图1 Φ-OTDR系统图
收稿日期:2023-12-14,
修回日期:2024-01-17,
纸质出版日期:2024-05-25
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电力管廊是城市里重要的基础设施,对其结构和状态进行监测及评估备受关注。本文针对电力管廊外力破坏监测的问题,提出一种基于衰减补偿的光时域差分曲线方差阈值定位振动事件的方案,并通过实验论证了其定位外破事件的准确性。该方案基于相敏型光时域反射系统架构,根据管廊外破事件引起的光纤振动导致光时域反射曲线在该事件位置处混乱度急剧增加的现象,对不同测量序列得到的光时域反射曲线作差,然后对光纤各位置对应的差值向量数据求方差,设定方差阈值定位外破事件。同时,考虑到光纤衰减导致方差阈值随距离增加而降低的问题,采用衰减补偿算法使光纤上任意散射位置对应相同的脉冲功率水平,从而修正光纤衰减对方差阈值的影响。实验搭建了分布式光纤振动传感系统,采用脉宽为30 ns、峰值功率为30 dBm的光脉冲,在约25 km的光纤范围,获得±3 m的定位精度。电力管廊外力破坏事件,具有低频、大扰动、持续长的特点,所提出系统方案通过曲线平均抑制瑞利散射衰落噪声,结合光纤衰减补偿校正散射信号幅值,从而将振动事件引起的光信号相位变化作为探测曲线波动的主导因素,以确保对外力破坏事件不漏报、不误报。
The power pipe corridor is an important infrastructure in the city, and the monitoring and evaluation of its structure and state have attracted much attention. Aiming at the problem of external failure monitoring of power pipe corridor, this paper proposed a scheme for locating vibration events based on attenuation compensation and variance threshold of optical difference time domain curve, and proved its accuracy by experiments. This scheme was based on the phase-sensitive optical time domain reflectometry system architecture. According to the phenomenon that the optical fiber vibration caused by the broken event outside the pipe corridor leaded to a sharp increase in the confusion of the optical time domain reflectometry curve at the location of the event, the optical time domain reflectometry curve obtained from different measurement sequences was differentiated, and then the difference vector data corresponding to each position of the optical fiber was evaluated and the variance threshold was set to locate the broken event. At the same time, considering the problem that the variance threshold decreased with the increase of distance due to fiber attenuation, the attenuation compensation algorithm was used to make any scattering position on the fiber correspond to the same pulse power level, so as to correct the influence of fiber attenuation on the difference threshold. A distributed fiber-optic vibration sensing system was set up in the experiment. Optical pulses with a pulse width of 30 ns and a peak power of 30 dBm were used to obtain a positioning accuracy of ±3 m in the optical fiber range of about 25 km. The external force damage event of the power pipeline gallery has the characteristics of low frequency, strong disturbance, and long duration. The proposed system scheme suppresses Rayleigh scattering fading noise through trace averaging, and combines fiber optic attenuation compensation to correct the amplitude of the scattering signal. Therefore, the phase change of the optical signal caused by vibration events is the dominant factor in trace fluctuations to ensure that external force damage events are not missed or misreported.
当前,电力管廊发展迅速,已成为城市中重要的电力设施[
相位敏感光时域反射仪(Phase sensitive Optical Time Domain Reflectometer, Φ-OTDR),常用于分布式光纤振动定位[
出于实时监测的需要,数据传输及处理速度应及时且高效[
虽然国内外学者在分布式光纤传感领域取得了许多研究成果,但还是存在许多难题没有解决,比如在长距离监测中提高系统的定位精度、降低系统的误报率和提高系统稳定性等问题[
实验系统如
图1 Φ-OTDR系统图
Fig.1 Schematic diagram of Φ-OTDR system
