Research Of Methane Sensor Based On Plused-LED

Coal and natural gas are the main energy sources in China,and our country's exploration and consurmption occupy a significant percentation in the world.With the inceasing consumption of coal and natural gas,the frequency of serious accidents such as methane brust and explosion increased,which are threatening the safety of worker's lives and mine properties.Approximate 80%mine gas is CH₄,so the on-line monitoring the CH₄ with high sensitivity is important for avoiding the explosion and gas brust.Based on the merits of high sensitivity,rapid response,selectivity,low cost and low consumption,near dispersive infrared?NDIR?spectroscopy has been widely used in atmosphere monitoring,breathing diagonoses and industrial safety.Therefore,the NDIR absorption spectrosocpoy based on pulsed LED source was developed for measuring the mine gas.The main content of the research is as follows:1.The principle of infrared gas detection technology was described.First,the physical principle and theoretical basis of infrared absorption technology was presented.Then the formulas of gas concentration were derived according to Beer-Lambert law,which is the basis of methane sening.2.Designed the Optical system.In traditional NDIR sensor,the gas concentration is derived by direct absorption signal.The signal is always disturbed by ambient noise.The pulsed-modulation LED was adopted in this paper,and then the gas concentration was obtained by an arithmetic.Besides the pyroelectric detector was replaced by photodiode to reduce the temperature compensation process.The cell was optimized in Trace Pro according to some parameters of light source and detector.The utilization efficiency of LED was increased by about 20%,because the parabolic mirror was equipped for lightsource and detector.Besides,the gas cell was coated by high reflective gold film.3.Designed the low power consumption and low cost methane sensor circuit.A light source drive circuit with feedback control was performed based on Hall-Libbrecht circuit,and the performance of the light source driver was evaluated in this paper.The maximum output current of the driver was 1 Amps,the current temperature coefficient was only 523 ppm/?,and the modulation frequency was up to 49 k Hz.The detector signal conditioning circuit was designed based on photodetector.The circuit consist of low-ripple power supply circuit,current-to-voltage conversion circuit,high-pass filter,low-pass filter and magnifying circuit.Then the FFT trasfromation algorithm,display circuit and communicaton circuit was designed on STM32H743 embedded system platform.4.This paper presents a concentration inversion methdology based on FFT transform and wavelet transform.Due to the multi-resolution characteristics of wavelet transform in the time and frequency domain,the output signal was denoised by wavelet decomposition,threshold denoising and wavelet reconstruction.Then the gas concentration was calculated by FFT transform.It is concluded that comparing with FFT transform,the accuary and fitting relevancy were higher when using FFT transform in coordination with wavelet transform.5.The methane gas sensing system was established and calibrated,thenand its performance was tested.The correlation coefficient between the detector's output signal and methane gas concentration reached 0.997,and the sensor's measurement error was 4.2%,and the long-term operating error was less than2.4%F.S.