| Among the various disasters,fire is the most frequent and common one that threatens public safety and social development.Every year,the number of deaths caused by fires is 20 times that of hurricanes,tornadoes,floods and earthquakes,which poses a great threat to people's lives and property.Fire prevention is a prioritized issue in any field.Therefore,it is of great significance to develop a fire detector to prevent fire.This paper originates from the national key research and development plan project,which name is: Research on reliability and safety monitoring methods in complex environment(project number: 2016YFD0700101);and the National Natural Science Foundation of China,major project special project,which name is Research on new infrared methane and carbon monoxide detector(project number: 61627823).In the early stage of fire,insufficient combustion occurs due to insufficient contact between the combustion products and oxygen,thus,the generation of carbon monoxide due to insufficient combustion is earlier than the flame.According to this,the aim of this paper is to develop a gas analyzer based on infrared absorption spectroscopy technology to detect carbon monoxide gas and prevent fire.Two key problems are focused on: 1)design and development of long-path gas absorption cell,2)high sensitivity,fast response and portable signal processing circuit.The main contents of this paper include:Firstly,the mathematical model of Herriott gas chamber is established.Based on this model,the position,size and reflection order of the reflecting spot on the mirror are calculated when the light continuously reflects in the gas chamber,and the direction vector of the reflecting spot is given.The design methods of single-ring Herriott chamber and multi-ring chamber with minimum volume are studied.On the basis of single reflection ring,the mathematical formulas for designing double or even multi-ring gas chambers are derived.Multi-ring gas chambers can be used to expand the detection range of single gas or to detect multi-component gas.The gas chamber can switch the detector between two optical paths,thus expanding the dynamic range of the detector.In order to extract the second harmonics of the detector signal,a spectrum analyzer is designed and developed,which extracts the second harmonics based on the optimized segment fast Fourier transform algorithm.The spectrum analyzer is controlled by dual-core chips(DSP and ARM).ARM chip works as a host,its main function is to control touch screen,respond to user's instructions,and exchange data with slave.DSP chip works as a slave.Its main function is to control ADC sampling,process the collected data with digital filter algorithm,orthogonal phase-locked amplifier(OLA)algorithm or optimized segment fast Fourier transform(OS-FFT)algorithm,finally,send the calculated data to the host.The infrared laser gas analyzer consists of a new type of Herriot gas chamber,spectrum analyzer and laser.As QCL(quantum cascade)laser with wavelength at mid infrared absorption peak of carbon monoxide is expensive,a distributed feedback(DFB)laser with central wavelength of 1.533 um(the first overtone absorption peak of acetylene)is used to replace QCL laser to test the performance of gas chamber and spectrum analyzer.In the experiment,the step was set as 100 ppm,acetylene gas with concentration of 0-1000 ppm was configured to verify the optical path,and the performance of OLA algorithm and OS-FFT algorithm were compared.The experimental results show that the optical paths obtained in the experiment are 6 m and 20 m,respectively,the error is very small comparing with the calculated optical path 6.12 m and 19.78 m in the simulation,which proves the accuracy of the design of the chamber model.In the calibration experiment,the linearity of the two algorithms is very good(99.7% and 99.79%),but the fluctuation range of concentration caused by OLA algorithm is-4 ppm to 2 ppm,which is less than that caused by OS-FFT algorithm-2 ppm to 6 ppm.From the calculation results of Allan variance,the detection limit of using OLA algorithm is 0.68 ppm,which is lower than that of using OS-FFT algorithm,which is 1.35 ppm.However,the computational complexity of the OS-FFT algorithm is less than that of the OLA algorithm,so the advantage of OS-FFT algorithm is faster signal processing speed.When the sampling rate of the system is very low,the computing time of the two algorithms is almost the same,but when the sampling rate is very high,the OS-FFT algorithm can significantly improve the computing speed,which can reduce the response time of the system.In summary,although the accuracy and stability of OS-FFT algorithm are not as good as the OLA algorithm,it has faster response speed and less hardware consumption.Therefore,when the detection accuracy is not required to be high,the detector based on OS-FFT algorithm can have a better response time and a higher cost performance.Innovation:1.A design method of single-ring Herriott cell with minimum volume is proposed.The minimum area of spot distribution is calculated when incident light is incident at a certain angle without changing the number of reflections.The minimum volume of single-ring Herriott gas chamber is obtained according to the distance between mirrors.2.A design method of Herriott cell with multiple concentric reflecting rings is proposed.The cell has multiple optical paths,which can be used to expand the detection range of a single gas or to detect multiple gases in a single cell.Based on this method,a double-ring Herriott gas cell was designed and fabricated.The optical paths of the two reflecting rings were 6 meters and 20 meters,respectively.3.A method of extracting the second harmonic signal by using the optimized segment Fourier transform algorithm is proposed.This method is different from the traditional phase lock-in amplification method.It has less calculation,faster speed and less hardware costs.It is suitable for developing gas detector with high performance-price ratio. |
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