Study Of Methane Detection System With Three Ellipsoid Absorption Chamber Based On MEMS Light Source

In the energy consumption of our country, coal has always been an important position. Most of coal mines in our country are in underground, with mining difficult and security risks, and although the government always paies attention to the safety production work, but the coal mine accidents caused by gas explosion were never interrupted.One of the necessary conditions for gas explosion is that the concentration of CH4 gas in the air must be reached 5%-16%. If we can timely and accurately detect the concentration of CH4, and when the concentration is in explosions or in high concentration above the upper explosive limit, the staff is in time withdrew from the mine, and CH4 is diluted by blowing air, so as to avoid explosion or personnel poisoning. Therefore, the development of methane sensor with high resolution and high sensitivity has important practical significance.At present, the domestic and foreign scholars are actively using infrared absorption method to realize the measurement of methane gas. Infrared absorption method overcomes the shortcomings of other methods such as constant calibration, large error and short life, and at the same time it also has many advantages, such as fast detection, good selectivity, wide spectrum and so on. Based on the principle of infrared absorption and differential absorption method of single optical path and dual wavelength, the methane sensor with high resolution is proposed, with MEMS infrared light of the wide wavelength, high modulation frequency and long life, and a designed three ellipsoid structure infrared air chamber. Theoretical analysis shows that the detecting minimum methane concentration of the sensor for 3.31μm wavelength infrared light can reach 1.58 ppm, and it theoretically derives that detection resolution of the infrared methane sensor has been greatly improved so as to enlarge its the application range,making it not only apply to detection in the mine, but also apply to detection of gas leakage in family using the natural gas and transportation natural gas in the pipeline with higher accuracy requirements.The main research contents of this paper are as follows:1) Designing the three ellipsoid structure air chamber. Infrared light can be reflected for five times in the new air chamber before reaching the detector to extend the optical path effectively. According to the simulating diagram obtained by using the Light Tools software to analyze the model of the air chamber, the air chamber realizes the light focusing effectively, and the light mainly distributes in the center strip area with good symmetry, for longitudinal to ±2mm, lateral to ±6mm around concentrated area and completely covering two receiving window range of the pyroelectric detector; If the position of pyroelectric detector is adjusted to consist the two receiving window direction with the light distribution, then the detector can realize the biggest reception. The analysis shows that the minimum methane concentration detected by the sensor has been reached 1.58 ppm, and it is theoretically verified that the structure can greatly improve the resolution of infrared methane sensor.2) Designing the MEMS infrared light source. In order to reduce the size of the light source and provide a high performance light source to the sensor,the design of the MEMS infrared light source is given in this paper, and then the structure of the light source and material selection of the membrane are expounded; In order to make deformation and thermal stress of the light as little as possible when it works in the high temperature, the result displays that the optimal thickness values of Si O2 support layer is 0.58μm and the optimal thickness of Si3N4 passivating layer is 0.72μm by using the COMSOL Multiphysics software to simulate and optimize thick parameters of the model, making the MEMS infrared light source having a higher working temperature and smaller deformation and stress; This provides the dimension parameters for the actual production of the light source.3) Designing the part circuit of infrared methane sensor. The circuit is designed, mainly including battery charging circuit, boosting circuit, bucking circuit, regulator driving circuit of light source, amplification and filtering circuit; It shows that the circuit has good amplification effect that the theoretical magnification obtained by calculating and the magnification obtained by using Proteus software to simulation prove to be in good agreement.According to the noise figure and amplitude-frequency figure,it is concluded that the amplification and filtering circuit has better inhibitory effect on the noise with the center frequency of the 2Hz before and after.4) Designing the software based on the modular approach. The designed software include the main program, the initialization program, and signal acquisition program, and MCU calls the program to achieve the function of infrared methane sensor.