Research On TDLAS Gas Detection System And Its Integration

The topic of air pollution is more and more serious in our country and the core of national pollution control issues is how to detect the air pollutants in real time.Among the many detection methods,Tunable Diode Laser Absorption Spectroscopy(TDLAS)technology has been widely used in gas detection field with its high sensitivity and high precision.The existed TDLAS gas detection system is mostly composed of several separate function elements which is space-occupation and very complex operationally.The main purpose of this thesis is to integrate and modularize all the functional modules of the separated TDLAS gas detection system,such as the laser and source driver,the gas chamber,the photodetector and signal conditioning,and digital signal processing.The main work of this thesis includes several items as follows.First of all,we used the FPGA to achieve a waveform generator based on DDS technique whose frequency,amplitude and phase can be adjusted.By using this waveform generator the modulation of the laser source driver are built up.The program control of laser working temperature is realized based on FPGA as well.The functional characteristics of the laser source driver and the temperature control module are investigated.Secondly,the anti-aliasing filter and oversampling technique are used in the photodetector and signal conditioning part,which can improve the signal to noise ratio of the harmonic signal and reduce the pressure of the back-end digital filtering.And the design details are also described in this paper.Thirdly,using FPGA,an improved direct digital acquisition mode lock-in amplifier is designed.With the design of the filter in the lock-in amplifier and square root module are described.And the filter effect caused by data truncation is simulated by the MATLAB Simulink tool.Finally,the integrated TDLAS gas detection system composed of all the designs above is built up.Direct absorption(DA)and wavelength modulation spectroscopy(WMS)techniques are separately used for detecting the concentration of mixed gases(CH4-N2).We mainly tested the stability and accuracy of the integrated system.The influence of temperature on the harmonic signal is also investigated experimentally.