Research On Digital Fiber Gas Detection Scheme

Gas explosion is one of the major threats to security of coal mine, and the fast and real-time detection of methane, which is proved to be the main component that caused gas explosion, has great significance. The spectral absorption gas sensor can be designed with all-fiber probe. It has high safety and reliability, and can be used in flammable and explosive gas detection. And the spectral absorption gas sensor has high precision and fast responsibility, can achieve the on-line monitoring of target gas without direct contact. It has broad application prospects in methane gas detection and environmental gas monitoring.Methane is selected as target gas to focus on the research of digital optical gas detection schemes. The absorption lines of methane gas are analyzed, and the basic principles of the spectral absorption gas sensor are also described in this thesis. The principles of tunable diode laser absorption spectroscopy (TDLAS) technology are further analyzed, and Fourier series decomposition theory is applied to get a theoretical analysis of absorption signal.Theoretical analysis and simulation both proved the feasibility of gas detection scheme with first harmonic signal.This scheme is chosen as the core to design the digital optical gas detection system architecture.The thesis focuses on the research of digital lock-in algorithm and the ambient temperature calibration scheme. In order to avoid the measurement error of concentration caused by DC drift from analog device, the improved orthogonal digital lock-in algorithm is designed for gas absorption signal.It not only avoid the DC drift from analog devices, but increase the detected harmonic signal amplitude without measuring the phase shift between the frequency modulation and intensity modulation of the laser. Thus, the measurement accuracy of the detected system is improved. The thesis also analyzes the influence of ambient temperature to the gas absorption. The calibration model of ambient temperature is established through theoretical analysis and simulation, and the influence to the detected gas concentration caused by ambient temperature is restricted. The designed gas detection model is tested by MATLAB simulation, the correctness of theoretical analysis and the feasibility of the scheme are verified.