实验中,激光波长为1 550.12 nm,线宽为1.6 kHz,脉冲频率为3kHz,脉冲宽度为30 ns、峰值功率30 dBm,数据采集卡采样率为100 Msps。电力管廊外破事件,通常由挖掘机、冲击锤、重型机车等引起,一般频率在50 Hz以下,且振动强度大,对于低频、大强度事件的捕捉,监测与定位应以“稳”和“准”为目标,因此,可以适当增加曲线平均次数,降低瑞利衰落噪声的影响,确保无外破事件时时域曲线“平稳”,进而凸显外破振动事件引起的光纤中光信号相位动态变化导致的光时域反射曲线混乱度增加的特性,保证电力管廊外力破坏定位的“准确”。本实验的数据平均次数为100。目前模拟微小振动事件常用的方法是将光纤缠绕在压电陶瓷上加载周期性电压信号来模拟振动事件。然而,在实际情况中光纤的振动情况没有压电陶瓷反映得那么完美。另外,在光纤附近走动[
实验采集128条光时域反射曲线进行对比分析。
图2 施加振动事件的Φ-OTDR曲线
Fig.2 Φ-OTDR curve with vibration event loaded
通过记录光脉冲进入被测光纤以及光纤沿线各位置处散射光返回的时间值,可以定位被测光纤沿线各散射位置。散射位置的距离与时间的对应关系如(1)式所示:
(1) |
其中:L为光纤沿线各位置对应的距离,t为光脉冲发送及光纤沿线某位置散射光返回的时间,
Φ-OTDR系统定位最常用的算法就是逐差定位算法,该方法简单且迅速,具备良好的实时性。其核心思想是将时域光信号强度的差值变化量与一个预设的阈值进行比较。如果变化量超过阈值,其对应的位置就被视为外界振动事件的发生位置。
图3 逐差曲线
Fig.3 Differential curve
从
本文运用数学统计学中的方差分析来对逐差曲线进行处理和分析。方差能够反映出随机变量离散程度,曲线振动处的混乱度大,而曲线非振动区域的混乱度相对于曲线振动处的混乱度几乎不变,如
图4 数据处理流程图
Fig.4 Data process flow chart
首先对系统进行参数设置,然后开始数据采集。采集到的数据转化为矩阵形式,矩阵形式可以更加清晰地表达出光时域反射曲线的条数以及长度距离信息,再通过衰减系数补偿方式使得背向瑞利散射曲线后端的功率补偿。然后,进行差分处理得到一个差分矩阵,再对差分矩阵进行方差处理最后得到一条方差曲线。通过阈值对时域差分曲线的方差曲线进行判断,确定是否有振动事件。若无振动事件则继续采集数据,若存在振动事件则对振动位置进行定位并发出报警信号。
算法的具体过程是先将采集到的原始数据进行时域差分,再将所得的曲线做方差处理进而得到方差曲线。具体步骤如下:
将采集卡采集到的一维数据进行矩阵处理,获得二维矩阵,用矩阵X来表示:
(2) |
X为一个M×N的二维矩阵,其中“M”代表光时域反射曲线条数,“N”代表光纤沿线各位置信息。如
图5 三维时域曲线分布
Fig.5 3-D time domain curve distribution
由于光脉冲功率会随着传输距离的增加而减小,为了在长距离监测中更加精准地判断出振动位置,对X矩阵中的各行(即光时域反射曲线数据)进行衰减系数补偿:
(3) |
其中:
(4) |
这样,(4)式可以表示为,k=N-n行的差分Y矩阵,每一行对应时域逐差曲线数据。
(5) |
接下来,计算Y矩阵中每一列的平均值,得到一个1×N的矩阵M:
(6) |
然后,对Y矩阵中各列对应的数据进行方差处理,得时域方差数据:
(7) |
最后,还可以对D求方根,得到时域差分曲线的标准差数据Z。
(8) |
于是,通过实验数据的统计与分析,选择一个合适的定位阈值能够有效地减少瑞利散射随机性的影响,同时保持对振动事件的敏感性。
实验中,施加振动事件,采集了128条有振动的光时域反射曲线数据。
图6 光时域反射曲线
Fig.6 Optical time domain reflecting curves
振动位置在24 225 m处开始,振动长度大约为20 m,因为实验是放在自然环境下,所以在实验过程中会有一些环境因素引起的振动干扰,但是这与人为敲击的干扰影响不同,不会影响最后的定位结果判断。从
由于光脉冲在传输过程中经历衰减时,信号强度会逐渐减小,导致信噪比降低。衰减系数补偿可以将被测光纤沿线各位置处的散射幅值校正到同一水平,从而使光信号相位成为引起时域探测曲线幅值波动的唯一(主导)变量,最终使得检测和分析信号时更加准确。
图7 衰减系数补偿前后曲线对比图
Fig.7 Attenuation coefficient compensation before and after curve comparison
图8 衰减系数补偿后的差分曲线
Fig.8 Difference curve after attenuation coefficient compensation
图9 差分方差曲线间隔对比图
Fig.9 Difference variance curve interval contrast diagram
因为对光纤进行衰减系数补偿,使得光时域反射曲线后端的功率得到修正,相应地也会使光纤后端的噪声放大。但是,从
图10 定位结果图
Fig.10 Results for vibration event location
在实时性方面与移动平均-移动差分算法进行比较[
分布式光纤传感技术对电力管廊的外力破坏监测相较于其他传统方法优势显而易见,分布式光纤振动检测系统能够覆盖大范围区域的电力管廊,并且可以同时监测多个位置的振动情况,大大简化了监测系统的布置和维护工作。因此,提高光纤振动检测系统的精确度和稳定性是十分重要的。
本文针对逐差定位算法在定位时无法有效地克服随机噪声的干扰导致系统容易出现误报或者错误定位的情况,提出一种基于衰减补偿的光时域差分曲线方差阈值定位算法。根据逐差曲线产生的随机噪声特性,通过运用方差阈值手段可以有效地克服随机噪声的干扰提高系统准确度。同时,考虑到光纤衰减会使散射信号幅值随距离增加而降低在进行方差阈值定位时后端阈值偏小的问题,通过采用衰减系数补偿算法,使光纤上任意散射位置对应相同的脉冲功率,进而使差分曲线在光纤上任意位置对应相同的波动幅度,解决了光纤衰减对方差阈值定位的影响。电力管廊外力破坏事件,具有低频、大扰动、持续长的特点,监测与定位应当力求“稳”和“准”,适当牺牲灵敏性而增加曲线的平均次数,可以获得更好的抗瑞利衰落噪声的效果,再结合衰减补偿算法,可凸显振动事件引起的光纤中散射光信号相位主导特性,再通过时域曲线差分及方差阈值判别算法可保证在外力破坏情况下不误报、不漏报。通过实验结果表明,在约25 km的光纤范围内实现了±3 m的定位精度,与传统的逐差定位算法相比差分方差阈值算法不仅能够准确定位扰动点而且可以降低误报率。这表明该算法在振动事件定位方面具有显著的有效性和可行性。在实时性方面相比于移动平均-移动差分算法提高了28.1%,利用FPGA数据处理模块,可以缩短第一次启动时的响应时间,在连续运行过程中,对后续振动事件的响应时间非常短,可以有效地提高系统的实时性。而且,该系统不仅成本较低且实施简单,具备较高的准确性,可以用于对电力管廊的安全监测。
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