【作者】 李彬；

【导师】 王一丁；

【作者基本信息】 吉林大学 ， 电路与系统， 2016， 博士

【摘要】 近年来,我国安全生产问题和环境问题十分严峻。一方面,煤矿瓦斯爆炸、危险气体泄漏等安全事故频发,给国家造成了重大的经济损失和恶劣的社会影响。另一方面,大气中温室气体浓度的不断升高也带来了一系列的环境问题。因此,先进的气体传感器研发工作十分重要。基于红外吸收光谱原理进行气体检测是目前国内外较为先进的方法。与传统的化学类传感器相比,其具有检测灵敏度高、响应速度快、选择性好、检测范围大、稳定性好等优点。红外气体传感器还具有非接触性测量和寿命长的特点,非常适用于煤矿、工业控制及大气监测等领域。可调谐半导体激光吸收光谱技术是一种高精度红外气体检测的方法。该技术利用波长可以调谐的二极管激光器作为光源,通过改变激光器的温度和电流使得激光器中心波长反复扫过被测气体的吸收峰。由此可获得高分辨率的气体吸收信号,以达到对气体定性、定量检测的目的。与可调谐半导体激光吸收光谱技术相结合,波长调制光谱技术通过对激光器进行高频调制,并提取气体吸收信号的谐波波形,极大地降低了系统中的固有噪声,进一步提高了检测灵敏度。本文采用可调谐半导体激光吸收光谱技术与波长调制光谱技术相结合的方法,设计并制作了以近红外分布反馈式激光器为光源的气体检测系统。首先,以红外吸收光谱理论为基础,针对本论文所设计的检测系统进行了理论分析。通过对系统中检测流程和数据处理的逐步推导,将激光器驱动与调制、光电转换、谐波信号、气体浓度等相关参数进行关联运算,验证了二次谐波信号与气体浓度的相关性。并使用MATLAB软件进行了系统仿真,为气体检测实验提供参考依据。其次,根据系统中使用的激光器的具体驱动特性,设计并制作了激光器温度控制电路和电流驱动调制电路,使其输出光谱满足波长调制的要求,这是本文的重点内容。基于模拟PID算法,对温度补偿网络电路进行了元件配置和参数优化。激光器在温度控制电路的恒温控制下,输出光谱稳定,长时间工作时中心波长不漂移。在室温环境下激光器长时间工作的温度波动在±0.02℃范围以内。研制了激光器电流驱动调制电路,通过低频锯齿波的电流驱动,可以使激光器波长周期性变化而反复扫过气体吸收峰。同时,叠加高频正弦波对激光器进行调制,用于谐波提取。另外,为提高激光器温度控制的稳定性以及降低系统噪声,研制了低纹波线性稳压电源作为系统电源,其纹波干扰远小于常规的开关电源。再次,针对光源系统输出的激光信号,研制了用于检测和处理气体吸收信号的检测系统。检测系统分为差分光路系统、透射型光纤气室、光电转换电路和数字式正交锁相放大器几部分,是本文的又一重点内容。差分光路对单光源进行分光处理,两路信号的差分处理可以有效地减小光源噪声的影响。以光纤准直器为核心光学器件,设计了结构稳定、使用光纤进行通信的透射型气室。使用高性能的In Ga As光电二极管制作了光电转换电路,将气体吸收信号与参考信号进行光电转换,通过差分电路得到的差分信号包含了气体浓度信息。设计并制作了数字正交锁相放大器,将用于谐波提取的相敏检波过程通过软件程序完成。在对谐波提取算法进行仿真分析的基础上,通过气体实验成功提取了谐波信号,并对谐波提取的稳定性进行了测试,从实验角度再次验证了谐波信号与气体浓度的相关性。最后,通过甲烷和一氧化碳的气体检测实验,对检测系统进行了性能测试实验。使用动态配气方式,针对系统检测下限、重复性、稳定性、响应时间和示值误差等性能指标进行了相关实验。通过阿伦方差进行数据分析,系统的甲烷检测下限约为29 ppm,一氧化碳检测下限约为148 ppm。实验结果表明,检测系统具有良好的检测灵敏度和稳定性,同时该检测系统还具有集成度高、成本低等优点,通过更换不同波长的激光器,还可以对其它气体进行检测。更多还原

