Design Of Non-cooperative Target Carbon Dioxide Detection System Based On TDLAS

Air pollution caused by energy consumption has become a social contradiction against people's yearning for a better living environment.Environmental issues have been one of the most important topics in the 21st century.As one of the main greenhouse gases,carbon dioxide in the atmosphere is of great concern to environmental protection.In this thesis,based on tunable laser diode absorption spectroscopy?TDLAS?,a remote detection system with non-cooperative target was developed for real-time monitoring of carbon dioxide using a 2004nm laser combined with wavelength modulation spectroscopy?WMS?.The main work is as follows:First,the remote CO₂detection system with non-cooperative target was integrated.The nanoplus2004nm laser was used and an absorption line at 4989.97cm^-1was selected for TDLAS measurement according to the requirements of non-cooperative detection.Based on the analysis of each hardware module,a single ended CO₂ detection system with non-cooperative target was designed,in which the transmitter and receiver of the light are at the same end.The CO₂ gas concentration was deduced by detecting the diffuse reflection/backscattering echo signal from the surface of a hard-topographic target.In addition,based on Lab VIEW platform,some programs including data acquisition,spectral shift correction,second harmonic?2f?signal normalized by direct absorption signal and gas concentration inversion are designed,which make the system achieve real-time and online measurement of CO₂concentration as well as avoid the influence of laser temperature drift and laser intensity fluctuation.Second,the automatic correction of gas pressure fluctuation was conducted.Aiming at the problem that the measured gas concentration by TDLAS is constantly affected by the pressure fluctuation of the atmospheric environment,a simple method was proposed for modification of the measured gas concentration without pressure sensor required.The analytic expressions characterized line broadening were established by the spacing between peak and valley of WMS-1f signal and the distance between two zero crossings of WMS-2f signal,respectively.The effect of pressure fluctuation on the measured gas concentration can be corrected by the WMS-1f or 2f signal,due to the relationship between line broadening and gas pressure.The influence of pressure variation on harmonic signals of CO₂ absorption spectrum was analyzed experimentally.The feasibility and effectiveness of the method are verified by correcting the concentration in the practical measurement process.Third,the performance of the remote detection system with non-cooperative target was analyzed and evaluated.Through the experiment of different surface and laser incidence angle and distance,the influence of measurement surface,angle and distance on signal-to-noise ratio is analyzed.The accuracy of the detection system was verified by comparing with the simultaneous measured data by a commercial CO₂sensor.The minimum detection limit,the repeatability and the concentration fluctuation were determined to be 8.7 ppbv,0.5%,and 15 ppm respectively.Finally,the detection system was employed to monitor the indoor CO₂ concentration continuously,which laid a foundation for the practical application of the system.Compared with the traditional CO₂ sensor,the developed CO₂ TDLAS system with non-cooperative target in this thesis has the characteristics of high precision,fast response time,large dynamic scale and flexible use,which can be extended to greenhouse gas monitoring,vehicle emission detection and other applications.In addition,the detection system is of good versatility benefiting from modular design.It can be used to detect other gases by replacing the laser and detector with different working wavelength band.