Research On Mid-infrared Carbon Oxide Detection System Based On Virtual Lock-in Amplifier Technology

Carbon monoxide (CO) is a toxic, flammable, explosive gas。 It may easilyexplode when mixed with air. In industrial production, if leakage occurs, itmay cause a fire, making heavy casualties. Research shows that, themaximum concentration of carbon monoxide that is allowed for a healthy adult is50ppm.It will endanger a person’s life if the monoxide concentrationinhaled reaches400ppm but is not treated in3hours. People will die within30minutes if inhaled carbon monoxide concentration rises up to3200ppm. But theNational Coal Mine Safety Supervision Bureau promulgated the "coal mine safetyregulations" provisions of the highest mine carbon monoxide concentration is24ppm. Therefore, it is of very important significance to research a real time,effectiveCO tester. This project is supported by the National Science and Technology SupportProgram“Infrared carbon monoxide detector (sensor) research and development inmine environment”,project number:2013BAK06B04。The CO system tester designed in this paper is implemented on the LabVIEWplatform. PCs have a great advantage in data processing when compared withthe embedded microprocessor. The system uses thermal light source to emit infraredlight, after absorption the Optical signal strength can be detected by the dualchannel pyroelectric detector, including the gas concentration related information. Thedetected signal is processed by a few simple signal processing circuits including afollower circuit, a preamplifier circuit and a DC blocking circuit. The USB dataacquisition card transmit the signal into the LabVIEW platform. Theamplitude signal extracts by the virtual lock-in amplifier based on LabVIEWplatform. The core of virtual lock-in amplifier is multiplier and amplitude extractionmodule. This paper also uses the signal generator generates a standard amplitudeadjustable sine wave as the signal to be measured to calibrate the virtual lock-inamplifier. the amplitudes of the output signals from the fabricated lock-in amplifier and the standard amplitudes are both tested and compared, and the linear fitting factoris up to99.978%.Innovation points of this paper: The virtual lock-in amplifier, compared with thetraditional lock-in amplifier, has better linearity and higher detection accuracy.Some experiments carried out using this detection system, and the gas is compoundedwith pure CO gas. The detection limit, sensitivity and stability of this system aremeasured. Experimental results show that, the system has a50ppm detectionlimit, and the detection error is less than4.2%. At the concentration of50ppm,3hours’ monitoring of the gas samples indicates that the fluctuation of COconcentration is in the range of6%~6%.