Research Of Early Fire Characteristic Gas Detection System Based On Cavity-enhanced Absorption Spectroscopy

Fire is one of the most universal disasters in human production and life,and also one of the greatest threats to people’s life and property safety.When a fire occurs,gas is generated earlier than smoke and flame,so developing a fire characteristic gas sensor is significant for early fire detection.CO and CO₂ are the most common fire gas products.If the gas sensor can only detect CO or CO₂,it is easy to be disturbed by the gas generated by candle burning,sandalwood burning and other non-fire conditions.The simultaneous detection of CO and CO₂ gases can reduce the false positive rate.Off-axis cavity-enhanced absorption spectroscopy is a type of infrared absorption gas detection technology.It has the advantages of high detection sensitivity,good gas selectivity,strong robustness,fast detection speed and in-situ detection.Therefore,a dual-gas sensor based on the off-axis cavity-enhanced absorption spectroscopy is reported in this paper.The sensor implements the simultaneous detection of CO and CO₂ gases by using a near-infrared DFBL with central wavelength of 1.573μm,an optical cavity and a photodetector.In this paper,the absorption lines of CO and CO₂ gases are determined according to the HITRAN database.Then,the overall structure of the system is given,and the hardware of the system is designed in detail from the optical part,the gas path part and the electrical part.In the optical part,the operating temperature and current of the laser are determined according to the selected CO and CO₂ absorption lines.Then a compact mechanical structure including an optical cavity is designed to improve the response speed and stability of the system.Finally,a photodetector module with high gain and miniaturization is designed to make the structure of the system more compact.To reduce the impact of water vapor and particulate matter in the environment on the stability of the system,a dust and dehumidification module is added in the gas path part of the system.In the electrical part,in order to make the output wavelength of the laser cover the absorption peaks of CO and CO₂ at the same time,the main control controller based on digital signal processor（DSP）that can generate the driving signal required by the laser is developed.In addition to generating the driving signal required by the laser,it can also collect the output signal of the photodetector and extract the absorbed signal amplitudes,as well as collect the operating temperature and current of the laser and achieve the function of controlling the working state of the laser.At the same time,to reduce the effect of environment temperature and pressure fluctuation on the detection accuracy of the system,a temperature and pressure controller based on STM32 is developed to keep the stability of the temperature and pressure in the cavity.In terms of software design,the direct digital synthesizer（DDS）configuration program,data acquisition program,baseline fitting program and absorption signal amplitude extraction program in the main controller are designed,and the whole software flow of the temperature and pressure controller and the PID dynamic control program are introduced comprehensively.The fire characteristic gas monitoring platform based on Lab VIEW is designed to receive,process,storage and display the data from the main controller and the temperature and pressure controller.By monitoring the changes in these data,we can judge the working state of the system and observe the trends of CO and CO₂concentrations in the environment in real time.Finally,we integrated the system and tested the performance.First,the temperature and pressure in the cavity were controlled by temperature and pressure controller.When the target temperature at the inlet of the cavity and in the cavity were set at 30℃and the target pressure in the cavity was set at 300 Torr,the temperature fluctuation range at the inlet of the cavity was from 29.941℃to 30.077℃,the temperature fluctuation range in the cavity was from 29.977℃to 30.045℃,and the pressure fluctuation range was from 299.12 Torr to 300.862 Torr through 30-minute measurement.Subsequently,the CO and CO₂gas calibration experiments were carried out,and the relationship between the concentration and the amplitude of absorbed signal of CO and CO₂ gas,respectively.Then the stability and response time of the system were tested.The experimental results indicated that when the average time was 1 second,the detection limits of CO and CO₂ were 23.67 ppmv and 20.82 ppmv respectively,and the response time of the system was 7-8 seconds.Finally,the concentrations of CO and CO₂ produced during smoldering experiment of cardboard were measured by this system,and the rise of CO and CO₂ concentration and the change of CO and CO₂concentration ratio in the environment were detected within 2 minutes after the heating table began to heat the cardboard.The results of this experiment verify the feasibility of this system in early fire detection.The innovation of this paper is as follows:Simultaneous detection of CO and CO₂is achieved by a near-infrared DFB laser.A temperature and pressure controller is developed to realize the precise control of temperature and pressure in the cavity,so as to reduce the influence of environmental factors on the detection accuracy of the system.