【Abstract】 In recent years, there are a series of domestic critical issues that cannot be neglected such as industrial safety and environmental pollution. On one hand, accidents including gas explosion in coal mines and hazard gases leakage has been frequently occurred. This causes tremendous economic losses and serious society impacts. On the other hand, the concentration of greenhouse gases has been increased every year and brings many environmental problems. Therefore, the research work of advanced gas detection sensors is very important.Infrared absorption spectroscopy technique is an advanced way for detecting gases in domestic and overseas. Contrast with traditional chemical gas sensors, it has many advantages including high sensitivity, fast response time, good selectivity, wide detection range and good stability. Infrared gas sensors also have the non-intrusive and long service life feature. In this way, infrared gas sensors are very suitable for application in coal mines, industrial control process and atmosphere monitoring fields.Tunable Diode Laser Absorption Spectroscopy(TDLAS) is a widely applied technique in the infrared gas detection field. It utilizes diode lasers, whose central wavelength can be tuned by modulation of laser temperature and injection current, to be light sources. The central wavelength is turned frequently to scan the target gas’ s absorption line to obtain the high-resolution spectroscopy for detecting the gas’ s type and concentration. Along with TDLAS, Wavelength Modulation Spectroscopy(WMS) modulates lasers with high-frequency signals and extracts harmonics signals to identify gas concentration. This method can effectively depress the influence of system noise and further increase the detection sensitivity. In this paper, based on TDLAS and WMS techniques, a near-infrared gas detection system has been developed using distributed-feedback lasers as light sources.Firstly, based on the infrared absorption spectroscopy theory, theory applied in this detection system is discussed. The data processing methods in the detection processes are presented step by step. The relative parameters of laser driving & modulation, optical to electrical conversion, harmonic extraction and gas concentrations are connected with formulas in order to deduce the relation of second order harmonic signal and gas concentration. Meanwhile, system simulation is implemented by using MATLAB for proving detection theory and guiding the experiments.Secondly, laser temperature control circuits and laser current driving circuits are self-developed for the lasers applied in the system. Under the control of those circuits, the output wavelength can be modulated to meet the demand of the WMS system. The above content is one of the core works in this paper. Based on analog PID algorithm, the parameters in the temperature compensation network are modulated and determined. The central wavelength is highly stable without shifts during a period of long time under the control of the temperature circuit. In the room temperature condition, the temperature fluctuation of the laser is within the range of ±0.02 ℃. Meanwhile, the laser current driving & modulation circuit is developed. The saw wave driving current varies the central wavelength periodically to scan across the absorption line. In addition, a high-frequency sine wave signal is superposed on the laser’s output for the extraction of harmonic signals. Moreover, a low-ripple linear power supply is self-developed to guarantee the stability of laser temperature and to decrease the system noise. The developed power module’s ripple noise is much less than normal switching power supplies.Thirdly, the detection system is developed on the basis of above work. The detection system is another core work in this paper. It consists of a few important parts including the differential optical paths system, fiber-coupled gas cell, optical to electrical conversion module and digital orthogonal lock-in amplifier(DLIA). The differential optical paths system divides the laser’s output into two power-equaled beams. The differential signal of the two beams can effectively avoid the light source’s noise. A fiber-coupled gas cell is developed by using fiber collimators that are the core optical part of the gas cell. The gas cell has advantages of stable structure, good sealing, cost-effective. The optical-to-electrical module is developed by using two high performance In Ga As photodiodes. The gas absorption signal and reference signal are converted into electrical signals via this module and the differential signal contents the information of gas concentration. A digital orthogonal lock-in amplifier is developed to extract harmonic signals. The phase sensitive detection is completed in the software programs rather than hardware circuits. Based on the result of simulation, gas detection experiments are carried out to test the performance of the DLIA. Experiment results show that the harmonics signals can be extracted successfully and stability is good. The experiment also proves the relation of harmonic signals and gas concentration.Finally, methane and carbon monoxide detection experiments are carried out to evaluate the performance of the whole detection system. By using dynamic gas distributing method, numerous characters of the gas detection system including detection limit, repeatability, stability, response time, indication errors and drifting are tested. The methane and carbon monoxide detection limit of the system are determined as 29 ppm and 148 ppm respectively by using Allen deviation. The experiment result indicates that the detection system has good sensitivity and stability. Besides, the system also have advantages such as high-integration and cost-effective. Other gas can be detected by replacing laser diodes with other central wavelength in the system. This gas detection system has the reference value for theory and application in the near-infrared gas detection research field.更多还原