| The subject of this thesis originates from the National Major Scientific Research Instrument Development Project of the National Natural Science Foundation of China:Research on new infrared methane and carbon monoxide detector(project number:61627823).In recent years,the total amount of coal mine accidents is a downward trend,but gas accidents in coal mines and fire accidents caused by spontaneous combustion of coal still occur frequently.These accidents seriously threaten people's life,health and property safety,and also bring many disharmonious factors to national development and social stability.Therefore,it is crucial to establish a gas sensing system,which can prevent and predict gas accidents and coal spontaneous combustion accidents.Methane(CH4)is one of the main components of coal mine gas,and carbon monoxide(CO)is the main product in the process of coal spontaneous combustion.High-precision analysis and dynamic monitoring of CH4 and CO in the coal mine environment can help to accurately identify the occurrence and process of gas accidents and coal spontaneous combustion accidents,which meets the requirements of coal mine safety production.Tunable diode laser absorption spectroscopy(TDLAS)technology has the advantages of high precision,good stability,fast response,and long-term continuous measurement,which meet the needs of in-situ gas detection in the coal mine environment.This dissertation developed a near infrared CH4 and CO gas sensing system in mine environment.Based on TDLAS,a dual-mode gas detection scheme combining direct laser absorption spectroscopy(DLAS)and wavelength modulation spectroscopy(WMS)was proposed.And a dual-gas sensing system based on a near infrared laser was designed by using a group of CH4 and CO absorption lines near 2.334?m.The main works are summarized as follows:Firstly,the optical part of system was optimally designed.The mathematical model of the Herriott cell was established;the mechanical structure of the fiber-coupled module was designed.The fiber-coupled Herriott cell(FC-HC)with an optical path length of?25 m was designed to enhance the absorbance of CO and CH4,and realized the high-precision gas detection order of parts per million in volume(ppmv).Using a single-mode fiber and a multi-mode fiber,the larger multi-reflection gas cell was separated from the optoelectronic module of the detection system,enabling remote and in-situ gas detection.The performance test experiments of the FC-HC were carried out through acetylene(1.533?m)and methane(2.334?m),respectively.The detection sensitivity of acetylene and CH4 were 0.38 ppmv and 1.1 ppmv,respectively.Secondly,the electrical part of the detection system was optimally designed.An integrated power converter and laser driver were used.Also,a safe board-level signal processing unit was developed based on digital signal processor(DSP),which was the core of the system control.Based on the signal processing unit,a software flow of a dual-detection-mode was designed and an adaptive algorithm for suppressing the electrical noise of the system was proposed.Also,the CH4 experiments were carried out.Using the Lagrangian interpolation(LI)filtering algorithm and the nonlinear least squares fitting(NLLSF)algorithm,the signal-to-noise ratio(SNR)of the CH4 sensing system was increased by 11 d B,the resolution of system was increased to 3 times,and the detection sensitivity was reduced from 1.5 ppmv to 0.5 ppmv.Thirdly,a dual-gas absorption spectral line separation algorithm was proposed.Based on the Lab VIEW platform,the extraction process of gas absorption lines were simulated and analyzed.With the two-step(2s)second harmonic(2f)algorithm based on polynomial fitting method(2s-2f-Polyfit),a dual-gas sensing system was developed.The detection limits of 62(parts per billion in volume)ppbv for CH4 and 11 ppbv for CO were achieved,the SNR of the 2f signal of CO was improved by 53 d B for the measurement of 500 ppmv CH4and 10ppmv CO.Finally,according to the gas detection requirements of the mine environment,a dust-proof and dehumidification device was integrated,and a near-infrared dual-gas sensor prototype was integrated.Field experiments were carried out to evaluate the sensor performance for early fire detection and CH4 gas leakage monitoring on the campus of Jilin University.Experiments showed that the relative error between the developed sensor and the commercial CO detector was less thaną1 ppmv.The main innovations of this thesis are as follows:1.A long optical path(?25 m),fiber-coupled Heriot cell was developed.A single mode fiber and a multimode fiber were used to separate the cell and the electrical devices in the detection system,realizing long-distance,in-situ gas detection.2.An intrinsically safe board-level signal processing unit based on DSP was developed.A signal processing platform based on DLAS and WMS was integrated in the signal processing unit,which expanded the dynamic detection range.Aiming at the background noise in the DLAS sensing system,a signal filtering algorithm based on LI and NLLSF was proposed,which improved the resolution of CH4 detection to 3 times.And the lower detection sensitivity of the CH4 sensing system was reduced from 1.5ppmv to 0.5 ppmv.3.A dual-gas absorption spectral line separation algorithm was proposed to suppress the cross interference between CO and CH4.Using the 2s-2f-Polyfit,the near-infrared dual-gas sensing system achieved a lower detection limit of 62 ppbv for CH4and a CO detection limit of 11 ppbv.For the measurement of 500 ppmv CH4and 10ppmv CO,the SNR with the 2s-2f-Polyfit algorithm was improved by 53 d B compared to the scheme without the algorithm. |
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