Research On Temperature And Pressure Control Of Laser Absorption Spectrum Measurement System

Since entering the 21st century,along with the rapid development of industrial construction and economy in China,"Carbon" consumption also increases with each passing day.Under the background that "peak carbon dioxide emissions" and "carbon neutrality" are paid more and more attention by the government,"carbon" monitoring is becoming more and more important.In particular,it puts forward higher and more accurate requirements for gas on-line and in-situ monitoring.At present,gas monitoring technologies based on optical principle has been widely used in various kinds of monitoring fields,such as atmospheric environment,industrial emission,smart cities and so on.Among these technologies,TDLAS technology has become a widely used trace gas detection method..In the TDLAS gas detection system,the signal source and driving circuit are generally used to drive the DFB(Distributed Feedback)Laser to generate the modulation signal,and the gas parameters are monitored through the inversion of the spectral signal absorbed by the measured gas.However,in order to meet the requirements of measurement accuracy in practical application,it is necessary to accurately measure and control the temperature and pressure of the target gas and the test system.According to the requirements of high precision and high stability control of temperature and pressure of laser absorption spectrum measuring instrument,the research and application of temperature and pressure control scheme of the system are carried out in this paper.Firstly,the effects of temperature and pressure on the measurement of laser absorption spectrum are studied based on HITRAN;Secondly,a temperature and pressure control system for laser absorption spectrum measurement is designed;Thirdly,the simulation model of temperature and pressure control system is established,and a variety of PID(Proportional-Integration-Differentiation)control algorithms are studied;Finally,using the laser control circuit and optical cavity,a laser absorption spectrum gas measurement system based on TDLAS is built,and the CO2 concentration and CH4 carbon isotope ratio are tested under the control of temperature and pressure.The following are the main research contents:1.The effects of temperature and pressure on the measurement of laser absorption spectrum are studied in this paper.According to the molecular motion characteristics,the changes of temperature and pressure will affect the physical quantities such as the absorption,spectral line shape and spectral line intensity of gas molecules,resulting in the error of spectral measurement accuracy.In addition,in the laser absorption spectrum measurement system,the change of temperature field will also cause the weak deformation of the optical cavity,which will affect the optical path,and introduce uncertain factors into the measurement results.2.The temperature and pressure control models of laser absorption spectrum measurement system are identified,and simulated with MATLAB.In the research,the output response curves of different control modes after inputting the same step signal are simulated and compared,and finally the fuzzy self-defined adaptive PID(Fuzzy Adaptive PID,FA-PID)parameter control algorithm were selected because it can meet the design needs in response time,overshoot and other parts.3.A temperature and pressure control system for laser absorption spectrum measurement is designed in this study.The box of the control system adopts the design scheme of "convex" multi-layer heat insulation and semiconductor cooler.Taking PID control algorithm as the core,the design involves high-precision temperature sampling module,analog-to-digital conversion module and driving circuit module in temperature control unit,flowmeter,high-precision pressure sensor and vacuum pump in pressure control units.Finally,it can achieve the temperature change for a long time shall not exceed 0.005?,the negative pressure can be controlled in the range of 100-200mbar,and the control accuracy can reach 0.2%.4.TDLAS gas monitoring system was set up to measure CO2 concentration,CH4 and isotope concentration at different temperatures and pressures.The results further prove the necessity of high-precision temperature and pressure control for the laser absorption spectrum measurement system.