the key technical problems found in the process of polyethylene material thickness and the rapid imaging direction of polyethylene material defects are carried out.The main research contents and conclusions are as follows:(1)Using a single-point thickness measurement model,the terahertz non-metallic material standard reference sample is calibrated by Teflon sample to quantify the systematic error,and the theory of the subsequent terahertz non-metallic material thickness is calculated.basis.(2)Using the Butterworth filter,the effective interval of the refractive index in the process of thickness determination depends entirely on the subjective judgment of the engineering staff on the stability interval of the refractive index spectrogram,which leads to the misunderstanding caused by subjective factors in the process of refractive index determination.The difference is increased,which in turn leads to an increase in thickness error.A kind of Butterworth bandpass filtering for different frequency bands of terahertz time domain signals is proposed.By establishing a signal-tonoise ratio model,the filtered signals are quantitatively analyzed,and then the frequency distribution interval with the smallest error of different samples is selected.This is a method for rationally selecting the effective range of the refractive index.The results show that this method effectively reduces the thickness measurement error caused by subjective selection of refractive index.(3)Using Gaussian deconvolution algorithm based on improved autocorrelation algorithm,there is obvious dispersion phenomenon due to the propagation of terahertz wave in polyethylene material,and its high transmittance signal is weak,resulting in signal-to-noise ratio.Lower,in the process of extracting the impulse response function,the proportion of the interference signal is amplified,causing the signal to be distorted,submerged,and unable to accurately extract the signal.In this paper,the improved autocorrelation algorithm is used to analyze the autocorrelation of the band-pass filtered time domain signal,and then the Gaussian filter deconvolution operation is combined to obtain the improved impulse response function.It aims to solve the problem of time domain signal characterization in the field of terahertz non-metal measurement.The experimental results of the improved algorithm show that the sharpness of the signal is enhanced,and the distortion or flooding of the impulse response signal caused by the interference of the clutter signal is solved,and the signal-to-noise ratio of the impulse response signal is significantly improved.(4)This paper studies the imaging method of terahertz technology by using THz-TDS point scanning method and CCD terahertz intensity imaging method respectively,and imaging test and quantitative analysis of standard polyethylene test block with defects.Compare the advantages and disadvantages of terahertz imaging methods in the detection of polyethylene defects.The results show that the effectiveness of the detection of polyethylene defects depends mainly on the frequency of the terahertz wave.