Research On Wavelength Modulated Laser Gas Detector Based On Programmable Logic Device

In recent years,the country has proposed a"dual carbon"strategy.In order to achieve continuous monitoring of global carbon emissions data,it is imperative to build a gas sensor network with wide coverage and long detection time,which puts forward technical requirements for gas sensors such as miniaturization,low power consumption,and networking.Currently,there are three common low power consumption gas concentration detection methods:electrochemical method,semiconductor method,and infrared absorption spectroscopy.However,electrochemical method and semiconductor method generally have problems such as poor selectivity and slow detection speed.However,infrared absorption spectroscopy has high accuracy,good selectivity,fast detection speed,and long service life,which has a high application prospect in the field of low power consumption gas detection.Laser gas detection technology is a kind of infrared absorption spectroscopy technology.By using laser as a detection light source,it has better selectivity and can effectively avoid interference of the target gas by other gases.However,currently common laser gas detection systems are generally large in size and high in power consumption,often used for high-precision gas concentration analysis in laboratories,and are difficult to serve as gas sensing nodes.Field Programmable Gate Array（FPGA）is a hardware reconfigurable logic device that is often used to customize hardware algorithmic logic to achieve acceleration of specific algorithms and improve system solution speed.By transplanting the data demodulation algorithm of Wavelength Modulation Spectroscopy（WMS）technology to FPGA to accelerate the analysis speed of gas sensing data,the detection speed of wavelength modulated laser gas detection systems can be further improved.In addition,through the hardware programming of FPGA,various functions of the system can be integrated on chip,which is of great significance for improving the integration of laser gas detection systems.Based on the development trend of laser gas detection integration and low power consumption,this thesis conducts research on wavelength modulated laser gas detection system integration and low power consumption based on FPGAIn terms of FPGA on-chip logic design,the orthogonal phase locked amplifier module,laser driver module,and temperature control module are designed and implemented through hardware reconstruction of related algorithms.Based on pipeline technology,quasi synchronization of the input and output signals of the quadrature phase locked amplifier module is realized;Using Direct Digital Frequency Synthesis（DDS）technology,the drive signal required by WMS technology with adjustable frequency and controllable phase is generated on chip,achieving synchronization of drive signal generation and harmonic signal extraction;This not only improves the detection speed of the system,but also lays the foundation for intermittent work.In order to achieve real-time monitoring and control of laser temperature,a laser temperature control module based on the Proportional Integral Differential（PID）algorithm was designed.In addition,a laser working current readback module is designed,and a full on-chip implementation scheme for controlling,driving,and data processing of a wavelength modulated laser gas detector is constructed.The working logic and program flow of the system in both continuous and intermittent working modes are studied.In terms of system hardware,based on FPGA programming requirements,specific hardware implementations of phase-locked amplification and drive generation circuits,power amplification and current reading back circuits,temperature reading back and temperature control circuits are proposed.Sorted out the power supply scheme of each circuit and evaluated the performance of each part of the hardware.The low power design logic in the hardware scheme is described,and the specific hardware driving method is given.Finally,based on the wavelength modulated laser gas detection electrical system developed in this paper,acetylene（C₂H₂）gas detection experiments were carried out to evaluate the gas detection performance of the system in both continuous and intermittent operation modes.In continuous operation mode,the system has excellent linearity and a response time of 7.6～5.4 s.Subsequently,long-term measurements were conducted in a pure nitrogen（N₂）environment,and Allan analysis of variance was performed on the experimental results.When the integration time was 0.1 s,the lower detection limit of the system in continuous operation mode was 26.1 ppm（part per millionth）.Under intermittent operation mode,the sensor’s ability to distinguish C₂H₂gas with different concentrations is similar to that under continuous operation mode.The long-term N₂ measurement results indicate that the lower detection limit of the system under intermittent operation mode is 16.3 ppm.Subsequently,by analyzing the operating currents in both modes,when intermittent operation is performed in a 1:9operating time and intermittent time configuration,the overall power consumption of the system is reduced by 61.27%compared to the continuous operating mode.The above results indicate that the scheme of reducing system power consumption through intermittent operation is feasible,and this operation does not significantly reduce the system gas detection performance.The innovation of this paper lies in:1.Based on FPGA,logic modules such as laser temperature control,laser temperature readout,drive signal generation,drive current readout,and gas absorption signal processing are constructed.On-chip integration of signal generation,acquisition,processing,and control functions for complex laser gas sensing systems is realized,and quasi-synchronous output of system scanning signals and harmonic amplitude is obtained.An intermittent working mode to reduce sensor power consumption is proposed.2.A wavelength modulated laser gas detection system for continuous and intermittent operation is designed from two aspects of logic code and hardware circuit.Comparative experiments for gas detection in both operating modes are conducted,verifying the feasibility of intermittent operation to reduce the power consumption of the wavelength modulated laser gas detector.