Research On Carbon Monoxide Gas Detection Technology Based On TDLAS System

Carbon monoxide gas is colorless and odorless,and is highly toxic and flammable,so it can be poisoned without any sense.In the coal mining industry,due to operational errors,a large amount of carbon monoxide gas is leaked.When exposed to an open flame,it will spontaneously ignite and cause an explosion.If the concentration is too large,it will cause casualties.In order to provide a reliable guarantee for safe production,it is necessary to design a carbon monoxide leak alarm system.Tunable Laser Diode Absorption Spectroscopy(TDLAS)is a widely used gas detection technology.This project designed a carbon monoxide on-line detection alarm drive control system for DFB(distributed feedback)laser with a wavelength of 1580 nm for carbon monoxide gas detection.The gas detection theory based on TDLAS technology is studied in detail,including: gas selective absorption theory,wavelength modulation theory,harmonic detection principle,phase sensitive detection principle.The design of this subject realizes high-precision laser drive circuit and temperature control circuit.According to the negative feedback principle,a high current constant current source driving circuit is designed to drive the laser to ensure the normal operation of the laser.PID control principle,the operating temperature of the DFB laser is adjusted,and the thermoelectric cooler drive chip LTC1923 drives the thermoelectric cooler(TEC).Ensure the operating temperature of the DFB laser;FPGA-based digital modulation and demodulation circuit,using the FPGA output high-frequency sinusoidal modulation signal and low-frequency triangular wave drive signal to be conditioned,control the voltage-controlled constant current source to drive the laser to realize the laser source Modulation.A gas detection and demodulation system based on digital phase-locked loop is designed and implemented.Aiming at the shortcomings of the phase-locked amplifier circuit used in the traditional TDLAS gas detection,it is difficult to adjust the phase and the stability is poor.A digital quadrature lock-in amplifier for gas detection is designed.The digital quadrature lock-in amplifying circuit extracts the first harmonic and the second harmonic signal of the extracted gas by filtering and calculating.The gas concentration measurement is realized by the relationship between the ratio of the second harmonic amplitude and the first harmonic amplitude and the concentration of the gas to be measured.On this basis,a carbon monoxide gas detection platform based on TDLAS was constructed.Carbon monoxide was used as an example to configure different concentrations of gas,so that the tunable diode laser was scanned near the gas absorption peak.The photoelectric signal was converted by the photodetector,and the gas was absorbed once.The harmonic and second harmonic signals are extracted by a digital quadrature lock-in amplifier signal processing module.And after passing the adjustment circuit,the FPGA controls the 12-bit AD conversion to acquire data.The STM32 MCU communicates with the FPGA to transmit data,and the data is transmitted to the computer through the Ethernet port.The extracted signal is fitted with the least squares data using MATLAB to calculate the gas concentration.