Design Of TDLAS Special Digital Lock-in Amplifier And Study Of CO₂ Detection

TDLAS technology is widely used in petroleum,chemical industry,environmental monitoring and other fields.It has the characteristics of fast detection speed,high accuracy and online detection.TDLAS technology needs digital lock-in amplifier for signal processing.Traditionally,the volume of LIA is too large to be integrated.Although some progress has been made in the research of miniaturization of digital lock-in amplifier,the volume of general-purpose laser controller is too large to be integrated.In this paper,the design and CO₂ detection of TDLAS special digital lock-in amplifier are studied from the point of view of miniaturization and integration.The research contents of this paper are as follows:1)The basic principles of gas detection and digital lock-in amplifier are introduced and analyzed,which provides theoretical support for the design of TDLAS special digital lock-in amplifier and the research of CO₂ detection.2)The hardware modules of TDLAS special digital lock-in amplifier and detection system are designed,including digital lock-in amplifier module,laser control module,Photoelectric conversion module,detection optical path and electromagnetic interference shielding device,which provide hardware basis for the construction of CO₂ detection system.3)According to the basic principle of digital lock-in amplifier,the detection program of digital lock-in amplifier is designed,including signal generator,data acquisition module,mixer multiplier and digital low-pass filter module.Through the combination of simulation and experiment,it is proved that the whole detection program can realize the function of second harmonic demodulation.4)The CO₂ detection system was built,and the system was tested and analyzed,and the CO₂ detection experiment was carried out.The experimental results show that at 20%CO₂ concentration,the signal-to-noise ratio of the detection system can reach 38 dB,the linear fitting degree between peak-peak voltage of second harmonic and CO₂ concentration can reach 99.664%,The maximum deviation between the mean and standard values of actual measured CO₂ concentration is 0.34,and the maximum variance is 0.019;when the integration time is 10.2s,the Allen variance reaches the minimum value of 0.07%,and in the long-term stability test,the detection is carried out.The output concentration fluctuation of the measuring system is less than 1%.It can be seen that the miniaturized and integrated TDLAS special digital lock-in amplifier designed in this paper has good CO₂ detection performance.