Study On The On-line Measurement Of Thickness And Defect Of Plastic Sheet Based On Terahertz Technology

The online detection of the thickness and defect of plastic sheet not only determines the quality assessment of the sheet itself,but also affects the production cost in the production process.Therefore,the scientific detection of the thickness and defect of plastic sheet is related to the project quality,project cost and enterprise benefit.At present,the existing thickness and defect detection methods at home and abroad have certain limitations in the aspects of safety,measurement objects,application scenes,etc.Terahertz wave is expected to become a superior measurement technology in thickness and defect detection due to its advantages of fast and non-destructive,safe and harmless,strong anti-interference,strong penetration,etc.Based on the transmissional terahertz time-domain spectroscopy system,this paper explores the on-line detection of the thickness and defects of plastic sheets.The work content of this paper is as follows:(1)The basic principle of transmitting and detecting terahertz waves based on the photoconductive antenna is analyzed,and the characteristics of the transmitted and reflected Terahertz time-domain spectral system are compared.Based on the transmissional terahertz time-domain spectrum system,the system is designed and its stability is evaluated.(2)The propagation process of terahertz waves in plates was analyzed,and the thickness measurement was carried out based on the terahertz time-of-flight thickness measurement method with two kinds of plastic plates of different materials as the research object.First,the thickness of the two plates was measured under normal temperature and dry environment according to the thickness measurement model.The results show that the overall absolute error of thickness measurement is less than 0.082 mm,and the relative error is less than 1.6%,which meets the requirements of thickness measurement of plastic sheet.Secondly,the feasibility of thickness measurement in the cooling process is studied.The results show that the temperature of the plate has no effect on the propagation of terahertz wave,and the thickness of the plate can be accurately measured by the thickness measurement model.Finally,the influence of ambient humidity on the thickness measurement is analyzed.The water vapor absorption model is used to eliminate the interference of humidity on the sample signal,and the on-line measurement of the thickness of plastic sheet under atmospheric environment is realized.(3)Research on defect detection was carried out based on terahertz technology.By preparing different defect samples,different time domain spectra are obtained.On this basis,defect identification is carried out based on support vector machine.The results show that it is feasible to use terahertz time-domain spectroscopy to detect the defects of plastic sheets.The recognition rate of defects can reach 91.89% by support vector machine,and the surface defects and internal defects can be accurately identified.