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许多气体分子都具有吸收光谱中某一特定频率的红外辐射的特性。其原理为当红外辐射入射在气体分子上时,当红外线的波长与分子的自然频率或共振相匹配时,会引起分子振动能级间跃迁[7]。非分光红外检测气体是根据所检测气体的原子振动频率及其跃迁能量的唯一性,即当一束具有连续波长的红外光透过待测气体时,气体会吸收相应波段的红外光,从而衰减了此波段的红外光能量,根据进出气室的红外光强不同进一步推出环境中气体分子的浓度。该吸收关系符合朗伯-比尔定律[1, 8-9],如公式(1)所示:
$$ I = I_0\times {\text{exp}}( - \varepsilon CL) $$ (1) 式中:
$ C $ 为气室内待测气体浓度;$ \varepsilon $ 为气体的吸收系数,根据波长不同而改变;$ L $ 为红外光透过待测介质的距离;$ I_0 $ 和$ I $ 分别为吸收目标气体前和后的红外光强。然而该定律只针对于单色光成立。从理论上来讲,所测CO2的浓度与红外光的衰减程度遵循朗伯比尔定律,但在实际的测试中,如果直接使用该定律进行计算,由于环境温湿度对其吸收系数影响较大,所得出的CO2浓度将不准确[10]。因此,文中采取用标准CO2气体标定的方法,建立气体浓度值与电压峰峰值的关系,通过函数拟合得到气体浓度与电压值的函数。鉴于任何函数都可以通过泰勒展开为高阶多项式,在数据处理时,应该优先选择多项式拟合。 -
搭建红外CO2传感器测试平台对所设计传感器系统进行测试,经过对多种情况下的多组测量数据的分析得出红外CO2传感器的精度与误差,并优化拟合系数,从而提高传感器系统的测量精度[13-15]。
搭建测试平台,将传感器放置到密闭气室内,标定实验在标准大气压和25 ℃环境下进行。为了达到测试的准确性,第一,保证密闭气室的密闭性良好;第二,确保测试环境稳定,浓度标定实验的环境稳定性是整组实验数据拟合的根本所在;第三,当气体浓度达到设定值时,等待1 min后再测量通道的峰峰值,并记录它们对应的浓度值。
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首先,在测试平台的环境下检测传感器性能的稳定性好坏。在密闭气室内充入浓度为10000 ppm的标准CO2气体,传感器接入上位机,持续工作12 h,每隔30 min保存一次数据,观测传感器采集峰峰值的波动情况,如图9所示。
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在实验开始之前,首先进行零点标定,利用真空泵将传感器内的气体抽出,密闭气室的注入气体通道接入纯氮气进行零点标定。然后将标气瓶中的浓度98%的标准气体经过气体分析仪器配置成标准的不同浓度的CO2气体,并依序分次注入密闭气室中。之后利用CO2传感器采集的峰峰值与注入的CO2浓度值之间的对应关系图,如图10所示,通过数据拟合得到25 ℃浓度拟合方程,如公式(2)所示。
方程为:
$$ {C}\left({{V}}_{{T}25}\right)=0.0072\times{{V}}_{{T}25}^{2}-105.2\times {{V}}_{{T}25}+384\;500 $$ (2) -
实验温湿度箱中的环境温度设置为可调温度分别为0、5、10、15、20、25、30 ℃,将标气瓶中的纯度0%、1%、2%、4%、5%的标准CO2气体注入密闭气室中。由于温湿度箱内空气流通较大,导致箱内不同位置的温度并不完全相同,所以温度值以温湿度传感器的输出值为准。采集温度传感器输出的温度值与CO2传感器的峰峰值,从而得到不同温度下的浓度值与峰峰值之间的关系,如图11所示。
图 11 不同温度下,浓度值与峰峰值的关系图
Figure 11. Relationship between the concentration value and the peak-to-peak value at different temperatures
由上图可知,温度主要影响CO2的吸收率,进而影响峰峰值大小,当温度上升时,峰峰值降低,未补偿的情况下测量浓度值偏大。以25 ℃为标准,找寻同一浓度下的峰峰值差值与温度差值的关系,如图12所示。
可以注意到,各个浓度下的峰峰值差值和温度差值近似为一条曲线,符合温度对CO2吸收系数的影响关系与探测器对红外辐射的吸收率的影响关系[6, 9, 13]。故将图10所示曲线拟合为一条曲线,其方程(3)为:
$$ {V}_{T}-{V}_{T25}=-0.100\;9 \times {\Delta T}^{2}-10.2\times \Delta T+0.752\;1 $$ (3) 将方程(3)代入方程(2)中,可以得到带有温度补偿功能的浓度计算方程。最终,将带有温度补偿功能的方程编写入STM32的程序中,再次测量,测量结果如表1所示。
表 1 重复测试结果
Table 1. Duplicated testing results
Sample gas concentration/ppm Detection of concentration/ppm Absolute error/ppm Relative error 1800 1773 −27 1.5% 12000 11873 −127 1.05% 24000 24269 269 1.12% 30000 30725 725 2.4% 35000 36326 1326 3.7%
Design of non-dispersive infrared CO2 sensor with temperature compensation
