| The production environment of coal mines is complex,and high-concentration gas(mainly methane)will seriously threaten the lives and property of workers in the mine.Therefore,it is necessary to monitor the concentration of methane gas at all times.Among a variety of gas detection methods,Tunable Diode Laser Absorption Spectroscopy(TDLAS)technology has been widely used because of its high detection accuracy,fast response speed and non-contact measurement.The core component of this technology is the lock-in amplifier,which is mainly used to extract the harmonic signal from the absorption signal of the gas to be measured,so as to achieve the purpose of detecting the gas concentration.Although commercial lock-in amplifiers have many styles and mature development,they are expensive and bulky,which is not conducive to system integration.Therefore,this paper has studied and designed a digital quadrature lock-in amplifier based on FPGA,which is dedicated to methane gas detection system.The designed lock-in amplifier is not only low in cost,but also simple in hardware circuit structure and small in size,which is convenient for the system integrated.This paper mainly elaborates from the following four aspects:1.The related theories of phase-locked amplification technology and TDLAS technology are summarized.Including the basic principles of related detection,the basic theory of lock-in amplifier detection of weak signals,the principle of infrared absorption spectroscopy,Lambert Beer's law,the principle of direct absorption spectroscopy and the principle of wavelength modulation spectroscopy.2.The programming process and simulation results of the quadrature lock-in amplifier module,laser drive module and data acquisition module are introduced in detail.The model of the gas absorption signal to be measured is established through MATLAB software and FPGA development platform,and the function test of the lock-in amplifier is carried out by using the gas absorption signal model to be measured.3.The hardware circuits of FPGA main control board,laser drive module and data acquisition module are designed.FPGA main control board mainly includes power supply circuit,JTAG download circuit and USB to serial port circuit,etc.The laser drive module mainly includes a low-frequency scanning signal circuit,a high-frequency modulation signal circuit and a waveform superimposing circuit.The data acquisition module mainly includes power supply circuit,A/D analog-to-digital conversion circuit and D/A digital-to-analog conversion circuit.The hardware structure block diagram and function debugging process of the three modules are introduced in detail.The three modules were integrated,and the functions of the overall system were debugged and verified.4.Related gas experiments were carried out to verify the function of the quadrature lock-in amplifier.Using the dynamic gas distribution system,the second harmonic signal and peak-to-peak information of gas samples with different concentrations are extracted.The extraction experiment shows that the peak-to-peak value of the second harmonic signal has a linear relationship with the concentration of the gas to be measured.Carried out the methane gas detection experiment,completed the concentration calibration and stability test of the system,and calculated the Allan variance.When the methane concentration is 0×10-6,the concentration fluctuation range is-0.16×10-6?+0.2×10-6.According to the Allan variance curve,when the integration time is 1 s,the 1?detection limit of the system is 64.5×10-9;when the integration time is increased to 17 s,the minimum 1?detection limit of the system is17.6×10-9.When the system has the best stability and the highest detection accuracy.Innovation points of this paper:1.For methane gas detection system,this paper designs a digital quadrature lock-in amplifier based on FPGA.The hardware circuit structure is simple,and the function expansion and hardware upgrade can be carried out flexibly without changing the hardware circuit structure.2.The design is more modular,and the hardware structure is smaller,which facilitates the integration of the gas detection system. |
PDF Full Text Request