| Because of the rapid development of national economy, a large number of industrialwaste gas produced by the petroleum chemical industry, electrical power and coal industryhave a serious effect on the environment pollution. It is common to come across someatmospheric pollution phenomenon, such as the greenhouse effect, acid rain and atmosphericozone hole. Nowadays, living environment and its impact to human health is increasinglybecoming a concern in our society. Therefore, the quality monitoring of the air is highlydemanded. How to detect timely and accurately those gases that are flammable, explosive,poisonous and harmful, with high sensitivity and reliability has attracted huge effort ofscientist to develop an intellectualized and portable infrared sensor system at low price.In this thesis, we will focus on the system design for methane gas detection. Theproposed system will be on the basis of the infrared absorption spectrum theory, and apply thedual-wavelength single optical path differential detection method. We will use a kind ofMEMS (Micro-Electro-Mechanic System) infrared light sources due to their broadwavelength range, high modulation frequency, and small size. For a enlarged opticalabsorption path, an integrating sphere is used, which will guarantee a high precision detection.In this thesis, the major research tasks are listed as the following:(1) The investigation on MEMS infrared light source used as the optical light source.Compared with the traditional infrared light source, MEMS infrared light source has aseries of advantages, such as small size, broad band wavelength, high modulation frequency,low price and so on, which qualifies the MEMS infrared light source for the compact andhigh performance infrared sensor system at low price. However, the MEMS infrared lightsource is a surface light source and with the Lamberation radiation characteristics. It thusbecomes necessary to do beam shaping in order to collect optical energy. The opticalcollimation system based on the free surface reflector and the aspheric collimation lens were studied respectively. We have built the simulation models in the commercial availablesoftware Zemax and TracePro, theoretically designed the beam shaping optics which canrealize to compress the divergence angle of the MEMS light source from180°to14°. Byusing Intex MEMS infrared light source, the simulation model have been approved。(2) The study of integrating sphere for the enlarged optical absorption path. With anintegrating sphere acting as a gas absorption cell, the length of the effective interaction opticalpath is increased because of the multiple reflections of infrared light by the inner surface ofthe sphere, and the detect sensitivity of the system improves. However, unlike the calculatingof effective optical path length of traditional gas cell, the random propagation of the infraredlight in the inner of the cell makes it difficult to calculate the equivalent optical path length forthe integrating sphere cell. In this thesis, the physical dimension is deduced for the equivalentoptical path according to the flux conservation principle in the process of light transmission,which help us to solve the calculation problem of equivalent optical path of the integratingsphere cell by establishing a mathematical model of the integrating sphere.(3) The design proposal for driving the MEMS infrared light source and signalprocessing. We propose to use a FPGA chip as the main control chip in the hardware circuitdesign. Modules and its integration are designed for building up the whole circuit system. TheMEMS infrared light source is droved by a high frequency modulation power signal which iscontroled by FPGA chip. For signal process, the voltages of the infrared detector of twochannels are filtered and amplified simultaneously. After the A/D conversion module, theanalog signal is converted to digital one which is transmitted to the PC for data processingand display. |
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