Research Of Gas Remote Sensing System Based On Wavelength Modulation Spectroscopy

In the process of coal mine production and natural gas transmission,the leakage of dangerous gas with flammable and explosive properties will lead to disaster accidents,which will seriously endanger the lives of miners and the masses.Therefore,only through taking real-time concentration detection of dangerous gas can prevent accidents.Considering the risk of gas leakage,and in order to ensure the safety of gas detection workers,remote sensing has become one of the most important research directions of gas detection.Among numerous gas detection methods,WMS technology which belongs to TDLAS technology is widely used because of its high sensitivity,strong ability to resist noise and the characteristics of short time.In view of this,a set of gas remote sensing system based on WMS which has the potential to be integrated into portable instruments with low costs is developed in this paper.Firstly,the research status of infrared gas remote sensing system in China and abroad were analyzed,and the infrared absorption spectroscopy and TDLAS theories were elaborated later.While introducing the principle of second harmonic detection,the orthogonal lock-in amplifier technology and segmental fast Fourier transformation technology were stated,and the two methods used in second harmonic extraction were simulated and analyzed by Matlab.Orthogonal lock-in amplifier technology was used as the harmonic detection technology in this pape because of its maturity and optimizable.Secondly,the digital orthogonal lock-in amplifier was designed.Hardware circuits include preamplifier and amplifier circuit,ADC circuit,SCI communication circuit and DAC circuit.In software programming,since the gas remote sensing system is prone to noise interference,in order to further improve the anti-noise ability of the system,after verified by Matlab simulation,the Savitzky-Golay filtering algorithm was added in the orthogonal lock-in amplifier to filter gas absorption signal.At last,the overall program flow of the software is introduced in detail.Thirdly,the optical path construction scheme of the gas remote sensing system was explained.The mathematical modeling and analysis of Fresnel lens,which is the core of remote sensing system were carried out.In order to verify the feasibility of applying the Fresnel lens to the remote sensing system,a Fresnel lens with a diameter of 200 mm,a focal length of 200 mm and a thickness of 2 mm was designed by Tracepro.For practical application,the possible problems in in optical path were also studied.Finally,CH₄ remote detection experiment was carried out.A comparative experiment was conducted in form of whether Savitzky-Golay filtering algorithm was used.Functional verification experiments showed that the system and related processing methods can extract the second harmonic signal,which proves that the system has the basic function of gas remote detection.Under the condition of 300ppm·m,the detection system with Savitzky-Golay filtering algorithm could increase the second harmonic SNR from 17.575 to 27.471.CH₄ concentration linear calibration experiment showed that the amplitude of the second harmonic extracted by the remote sensing system was proportional to the gas concentration,and the goodness of linear fitting without Savitzky-Golay filtering algorithm is 0.99861,compared with the system with Savitzky-Golay filtering algorithm is 0.99989.Allan variance test showed that the 1?detection limit of the detection system without Savitzky-Golay filtering algorithm is 14.985 ppm·m,compared with the detection limit of the system with Savitzky-Golay filtering algorithm is 7.443ppm·m.Using Savitzky-Golay filtering algorithm can improve the performance of the detection system was therefore proved.At the same time,the experiment of on-line remote detection of methane confirmed that,the system has the ability to respond quickly to the change of concentration and meets the basic requirements of gas remote detection.The Innovations of this paper are:1.The second harmonic signal is extracted by segmental fast Fourier transformation technology.2.The Savitzky-Golay filtering algorithm is adopted to improve the anti-noise ability of the gas remote sensing system.