The Digital Signal Processing And Analysis Based On TDLAS

Casualties and property which is caused by CH4 gas explision is offen happend in our country’s coal mine acquisition activity, so it’s important to measure the concentration of CH4 accurately. The technology of tunable diode laser absorption spectroscopy detects the gas concentration by measuring the intensity changes of reflected light or transmitted light. TDLAS has been widely used in the field of the gas concentration detection and it has the advantages of high-sensitivity, high-stability and good-selectivity.First of all, according to Beer-Lambert law and the database of HITRAN, three methane absorption lines in the R3 transitions of 2v3 band are chosen to investigate.Wavelength modulation spectroscopy theory and harmonic detection theory are then studied, and the intensity-frequency modulation signal which is based on Lorentz lineshape, is analyzed based on the Fourier series expansion and Taylor series expansion.The expression of non-linear modulation model are given and analyzed the cause of the harmonic signal baseline drift.Secondly, the source and the characteristics of the noises in TDLAS are studied and the use of noise elimination with wavelet threshold de-noising is studied intensively.The algorithm, decomposition layers, threshold function selection and threshold selection are analyzed particularly, and a threshold function which has a better effect compared with the traditional hard and soft threshold function, is proposed in the thesis based on the hyperbolic function. Several concentration inversion scheme are analyzed and a scheme compared least square method withffII12 is proposed and we discuss the effect of detection scheme by the light wavelength drift, frequency modulation amplitude and wavelength scanning range. Detection precision of the system increased three times after de-noising.Thirdly, the influence of temperature fluctuation on the gas absorption spectral line parameters(line strength, line width, density of gas molecules and the absorption coefficient) and the concentration measurement is analyzed. The advantages and disadvantages of temperature correction scheme are discussed, and the measurement error from temperature fluctuation is reduced using a temperature correction scheme.Combining with the design of the absorption cell from which the temperature andpressure are extracted from, to improve the accuracy of measurement, and the temperature drift reduce 25 times, reached 2ppm/℃.Finally, using Matlab Simulink simulation tool to build the model of the TDLAS gas detection system. According to the laboratory work parameters of laser and photoelectric detector, light source module, gas cell module, data detecting and signal processing module are built, and the whole system model is tested and analyzed. At last,the simulation results are given, and improved that the system performance is greatly improved after wavelet denoising and temperature correction,finally the data processing software interface of the TDLAS system is designed and wrote, and each functional module is introduced detailedly.