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反射式光纤传感实验的光纤探头T由两根光纤组成,一根用于发射光,一根用于接收反射回来的光,R是反射材料的反射率[13]。由发射光纤发出的光照射到反射材料上,通过检测反射光的强度变化,就能测出反射体的位移。采用的光纤传感器的原理如图1所示。光纤探头A由两根光纤组成,一根用于发射光,一根用于接收反射回的光。
由光纤端出射光场的场强分布[13]函数,即在(r, x)处的光通量密度为:
$$ \begin{split} \varphi(r, x) = & \frac{I_{0}}{\pi \sigma^{2} a_{0}^{2}\left[1+\xi\left(x / a_{0}\right)^{3 / 2}\right]^{2}}\;\cdot \\ & \exp \left\{-\frac{r^{2}}{\sigma^{2} a_{0}^{2}\left[1+\xi\left(x / a_{0}\right)^{3 / 2}\right]^{2}}\right\} \end{split}$$ (1) 式中:I0为由光源耦合入发射光纤中的光强; σ为一表征光纤折射率分布的相关参数,对于阶跃折射率光纤,σ=1;r为偏离光纤轴线的距离;x为光纤端面与反射面的距离;a0为光纤芯半径;ξ为与光源种类、光纤数值孔径及光源与光纤耦合情况有关的综合调制参数。
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系统采用光纤网络完成测试端与监控平台的连接。测试探头部分采用的是反射式光纤传感结构,为无源探测,避免了系统受电磁干扰的问题;探头端采用非接触测量,降低了对待保护物品的影响。系统结构如图2所示。
图中处理系统控制激光器(LD,@650 nm)产生初始信号光,信号光通过传输光纤进入反射式光纤传感探头部分。探头部分在图中用虚线框标注,同时在虚线框下给出了虚线框对应的具体工作环境中传感探头的安装位置。控制平台与采集模块相连接,采集模块可以同时获取多个传感探头位置的位置偏移信息。当用于保护防盗目标的保护外壳发生位移时,则偏移信号使回波光发生变化,再根据分析算法完成对偏移程度的分析,最终给出是否需要报警的判断[14-15]。
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传感模块结构的本质可以理解成一个微位移测量结构,当遇到非法开启保护装置时,保护挡板的位置发生大幅偏移,传感探头部分可以测试得到偏移程度信息,通过偏移量分析外部情况。其传感模块在实验室的等效结构如图3所示。
如果将同种光纤置于发上发射光纤出射光场中作为探测接收器时,所接收到的光强可表示为:
$$ I(r,x) = \iint\limits_s {\frac{{{I_0}}}{{\pi {\omega ^2}(x)}}\exp\left \{ \frac{{{r^2}}}{{{\omega ^2}(x)}}\right \} }{\rm{d}}s $$ (2) 式中:
$ \omega (x) = \sigma {a_0}\left[1+\xi\left(x / a_{0}\right)^{3 / 2}\right]$ ;s为接收光面,即纤芯端面。在纤端出射光场的远场区,为简便计算,可用接收光纤端面中心点处的光强作为整个纤芯面上的平均光强[14]。在这种近似下,得到接收光纤终端所探测到的光强公式为:
$$ {I_A}(x) = \frac{{Rs{I_0}}}{{\pi {\omega ^2}(2x)}}\exp \left \{ - \frac{{{r^2}}}{{{\omega ^2}(2x)}}\right \} $$ (3) 在此基础上,对不同位置到探头处的光强响应曲线进行测试。该测试目的在于通过已知强度的光源照射,获得该系统探头结构关于距离的准确响应值,从而选择最优检测位置。结果如图4所示。
由图4的测试曲线可知,在光纤探头测试端与反射结构距离为1.2 mm时,反射光强能量最大,其余位置光强均不同程度的衰减。通过光功率计的测试值完成对位置的计算,由此确定探头与待测结构的最佳测试位置范围。同时,经过计算可知在0~1.2 mm和1.2~3.0 mm区间其对应的拟合函数有:
$$ \left\{ \begin{gathered} {y_1}= 146.37{x_1}-13.901 \\ {y_2} = -68.658{x_2}+212.67 \\ \end{gathered} \right. $$ (4) 由上式的斜率值可知,上升曲线与下降曲线的斜率具有明显差异。可以通过计算两次测试数据之间的差值完成对位置方向的识别,再通过对测试值的对比可以得到其偏移位置的具体值,从而分析其是否存在非法打开的现象。
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反射式光纤传感实验装置包含五个部分:光源(功率2.5 mW,波长650 nm)、功率计、Y型光纤探头,其中心波长为62.5 μm。反射式光纤传感调整架和反射镜(直径25.4 mm),连接时注意反射光纤头不要触碰反射镜或其他物体,以免损坏。两光纤的头分别接光源和功率计,反射头固定夹持架上。逐渐将Y型光纤出射端和反射镜靠近(光纤输出端不要与反射镜接触),靠近过程中会出现功率最大值,在此状态下,调整调节Y型光纤的姿态(四维调整架的二维俯仰旋钮)和反射镜的反射角度使功率计示数最大,然后移动平移台继续靠近反射镜,直至示数最小,完成测试标定。系统整体如图5所示。
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在标定实验中,采用定量改变测试位置的方式完成对传感单元进行标定,每0.5 mm步进一次,在同一个位置上测试5次数据,并取平均值,并且计算测试值对应的方差,从而分析测试数据的波动情况。利用插值算法对测试数据进行插值分析,得到该传感模块的实际位移应变曲线,如图6所示。该曲线与理想曲线之间的差值就是用于校正测试结果的标定数据表,如表1所示。其中光功率用于计算测试位置,方差用于计算测试位置的波动范围。在标定数据表的基础上,可以实现对测试数据的预处理,从而为干扰对比实验提供技术支撑,为识别是否是非法移动保护系统提供判断依据。插值最小值为0.05 mm,即原始测试数据的1/10,由此获取细致的回波强度曲线。
表 1 反射式光纤传感测试数据
Table 1. Test data of reflective optical fiber sensing
