A Study Of Methane Gas Sensor Based On The Method Of Non-dispersive Infrared

In recent years,mine disaster?oil and gas pipeline leakage frequently occurred which brings terrible influence to our lives.Consequently the accurate and real-time methane gas detection shows its significance to us and the methane gas sensor has been designed based on the principle of NDIR.This sensor has some advantages including tiny size?high accuracy and non-poison etc.According to this dissertation,several common gas detection methods have been illustrated and the benefits and drawbacks of them have been analyzed as well.The detection method of this system is selected as the non-dispersive infrared gas detection based on the conclusion.Moreover,this thesis reveals the theoretical basis of this system is based on lambert beer law.And for the purpose of eliminating the influence by light intensity,the two-channel detection method is proposed and the theoretical formula of gas concentration can be derived accordingly.Based on the analysis of methane gas molecular absorption line,the characteristic absorption wavelength of methane gas is confirmed to be 3.31 um.Meanwhile,the central wavelength of reference channel is determined to be 3.93 um according to the selection of optical filter as well.According to the system detection solution,proper infrared light source?detector and the appropriate chip for the amplified circuit are determined and the air chamber structure design of sensor is complete as well.Due to the fact that pyroelectric detector is highly sensitive to the vibration of infrared light source,hence the modulation of infrared light source is required;As for the software and hardware design,the STM32 series single chip microcomputer is selected to be the controller for this system.Meanwhile,corresponding control circuit is designed and host computer program design is accomplished.The driving circuit of light source is designed on the basis of the features of infrared light source.Also the design of signal processing circuit aim at the small-signal output from pyroelectric detector,the output small-signal can be amplified and processed and the resulting signal will be sent to AD conversion circuit.In order to complete the acquisition of the signal,the resulting signal will be sent to SCM and it will compute the difference value between gas detection channel and reference channel as well as the ratio between these two parameters.Along with the compensation value of the environmental temperature,gas concentration can be reversely derived.Besides,power circuit of the system and environmental temperature detection circuit are designed for the purpose of reducing the external and internal interference by adapting the combination of software and hardware anti-interference measures.Digital signal processing program is provided as well.Calibration and performance test for the sensor system are performed in these experiments.By measuring the standard methane gas in different concentration range from 0 to 5.05%,the relationship between the differential value of various detector output voltages and gas concentration can be calculated.And the gas concentration can be computed by stepwise method based on the resulting curves and data analysis.Furthermore,according to the temperature compensation formula results from the experiments,how the test results are influenced by the temperature can be derived hence the temperature compensation enforced by the system can be complete.After the calibration,start the performance test for the sensor.