| Methane gas detection is an important research topic in field of industrial production, environmental monitoring and scientific research. Methane is a kind of flammable and explosive gas. The rapid and real-time monitoring of methane has great significance in coal mine safety and environmental protection. With the development of optical fiber communication technology, the optical fiber gas sensor has achieved more and more attention due to the incomparable advantage of traditional sensors.Tunable diode laser absorption spectroscopy (TDLAS) harmonics detection technology is a new high sensitivity detection scheme developed from the combination of laser and modulation technology. It has been widely applied in gas detection. Compared with ordinary detection method, TDLAS has the characteristics of high selectivity, and sensitivity, rapid response, anti-noise, non-contact measurement and don't need the pretreatment of gas.In this paper, TDLAS technology is applied to detect the near-infrared absorption lines of methane gas. The combination of triangular wave and sine wave modulation is used to modulate the vertical-cavity surface-emitting lasers. The mathematical model first harmonic and second harmonic from absorption signal is analyzed, two kinds of solution schemes are proposed.The optical path and circuit is analyzed in detail, the signal generator circuit, drive circuit of laser, 5×10cm optical path absorption air chamber, weak signal detection processing circuit and the A/D and D/A transform and processing circuit are designed. The balanced modulator/demodulator AD630 is chosen as a lock-in amplifier to extract the amplitude of first harmonic and second harmonic. The C8051F020 MCU is chosen as the core of data processing and concentration display. Methane gas detection with concentration varied from 0 to 5000ppm is realized. The system calibration and performance testing is described at the end of paper.In the detection system, the first harmonic detection scheme is superior to the second harmonic detection scheme, as a result, the first harmonic detection scheme is chosen as the detection scheme of the system. The factors influenced gas detection is analyzed and the improvement method is proposed.The test results show that the amplitude of first harmonic has good linearity with gas concentration, and the feasibility of designed scheme is proved. The gas detection system performs well, and the system structure is simplified. The influence of laser fluctuations is eliminated by ratio of harmonic signal. The resolution and accuracy of the system can achieve 20ppm, 100ppm. |
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