Research On High-Precision Detection Technology Of Field Near-Infrared Spectrometer

For in-situ analysis,the near-infrared(NIR)spectroscopy technology can assess a variety of chemicals while being non-destructive,quick,and pollution-free.Field analysis frequently makes use of the NIR spectrometer.In addition,modulation and demodulation technology matters considerably in weak signal detection of NIR spectrometer.The modulated signals can be divided into two categories: equal amplitude modulation optical signal with an equal amplitude and variable amplitude modulation optical signal with varying amplitudes in each modulation period.Two different types of modulated optical signals are transformed into equal amplitude and variable amplitude modulated signals by the sensor.The demodulation techniques are primarily divided into analog and digital demodulation,and digital demodulation technology has gained popularity recently due to its benefits in streamlining hardware circuits,lowering volume,cutting costs and raising instrument reliability.Field NIR spectrometers mainly includes portable and on-line instruments,which are constrained by the working environment,application requirements,instrument volume,and power consumption,as well as subject to the issue of low precision.In particular,the field instruments can’t be warmed for a long time because they need real-time detection.The poor signal-to-noise ratio(SNR)and stability of the spectrometers have traditionally been the technological roadblocks preventing the development of field NIR spectrometers.Based on the digital demodulation method of frequency correction,the focus of the present study is placed on the high-precision conditioning,demodulation technology of the modulated signal and the high-stability field detection technology of the spectrometer without preheating or short-time preheating.The following research work is mainly conducted:(1)Variable amplitude modulation signal conditioning method research and system development.Targeting the issue that the demodulation data are noisy and the conditioning process for detecting variable amplitude modulation signals is easily distorted,resulting in the low SNR of on-line filter-type NIR spectrometer,the low frequency fitting conditioning method of the variable modulation signal based on spectrum analysis is proposed here.By creating the mathematical model of the variable amplitude modulation signal and the ideal signals that can be demodulated using the numerical accumulation digital demodulation technique,it is found that the amplitude of each modulation period of the amplitude modulation signal has no effect on the spectrum conversion properties of two signals.At low frequency,the energy is concentrated and the amplitude-frequency characteristics are proportionate.Based on this,a low-frequency fitting band-pass filtering amplitude modulation signal is constructed using low-frequency fitting to complete the fidelity conditioning of amplitude modulation signals.Additionally,the demodulation of variable amplitude modulation signals is expanded using numerical cumulative digital demodulation technology.Experimental research is conducted using the design system.And the results demonstrate that in comparison to the peak detection approach.The SNR of the on-line filter NIR spectrometer has increased from 2,320 to 5,594.(2)Research on multi-channel acquisition and high SNR digital demodulation method and system development.A digital demodulation method of micro-delay multi-channel parallel acquisition is thus presented since the SNR of the field NIR spectrometer is low.To boost the SNR,the sampling rate is increased through several acquisition channels using micro-delay sampling to increase the quantity of data available for numerical cumulative digital demodulation of equal amplitude and variable amplitude modulation signals.Based on this method,two digital demodulation systems for equal amplitude and variable amplitude modulated signals are accordingly conceived and constructed,and experimental study is also conducted.The experimental results demonstrate that this modulation method increases the SNR of the portable raster scanning NIR spectrometer from 2,500 to 3,000,the SNR of the on-line filter type NIR spectrometer has risen from 5,594 to 7,414,when compared to the digital demodulation method based on frequency correction.(3)Research and system development of dual-beam contrast correction for the high-stability spectral detection method.Taking into account the measurement requirements of a portable raster scanning NIR spectrometer with energy storage,the light source cannot be heated for an extended period of time,the measurement data has issues with long-term drift,and the spectrometer’s stability is poor.A spectral measurement method for comparison and correction of the homologous double optical beams is hereby proposed.The measurement is the ratio of the demodulated value of the homologous contrast beam to the sample beam.And this method eliminates the effect of drifting the amplitude of the equal amplitude modulation signal on the recorded spectral data by light intensity modulation.The mechanical structure and circuit system are designed using this method,and experimental research is conducted.The outcomes demonstrate that the baseline drift of the spectral data is decreased from 0.28%and 2.66% of the original instrument to 0.07% and 0.23%,respectively.(4)Development of the system and research on the high-stability spectral detection method based on light source rapid stabilization.The rapid stabilization method of constant current power supply light source based on process control is proposed with the method of rapid preheating starting with large preset values and low temperature drift high stability constant current power in order to address the issue of long preheating time and long time drift of measurement data of on-line filter type NIR spectrometer.So that the luminous light intensity of the source light is rapidly stabilize enough to increase the stability of measurement data with the reduced warm-up time.According to the method,a quick stabilization constant current system based on process control is created,and experimental research is carried out.Experimental results demonstrate that the drift of the light intensity data baseline is reduced from 0.5% to 0.06% when the light source powered by the design system is compared to the original constant current system.The system is also applied to an on-line filter type NIR spectrometer at the same time.When compared to the original spectrometer,the baseline drift is reduced from 1% to 0.19% after a 2-minute preheating period.Finally,two different types of field NIR spectrometer moisture analysis comparison studies are carried out.(1)The portable raster scanning NIR spectrometer.Modeling experiments demonstrate that the moisture content determination coefficient for the single model using the combination of the micro-delay multi-channel parallel acquisition digital demodulation method and the comparison and correction of the homologous double optical beams is 95.75%,the moisture content determination coefficient of single-channel single-beam measurement is 93.20%,the sum of squared residuals of the projected residuals are 0.149‰ and 0.096‰,under the condition of no warm-up.The experiment on the impact of SNR demonstrates that,under the condition of a 20-minute warm-up,the micro-delay multi-channel parallel acquisition digital demodulation method reduces the maximum deviation of measuring maize grits moisture from0.90% to 0.52% compared to single-channel acquisition.The stability effect experiment shown that the method of comparison and correction of homologous optical beams with double beams reduced the maximum deviation of measuring maize grits moisture from 1.67%of single beam measurement to 0.70%,under the condition of no warm-up.(2)The on-line filter type NIR spectrometer.Modeling experiments demonstrate that combining the two methods of the rapid stabilization method of constant current power supply light source based on process control and a digital demodulation method of micro-delay multi-channel parallel acquisition which is preheated for 20-minute,compared to the peak detection method under the condition of a 20-minute preheating,the determination coefficient of model is increased from 85.98% to 95.59%,the sum of squares of the projected residuals was reduced from 1.4‰ to 0.99‰.The SNR effect experiment demonstrates that the low frequency fitting conditioning method of the variable modulation signal based on spectrum analysis and the micro-delay multi-channel parallel acquisition digital demodulation method were used to reduce the maximum deviation of measuring white paper moisture from 0.2% to0.05%,compared with the peak detection method.Stability effect experiments show that under the condition of 2-minute warm-up,the rapid stabilization method of constant current power supply light source based on process control,compared with original constant current source reduced the fluctuation of the moisture value of the measured white paper from 0.68% to0.07%.From the aforementioned research,it can be seen that the presented detection method used to equal amplitude and variable amplitude modulation signals has significantly enhanced the SNR and stability of portable raster scanning and on-line filter-type NIR spectrometers.And may achieve high SNR and great stability without preheating or short-term preheating,which is of great significance for maximizing the benefits of in-situ detection of field spectrometers,enhancing the detection efficiency of spectrometers,and encouraging the development of NIR spectrometers.