Design And Temperature Compensation Of Gas Concentration Detection System Based On TDLAS Technology

Carbon monoxide and carbon dioxide are important products of fuel combustion,which not only pollute the environment,but also threaten people's health.It is of great significance for the detection of carbon monoxide and carbon dioxide in industrial production,mining,and environmental pollution monitoring.Traditional gas detection technology has low sensitivity,long response time,and is susceptible to interference.It cannot achieve continuous measurement and adapt to complex industrial environments.Gas concentration detection based on tunable diode laser absorption spectroscopy(TDLAS)technology can overcome the shortcomings of the above traditional gas detection,and has attracted more and more attention in the field of gas detection.Aiming at the problem that the current TDLAS gas detection system is mostly built with large-scale separated instruments,a miniaturized TDLAS gas concentration detection system was designed for carbon monoxide and carbon dioxide gas detection.Firstly,the basic principle and detection method of TDLAS gas detection were studied in detail,and two absorption spectra near 1580 nm were selected as the detection objects of this system.The influence of temperature on gas absorption coefficient and the second harmonic was analyzed.When the temperature changed from-20? to 60?,the amplitude of the second harmonic of carbon monoxide and carbon dioxide decrease by 23.5% and 17.7% respectively at the absorption center frequency.Based on wavelength modulation and harmonic detection technology,an overall system design scheme based on STM32+FPGA was proposed.The circuit design of the laser temperature control module and the fuzzy adaptive PID software design were realized.The temperature control deviation is less than ± 0.015 ° C and it has good adaptability to DFB lasers with different structures.The waveform generator circuit and harmonic detection module circuit were designed to realize laser drive current modulation,laser transmission signal acquisition and harmonic extraction.A digital quadrature lock-in amplifier was designed based on FPGA,and the DDS sine / cosine waveform generator,FIR filter,multiplier,vector operation,A/D acquisition,and D/A output functions were simulated and implemented.After completing the above hardware circuit design and programming of STM32 and FPGA,the TDLAS gas concentration detection system platform was set up.Carbon monoxide and carbon dioxide standard gas were used for concentration calibration,the second harmonic amplitude output from the digital quadrature lock-in amplifier was extracted.The gas concentration calibration curve was obtained by the least square method and the results were analyzed.The second harmonics of carbon oxide and carbon dioxide at different ambient temperatures were measured,and the effect of temperature was corrected using empirical formulas,which effectively improves the detection accuracy and achieves the accurate measurement of carbon monoxide and carbon dioxide in a wide temperature range.