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摘要: 近年来,温室效应愈发明显,环境二氧化碳(CO2)浓度增加带来的副作用严重影响了人们的生产生活。目前,商业化的CO2传感器体积较大,便携化、高精度、模块化等方面不尽人意,设计了一种基于热释电的非分光红外法(NDIR) CO2浓度测量系统,主要分为气室结构设计、信号电路设计、软件控制设计与数据处理四个部分。气室结构设计方面采用单通道结构设计,并对气室进行了光学仿真,最终确定了气室的尺寸,有效提高了系统的测量精度。信号电路设计方面设计了一种基于差分方式的小信号放大电路,将有效信号从噪声中提取并放大,提高分辨率。软件控制设计方面应用数字滤波算法,滤除干扰与杂波,提取数据中的有效值,提高信噪比。数据处理方面,搭建CO2气体测试平台,通过温湿度与峰峰值补偿公式补偿温度对峰峰值产生的影响,再使用25 ℃下曲线拟合法计算气体浓度值,最终通过串口输出。经过测试,该系统测量范围为5%,相对误差值在1500 ppm以内,能够满足防火报警、矿下监测等场合的安全测试需求。
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关键词:
- 非分光红外(NDIR) /
- 气室光学仿真 /
- 信号处理电路 /
- 温度补偿浓度算法
Abstract: In recent years, the greenhouse effect has become more and more obvious. The side effects brought by increasing of CO2 concentration have seriously affected people's production and life, and are even closely related to everyone's healthy. In defect of the large size, portability, high precision, and modularity of CO2 commercial sensors currently, a pyroelectric-based non-dispersive infrared (NDIR) method measuring CO2 concentration system was designed. The system design was mainly divided into four parts: air chamber structure design, signal circuit design, software control design and data processing. The air chamber structure design adopted a single-channel structure design, which was optically simulated. Finally, the size of the air chamber was determined, which effectively improved the measurement accuracy of the system. In terms of signal circuit design, a small signal amplifying circuit based on the differential method was designed to extract and amplify the effective signal from the noise to increase resolution. The software control design employed digital filtering algorithm to filter out interference and clutter, extract the effective value in the data, and improve the signal-to-noise ratio. In view of data processing, a gas test platform was built, and the temperature and humidity and peak-to-peak compensation formulas were used to compensate for the influence of temperature on the peak-to-peak value. Then, the gas concentration value was calculated using the curve fitting method at 25 ℃, and finally output through the serial port. After testing, the measurement range of the system is 5%, and the relative error is within 1500 ppm, which can meet the safety requirements of fire alarm, underground and other occasions monitoring. -
表 1 重复测试结果
Table 1. Duplicated testing results
Sample gas concentration/ppm Detection of concentration/ppm Absolute error/ppm Relative error 1800 1773 −27 1.5% 12000 11873 −127 1.05% 24000 24269 269 1.12% 30000 30725 725 2.4% 35000 36326 1326 3.7% -
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