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如图 2所示,为了显示基于FBG的光纤传感网络的分布结构,将巷道划分了3个截面,其中在巷道拱起段中45°和60°的固定锚位置上安装FBG传感器,因为该位置可以很好地监测围岩拱顶压力分布。利用分布在固定锚上的FBG可以实时采集拱段的应力场分布,从而为坍塌风险评估提供预警数据。
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基于FBG的应力监测需要解决两个问题: 一是温度变化对应力监测的交叉敏感;二是开采层大型机械振动引入的噪声对FBG应力监测的影响。为此设计了新型的FBG封装结构,采用两个不同FBG完成应力与温度数据的获取,同时将两组信号做互相关运算, 实现对振动噪声的差分消除,其结构如图 3所示。
FBG1与拱段固定锚紧接,从而可以通过FBG1获取测试点应力值,实现应力实时监测。FBG2与FBG1均在保护外壳中,认为其温度一致,但由于FBG2采用单端固定的方式,所以当围岩应力发生改变时不影响FBG2,由此可知,FBG2可用于温度标定。
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FBG的波长偏移量Δλ解算函数为:
$ \frac{{\Delta \lambda }}{{{\lambda _0}}} = \left( {{\alpha _f} + \xi } \right) \cdot \Delta T + \left( {1 - {P_\varepsilon }} \right) \cdot \varepsilon $
(1) 式中,λ0为初始波长(FBG1的回波); ε为FBG的应变量产生的应变,其与应力F(F=kε, k为应变系数)成正比;ΔT为温度变化量;Pε为弹光系数; αf和ξ分别为对应不同光频率f的热膨胀系数和热光系数[20]; k, Pε, αf和ξ均为常数; λ0即为中心波长; Δλ为波长偏移量,由解调仪[21]解出。由上式可知,仅有温度与应力为未知量,又因为FBG2为单悬臂,无应力干扰,故其ε=0,则温度可由FBG2解算得到,满足下式:
$ \Delta T = \frac{1}{{\left( {{\alpha _f} + \xi } \right){\lambda _0}}}\Delta {\lambda _{{\rm{FB}}{{\rm{G}}_2}}} $
(2) 式中,ΔλFBG2为由FBG2解算得到的波长偏移量值。将(2)式代入(1)式,再利用FBG1的波长偏移量就能求出F,从而实现应变与温度的解耦。
针对测试过程中存在振动干扰,采用差分运算的方式进行抵消处理。由于振动在FBG1和FBG2中均存在,并且在时间上是一致的,所以对两个FBG的回波数据做傅里叶变换,将两组信号的频域信息做互相关运算,从而将具有明显振动周期存在的光谱波动找出,并将其对应的光谱相互抵消,就能最大程度地消减由于振动造成的干扰了。则光功率谱G有:
$ G(\lambda ) = \int_{ - \infty }^\infty A (t)\exp ( - {\rm{i}}2{\rm{ \mathsf{ π} }}\lambda t){\rm{d}}t $
(3) 式中,t为时间,λ为波长,A(t)为t时刻的功率值。然后对两个FBG的频域数据进行互相关运算,互相关函数有:
$ {\left. R \right|_{{\rm{FB}}{{\rm{G}}_1}, {\rm{FB}}{{\rm{G}}_2}}}(\lambda ) = \int_{ - \infty }^{ + \infty } {{G_{{\rm{FB}}{{\rm{G}}_1}}}} (\lambda ){G_{{\rm{FB}}{{\rm{G}}_2}}}(\lambda ){\rm{d}}\lambda $
(4) 然后将相关度高的波长波动位置数据组成集合,从而依据集合剔除由于振动引入的应力测试值变动。
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为了验证系统的可行性,在太原西山煤矿南矿区某一非工作状态的巷道中进行了环境参量监测实验。实测井下平均温度21.4℃,相对湿度69%,巷道内本身没有开采设备工作,但临近巷道有开采作业,存在明显振动噪声。并将监测数据通过光纤传感网络与互联网相连,使办公区域的主机可以直接巡检开采层环境监测数据,系统结构如图 4所示。
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在同一个拱形截面中(如图 2所示),采用4个FBG可以分别获取拱段两侧的压力分布。以一侧为例,两个FBG应力传感器的监测数据就能够判断出拱段位置上是否存在应力异常变化,从而实现风险预警。由于数据具有一定的线性特征,故测试时可采用可测区间内两个测试值完成测试曲线的标定,标定后压力F与FBG的回波波长λ的函数可表示为:
$ F = C\left( {{\lambda _{{{45}^\circ }}} - {\lambda _{{{60}^\circ }}}} \right) $
(5) 式中,C表示土体压力系数(kPa/nm),λ45°和λ60°表示FBG在45°和60°的回波波长。
实验监测时间为96h,共计4d的测试数据,期间第1天中有2次相邻开采层的爆破、第2天中有2次在更远一些的巷道的爆破以及第4天有1次另一个稍远的巷道的爆破。依据FBG测试回波信号反演的压力变化曲线如图 5所示。
由土体压力数据可知,在9:00~10:00之间及15:00~16:00之间相邻开采层的爆破引起的应变传感FBG明显的波动,分别导致45°测试点位置应力最大值为949kPa和863kPa,60°测试点位置应力最大值为757kPa和653kPa。两个FBG测试曲线的变化时间及振幅差基本相同。系统通过差分计算完成应变测试数据中对爆破振动造成的响应误差的消除。
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将用于温度传感的FBG的数据采集至处理系统,依据(3)式和(4)式将具有相关度超过预设阈值的波长偏移数据滤除后,再将整个时间段的波长进行温度反演,反演后温度波动如图 6所示。
由实验数据分布可以看出,在96h的数据采集过程中,巷道内温度变化很小。整个过程温度测试精度为0.5℃,温度最大值为21.7℃,最小值为21.0℃。相比之下,在未经振动差分滤噪的温度测试数据中,计算获得的温度最大值为49.8℃,最小值为12.4℃,显然计算值与实际情况不符,说明未滤波的条件下振动对测试数据影响明显。而未经差分滤噪的错误数据是由于测试过程中相邻开采层爆破产生的振动导致的,可见,本差分算法在应变、温度与振动的解耦方面具有明显效果。
面向井下环境参量的光纤传感物联网系统
Optical fiber sensing internet of things system for underground environmental parameters
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摘要: 为了能够大范围准确获取井下环境参量并有效抑制噪声, 研究了基于新型光纤光栅(FBG)封装的光纤传感物联网系统。设计了一种可同时测试应变与温度的传感模块, 试制了新型FBG封装结构, 搭建了符合巷道结构的光纤应力监测分布网络。采用基于互相关差分计算的方法实现了对振动噪声的消除, 并对井下巷道的应力和温度进行了连续监测。结果表明, 拱段45°与60°的FBG可以准确记录应力、温度变化数据。该系统在大范围井下环境参量监测中具有更高的稳定性、更好的适应性。Abstract: In order to obtain accurate parameters of the downhole environment in a large range and effectively suppress the noise, an optical fiber sensing internet of things system based on a new fiber Bragg grating(FBG) package was studied. A sensor module that can test strain and temperature simultaneously was designed. The new FBG package structure was trial-produced. An optical fiber stress monitoring distribution network conforming to the tunnel structure was built. A method based on cross-correlation difference calculation was used to eliminate vibration noise. The experiment continuously monitored the stress and temperature of the underground tunnel. The results show that the FBG at 45° and 60° in the arch can accurately record the stress and temperature change data. It can be seen that the system has higher stability and better adaptability in large-scale downhole environmental parameter monitoring.
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