L-M非线性拟合的TDLAS氧气测量研究

The Research of Oxygen Measurement by TDLAS Based on Levenberg-Marquardt Nonlinear Fitting

氧气浓度是工业生产过程中重要监测参数,采用可调谐二极管激光吸收光谱法(tunable diode laser absorption spectroscopy,TDLAS),结合波长调制技术,可以实现对现场氧气浓度的高精度在线监测,利用氧气位于760nm处的特征吸收峰进行了氧气浓度的测量。由于激光具有很强的相干性,所以TDLAS技术的检测灵敏度受到光学干涉噪声的严重制约,特别在低浓度时,光学干涉引起的基线起伏使得提取吸收峰波形信号时出现较大误差,影响了TDLAS分析仪的监测灵敏度。针对这一情况,采用了Levenberg-Marquardt非线性拟合算法,并且利用了吸收谱线线型——洛伦兹线型的导数形式对波长调制后获得的二次谐波波形信号进行拟合,提取波形信息。另一方面Levenberg-Marquardt非线性拟合方法需要有大量的计算,为了使研制的TDLAS分析仪能够实现现场的实时监测,采用了支持浮点运算的DSP的C28系列芯片进行数据处理,实现仪器在现场实时监测的功能。实验结果表明,该算法能够有效提取二次谐波信号的吸收峰特征值、克服背景噪声影响,由算法反演得到的氧气浓度与实际浓度的线性比值为1.01,浓度测量的线性误差为1.18%。

Oxygen concentration is an important monitoring parameter in industrial process.Wavelength modulation spectroscopy of tunable diode laser absorption spectroscopy（TDLAS）was used to measure concentration of oxygen gas in industrial process by online monitoring.In this paper,we use the characteristic absorption peak of Oxygen at 760 nm to measure the oxygen concentration.Because of the strong coherence of laser,the detection sensitivity of TDLAS is severely restricted by optical interference noise.Especially at low concentrations,there is larger error by extraction signal in the absorption peak waveform because of the background fluctuation caused by optical interference.In response to this situation,Levenberg-Marquardt nonlinear fitting algorithm was proposed,and the use of the absorption line-derivative form of Lorenz line to fit the second harmonic signal and to extract the peak amplitude.On the other hand,Levenberg-Marquardt nonlinear fitting method needs a large amount of calculation.In order to develop the TDLAS analyzer can achieve real-time monitoring of the site,we use the C28 series of DSQ for data processing which support floating-point arithmetic,and the instrument achieve real-time monitoring capabilities in industrial process.Experimental results show that the algorithm can effectively extract the absorption peak characteristic value of the 2nd harmonic signal and overcome the background noise,The ratio of calculated by algorithm to actual oxygen concentration is nearly1.01,the linear error of the concentration measurement is 1.18%.