Study On Terahertz Technology For Paper Thickness Measurement

The development of paper industry has an important impact on national economic construction and people's life.Thickness is the main index of paper quality problem,and controlling the thickness of paper in a reasonable and controllable range is the primary condition to ensure other performance requirements of paper.Terahertz(THz)technology is an emerging technology in recent years.It is between electromagnetic wave and infrared wave,and its special properties make terahertz technology widely studied in the fields of material identification,quantitative detection and environmental monitoring.In this thesis,the paper thickness measurement based on terahertz spectroscopy is studied,and the theoretical model of interaction between paper and terahertz wave is established.In addition,the paper thickness detection methods based on experimental data and theoretical model are studied respectively,and satisfactory results are obtained.The main research work and results of this paper are as follows:1.The development of paper thickness measurement and terahertz technology are reviewed and the research significance of this studyis stated.The terahertz time-domain spectroscopy is introduced in details from three aspects: terahertz generation and detection principle,terahertz time-domain spectroscopy system and the transmission terahertz system adopted in this paper.2.The theoretical model of interaction between paper and terahertz signal is established.Based on Fresnel formula,considering the influence of multiple reflections,the transmission model is established when terahertz wave incidents on paper sheet.Based on the resctructed model,simulation is given to study relationship between terahertz signal and paper thickness.The characteristic of different features,both in the time domain and in frequency domain,are simulated to provide theoretical guidance the thickness measurement based on terahertz techniques.The feasibility of this method is shown by the simulations,too.3.The paper thickness measurement method based on terahertz experimental data is studied.First,the paper sheet of terahertz spectrum is measured by terahertz timedomain spectroscopy(THz-TDS)setup,then the time domain and frequency domain characteristics are extracted respectively from the paper sheet of terahertz spectrum to predict the thickness of the sheet.The predicted thickness are compared,and the results show that phase spectrum in the frequency domain has best prediction results.In order to further improve the accuracy of detection,particle swarm optimization(BPSO)is used for frequency selection,then partial least squares(PLS)are adopted to predictes the paper thickness.Compared with the commonly used quantitative methods,this method can improve the predictive accuracy of paper thickness.Furthermore,considering that amplitude spectrum is still related to the paper thickness,study on the fusion of amplitude spectrum and phase spectrum method is done to predict the thickness of the sheet.The method combines empirical mode decomposition(EEMD)and multidimensional partial least squares(N-PLS),in which the EEMD decomposition extracts the main signal denoising processing while N-PLS method is used to build the relationship between the thickness of the sheet and the amplitude & phase spectra.The predicted results show that the accuracy of paper thickness prediction can be further improved by combining amplitude spectra and phase spectra of paper.4.A model-based thickness measurement is studied on the theory of terahertz transmition model.This method can be described as follows: the transmitted THz signal,which depends on the parameters of paper sheet,is simulated by the transmission model.By comparing the simulation to the measured signal of paper sheet,the unknown thickness of paper sheets can be measured in a numerical optimization step.The NelderMead iterative algorithm is selected to seek for the optimal solution.In our work,two kinds of paper are used to show the performance of the method.The results show that the terahertz model can be used to measure the thickness of paper effectively,and this method needs only one experiment to relize the advantages of measuring the thickness of paper accurately.