Research Of Terahertz Time Domain Spectroscopy Thickness Measurement Technology Based On Evolutionary Algorithm

Non-destructive thickness measurement technology has a broad application in manufacture industries like aviation,aerospace and automotive manufacturing.The time-of-flight based terahertz thickness measurement technology could obtain thickness information of micron-level coatings in a non-contact manner.However,due to limitations of terahertz pulse width and multiple reflection effects,this technology has a measurable limit about 60μm and could not extract the thickness of each layer in coating samples with multiple layer structure.In order to improve current terahertz thickness measurement technologies,the following works have been done:(1)Considering multiple reflection effects of terahertz pulses in a single-layer coating sample,the Multiple Reflection Correction Model(MRC Model)was built and the Ensemble Empirical Mode Decomposition(EEMD)is adopted to improve the signal-to-noise ratio(SNR)of sample signals.Combining both methods,thicknesses as low as 40μm could be measured,which means the minimum measurable limit is improved.(2)Based on Rouard’s Method,the Debye Rouard(DR)model and the Lorentz Rouard(LR)model of terahertz signals are built,into which the Debye model and the Lorentz model are introduced to describe the dielectric constant s of coating materials.The thickness of each layer in one or two-layer coating samples was measured and thickness as low as 4.6μm is distinguished.These models could also simultaneously obtain dielectric constant information of coating materials,which could be used to identify the kinds of different paints.(3)To verify the feasibility of above models in case of monitoring the real-time drying process,the MRC model and DR model were applied to measure the real time thicknesses of different wet samples.With acquired thickness information,the drying curves of painting and paper samples were calculated.For the two drying curves of painting sample,calculated by MRC model and DR model,it was found that both of them are in exponential form with time constants 25.5s and 26.1s,respectively.It was observed that the second-order Debye model has a better performance than its first-order form in case of describing the dielectric feature of water in the terahertz band.The time constant of this wet paper sample measured by DR model is 30.8s.The terahertz time-domain spectroscopy technology is able to measure the thickness of micron-scale coatings effectively.With appropriate signal processing methods,the dielectric constant information of the coating materials could be obtained,and the drying process of the coating could be monitored in real time,which meets the non-destructive and non-contact requirements of online applications.