Displacement/mm Power/μW Variance 0 1.00 0.045 0.5 18.06 0.082 1.0 18.76 0.124 1.5 15.41 0.102 2.0 15.12 0.084 2.5 11.59 0.069 3.0 8.47 0.071 3.5 7.06 0.052 4.0 5.41 0.049 4.5 4.29 0.069 5.0 3.12 0.055 由图6可以看出,插值后的标定曲线与反射光强分布曲线相似。为了使其适应更多类型的测试条件,标定实验中去掉了位置定标实验中采用的聚焦透镜,从而使其探头结构非常小巧,由此其回波响应强度约为距离实验中的1/10,但其曲线分布几乎没有改变。当测试环境要求更高信噪比时,可选择再次加装聚焦透镜组。在1.0~2.0 mm范围内有一段较好的平缓反射区。分析认为该部分是测试区域平面度相对较小造成的,由于超过3.0 mm以后的测试曲线的单调性仍然很好,所以该处的曲线波动并不影响测试分析。
Design of reflective optical fiber sensing anti-theft system for sensitive environment
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摘要: 为了解决在敏感环境中位置测试容易受到干扰的问题,提出了一种基于反射式光纤传感防盗系统。该系统采用T型光纤探头作为无源传感探头,反射配件安装于安防外壳表面,通过实时测试位置偏移量提供安防预警。探测模块通过预先测试最佳反射位置的方式,将光纤探头与反射配件固定于安防位置上。在获取激光回波信号的基础上,计算了关于位置的场强分布函数。在标定实验中,测试位置由0递增至5.0 mm,响应光功率最大值为18.76 μW,方差最大值0.124。当超过3.0 mm后的曲线单调性良好,符合系统设计要求。在不同干扰类型对比实验中,分别对比了湿度、温度、振动对系统测试结果的影响程度。实验结果显示,干扰项的光功率波动无论是线性还是非线性的,其波动范围小于4 μW,而真实人为打开条件下,其波动程度大于18 μW。与此同时,系统反演计算的位移偏移量也发生显著改变,相比干扰项的位移偏差均值大8倍以上。系统具有传感端无源、灵敏度高、实时性好等优势,可有效应用于敏感环境中的安防监测。Abstract: In order to solve the problem that the position test is easily interfered in the sensitive environment, an anti-theft system based on reflective optical fiber sensing is proposed. The system uses a T-type optical fiber probe as a passive sensing probe, and the reflective accessories are installed on the surface of the security enclosure to provide security early warning through real-time testing of position offsets. The detection module fixes the optical fiber probe and the reflection accessories in the security position by pre-testing the optimal reflection position. On the basis of acquiring the laser echo signal, the field intensity distribution function with respect to the position is calculated. In the calibration experiment, the test position was increased from 0 to 5.0 mm, the maximum response optical power was 18.76 μW, and the maximum variance was 0.124. When the curve exceeded 3.0 mm, the monotonicity of the curve was good, which meets the system design requirements. In the comparison experiments of different interference types, the influences of humidity, temperature and vibration on the system test results were compared. The experimental results show that whether the optical power fluctuation of the interference term is linear or non-linear, the fluctuation range is less than <4 μW, and the fluctuation degree is greater than 18 μW under the real artificial opening condition. At the same time, the displacement offset calculated by the system inversion is also changed significantly, which is more than 8 times larger than the average displacement deviation of the interference term. The system has the advantages of passive sensing end, high sensitivity and good real-time performance, and can be effectively applied to security monitoring in sensitive environments.
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表 1 反射式光纤传感测试数据
Table 1. Test data of reflective optical fiber sensing
Displacement/mm Power/μW Variance 0 1.00 0.045 0.5 18.06 0.082 1.0 18.76 0.124 1.5 15.41 0.102 2.0 15.12 0.084 2.5 11.59 0.069 3.0 8.47 0.071 3.5 7.06 0.052 4.0 5.41 0.049 4.5 4.29 0.069 5.0 3.12 0.055 -
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