硬件可重构近红外波长调制甲烷传感器及应用(特邀)

Hardware Reconfigurable Near Infrared Wavelength Modulated Methane Sensor and Its Application(Invited)

为了加快气体吸收光谱分析算法的求解速度、重构算法的硬件逻辑,采用现场可编程门阵列(FPGA)器件,结合波长调制光谱技术,研制了一种硬件可重构波长调制甲烷传感器。根据应用功能需求,可在硬件层面重构逻辑电路,从而更新系统工作模式和探测参数。采用流水线技术,对光谱分析算法进行了硬件加速,光谱分析部分的输入与输出之间的时间延迟仅为4.05 ms。实验分析了该传感器在连续、间歇两种工作状态下的波形与电流,证明了该系统工作模式的可重构特性,也证实了采用间歇式工作降低激光气体传感器功耗的可行性。采用正弦信号对FPGA算法部分的功能进行了测试,结果表明,测得的信号幅值与输入信号幅值的线性度达到了99.99%。采用中心波长为2334 nm的分布式反馈激光器作为光源、光程为25 m的赫里奥特气室作为气体吸收池,开展了甲烷检测实验。传感器的线性度为99.97%,响应时间约为4.9 s。艾伦方差结果表明,当积分时间为0.5 s时,传感器的检测下限为7.8×10^(-6)。开展的甲烷泄露实验证实了该传感器的现场应用能力。设计的FPGA算法,通过硬件编程,可实现传感器工作模式与参数的灵活重构,在激光气体传感领域具有较好的片上集成应用前景。

At present,the demand for gas sensors is moving towards miniaturization,low power consumption and networking.Among them,laser absorption spectroscopy has wide application prospects in related fields with advantages of fast detection speed,high accuracy,good stability,and long service life.However,the commonly used laser gas sensor system is generally large in size,high in power consumption and high in cost,which is difficult to be widely used as a gas sensor node.Therefore,it is urgent to develop a new spectroscopy gas sensing system with small volume,low power consumption and low cost to promote the application of laser gas sensing technology in“carbon neutralization”.Based on the above social needs,we developed a hardware reconfigurable Wavelength Modulation Spectroscopy(WMS)methane sensor.Based on the characteristics of Field Programmable Gate Array(FPGA)hardware algorithm acceleration,the hardware-based wavelength modulation gas absorption spectroscopy analysis algorithm,realizes quasi-synchronous output of scan signal and harmonic signal amplitude as well as on-chip integration of WMS.The on-chip logic circuit is mainly divided into WMS analysis module and WMS drive signal generation module.Based on Direct Digital Synthesis(DDS)technology,the WMS drive signal generation module is used to generate sine wave and triangular wave signals required by WMS,and both signal frequency and phase can be tuned in a wide range,which can flexibly control the sensor operation according to an enabling signal.The core of WMS analysis module is an orthogonal phase-locked amplifier.This work focuses on enhancing the hardware reconfigurability to enhance the flexibility of the sensor system.Only when the enabling signal is in high level,the WMS analysis module and the drive signal generation module start to work,which lays the foundation for intermittent work,scanning signal alignment,and gas absorption fixed point analysis.The feasibility of the intermittent work is verified by the actual measurement.The system has a unified sampling clock,and an on-chip clock generation unit based on external reference signals is constructed.On the basis of avoiding the frequency offset problem,the flexible reconstruction of the system sampling clock is realized.A standard orthogonal signal generation module based on DDS technology is established to realize the generation of standard orthogonal signals at any frequency.Combined with the characteristics of finite impulse response filter that the frequency response can be reconstructed by modifying parameters,the phase-locked analysis at any target frequency is realized.The on-chip system logic part is developed in hardware logic language as a whole,and there is no problem of“program runaway”.After the design of the system logic part,the software simulation is carried out to verify the accuracy of the system logic function.In order to verify the linearity of harmonic signal extraction of the designed hardware reconfigurable wavelength modulation methane sensor,a sinewave verification experiment is carried out.By analyzing the amplitude extraction results of different standard sinewave signals,a step output of the system under different inputs is observed,and a linear response with a goodness of fit of 0.99999 is obtained.The measured sinewave amplitude fluctuation range is about±0.5 mV.In order to verify the actual sensing performance of the methane sensor,methane experiments are carried out using a Herriot gas cell with an optical path length of 25 m and a laser with a central wavelength of 2334 nm for targeting the absorption line of methane at 4284.5 cm^(-1).It can be seen from the experimental results that the gas detection linearity of the methane sensor is 99.97%.The stability analysis shows that the sensor resolution is better than 25 parts-per-million in volume(ppmv),and the response time is~4.9 s.In addition,the Allan variance results show that when the integration time is 0.5 s,the detection limit of the sensor is 7.8 ppmv.Methane leakage test is carried out.Through the analysis of the measured CH4 concentration levels when CH4 leakage occurs,the detection ability of the sensor for methane leakage is proved.Compared with the existing methane sensor based on software architecture,the extremely low data delay lays a foundation for intermittent work,and provides the possibility to realize the application of node-type low-power gas detection.At the same time,the sensor has high accuracy,fast response speed,and can be used flexibly and conveniently based on actual needs.The developed sensor shows a good application prospect with high electronic integration based on FPGA.