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滑觉传感标定实验系统如图3(a)所示,该实验系统主要由解调仪、传感模块、压力计组成。解调仪最小分辨率为0.5 pm。实验中所用光源波长解调范围为1525~1565 nm,最小分辨率为0.5 pm。实验施力物体为一半径2 mm的金属施力杆,受力面尺寸较小,可看作点施力。硅胶块内铺设的FBG写入了8个不同峰值的光栅,每一个光栅长度为10 mm,光栅间距为8 cm,每一个光栅的反射率都能达到90%以上,且边模抑制比都大于15 dB。8个光纤光栅传感器带宽均为0.195 nm,其中光纤弹光系数、光纤热光系数分别为0.120、0.275,泊松比为0.17。
Figure 3. (a) Calibration experiment device; (b) The actual points and the predicted points of the static object position
实验过程中分别在36个小区域处施加20 N的力,反复多次进行实验,通过解调仪采集回36组光纤光栅中心波长偏移量值,用MATLAB对实验数据进行拟合,得到每一个光纤光栅传感器在每一个区域处的分辨率值,完成对光纤光栅传感模块的标定。表1为标定实验所得每一个光纤在每一个小区域处的分辨率k。
k/nm Area 1 Area 2 Area 3 Area 4 Area 5 Area 6 Area 7 Area 8 Area 9 FBG1 0.06 0.48 1.92 0.36 0.44 0.29 0.45 1.93 1.07 FBG2 0 0.01 0.54 0.32 0.09 0.76 0.29 0.84 0.45 FBG3 0 0.26 −0.37 0 0 0 0 0 0 FBG4 0.26 0.35 −0.97 −0.46 0.02 0 0 0 1.95 FBG5 0 0 −0.36 0 0.07 −0.46 0 0 0.39 FBG6 0 −0.17 −0.23 0 0 0 0 0 0 FBG7 0 −0.48 −0.20 9 10 0.74 −0.02 −0.04 0 FBG8 0.36 1.96 0.01 −0.19 −0.04 0 0 −0.07 0.04 Table 1. Different resolution values of different FBG in different regions
按照仿真图中的轨迹点用金属施力杆模拟静止物体,使其对硅胶块施加20 N的力得到如图3(b)所示的实际坐标点与预测坐标点对比图,可见以该实验标定的各个FBG响应区域能够较准确地判断物体位置坐标。
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在实际轨迹监测实验中所用运动物体为0.255 kg的钢制小球,实验装置如图4(a)所示。假设物体移动过程中对硅胶作用力集中于点,给物体一定的初速度并使物体任意运动一段轨迹。物体在不断移动的过程中,对硅胶块的不同位置施加相同的力,使硅胶块发生一定的形变。与此同时,光纤光栅传感器产生相应的响应,通过解调仪记录物体移动时间以及光纤光栅中心波长实时偏移量。以公式(5)为依据,对获得的光纤光栅中心波长偏移量进行数据处理,获得铁球运动过程中实际位置坐标与预测坐标间对比图,如图4(b)所示。
Figure 4. (a) Experimental installation of monitoring small ball sports trajectory; (b) Actual trajectory comparison and measurement of small ball sports
结合标定实验与小球实验结果可知,对硅胶传感模块施加不同大小的力只是改变光纤光栅传感器响应范围以及信噪比。质量越大,压力越大,则传感器相应范围更广,其信噪比更好。由于文中是通过FBG间响应参数比值进行位置解算,即使重物的质量发生改变也不会对传感模块的解算精度产生影响。实验对质量为0.255 kg的小球进行监测,光纤光栅传感器已具有很理想的响应结果,足以证实该传感模块可应用到仿生体滑觉监测系统中。
图4(b)中红色为实际运动轨迹点,蓝色为预测运动轨迹,可以看出,以6×6来划分硅胶块,通过光纤光栅传感器的实时监测能够较准确地确定物体运动的大致路线,因此该传感组能够用来监测物体的非线性运动轨迹以及运动方向。
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小球运动轨迹实验中选取7个点,确定每一个点所处区域以及具体位置坐标,并求得实际位置与测量位置间的距离误差,如表2所示。
Actual location/mm Section number Measuring position/mm Section number Range error/mm (14,0) A11 (12.5,12.5) A11 12.5 (22,25) A11 (12.5,12.5) A11 15.7 (31,50) A23 (37.5,62.5) A23 14 (43,75) A23 (37.5,62.5) A23 13.6 (57,100) A35 (62.5,112.5) A35 13.6 (75,125) A45 (87.5,137.5) A45 17.6 (100,150) A55 (100,150) A55 0 Table 2. Ball trajectory and prediction trajectory
实际点与测量点间的距离误差为:
实际点与测量点间的角度误差为:
由计算结果可知,运动过程中小球实际运动位置与该系统预测位置距离最大误差为17.6 mm,该系统预测的小球的区域位置与实际区域位置完全一致,而位置误差大小均小于所划分区域数值大小。由最后的实验结果可知,在将光纤传感模块划分为6×6的分辨率的情况下,可以较准确地确定物体的具体位置和运动方向,并且系统在位置角度的测量过程中可以通过调节FBG铺设密度来实现更准确的测量。如果想要更进一步提高位置坐标的准确度,可以以同样的粘贴方式增加FBG的数量或者划分成更小的区域。由于光纤光栅传感器是嵌入到硅胶中,在实际应用中可根据测量现场具体问题对硅胶的形状、大小以及厚度进行调整,实现了某些特定测量场合对传感器微型化的要求。
On-line slippage measurement system for optical fiber sensing array
doi: 10.3788/IRLA20210278
- Received Date: 2021-04-28
- Rev Recd Date: 2021-06-23
- Publish Date: 2022-04-07
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Key words:
- fiber Bragg grating sensor /
- displacement monitoring /
- state analysis /
- perception array /
- simulation analysis
Abstract: Aiming at the real-time monitoring of position and velocity in smart flexible sliding sensor, a fiber optic sensing module for real-time monitoring of target displacement and velocity was designed. First, the temperature compensation module was used to solve the temperature cross-sensitivity problem; then, based on the analysis of the stress distribution simulation results of the sensing unit, the "meter" type FBG network structure was proposed; finally, the standard pressure gauge and the ball were used to complete the test of the position direction and the speed state, and a target state solution model suitable for this structure was proposed. The simulation results show that the average deformation of the target trajectory is about 0.32 μm, and the attenuation distance width is about 3.0 mm. The experiment carried out a sliding test on a 0.255 kg steel ball. The FBG stress sensitivity was better than 0.0206 nm/N. The FBG center wavelength offset can accurately determine the location of the object and the direction of the object's movement. The sensor module can monitor the movement state of the object in real time, and intelligently adjust the force and posture of the object. The results show that the plane positioning accuracy, motion angle and speed conversion of the sliding sensor system meet the design requirements, and according to the model function relationship, it can be known that the control of accurancy of the position, angle and speed can be achieved when adjusting the total amount of FBG in the sensor network. In summary, the system has the capability of real-time monitoring of the target position and movement status in the detection area, and is suitable for technical fields such as flexible intelligent assembly and intelligent bionic skin.