Key Technology Of Liquid Leakage Sensing Based On Plastic Optical Fiber Side-coupling Effect

For the underwater vehicle,it is necessary to have good water tightness in the process of underwater navigation or the emergence from the water to prevent seawater immersion.Therefore,the leakage test to the vehicle is essential.However,the traditional water-tightness test cannot simulate the actual environment of the emergence from water,the means of in-situ leakage testing needs to be added to obtain the specific time and accurate position of leakage in real time,so as to provide the most authentic and reliable test data for the optimization of the structure design of the vehicle.The internal environment of the underwater vehicle is complex,and the number of leakage points to be monitored is very large.The applicable liquid leakage sensor must be immune to electromagnetic interference,easy to arrange and have a simple structure.Under such conditions,the traditional electric-measurement methods almost fail,and the liquid leakage sensors for underwater equipment have not been reported.Therefore,it is of great practical significance and application value to independently develop a liquid leakage sensor which is specially designed for underwater equipment with high real-time and high spatial resolution.Considering the specific requirements of in-situ leakage testing of underwater equipment,fiber optic sensing technology has more advantages.It is immune to electromagnetic interference and suitable for distributed measurement in complex spatial structures.Nonelectrical measurement will not cause short circuit due to liquid contact,electric leakage,etc.Thus,a solution based on the side-coupling effect of plastic optical fiber(POF)is proposed.Firstly,as the liquid leakage sensing can be realized by measuring the change of refractive index(RI)of the medium around the monitored point,a side-coupling structure of POF,directly sensitive to RI,is designed.The influence of the RI of the coupling medium on the sidecoupling ratio is analyzed qualitatively from the perspective of optical wave theory,which provides a foundation for the design of high real-time liquid leakage probe.According to different input waveguides,the side coupler of POF can be divided into fiber-fiber side coupler(FFSC)and prism-fiber side coupler(PFSC).Secondly,a liquid leakage probe based on FFSC is designed and implemented.From the perspective of ray theory,the coupling ratio of FFSC increases with the increase of RI of the coupling medium.In order to realize the continuous measurement of fixed point monitoring of liquid leakage,the POFs are respectively wrapped around a cylinder column and a race-track column.The experiment verifies the method of improving the sensitivity and measuring range of the probe by filling the uv-curable adhesive with high RI into the gap of the side coupler.The effects of different structure parameters on the performance of the probe and the temperature dependence of each probe are analyzed.The experimental results show that the sensitivity of the twisted and continuous liquid leakage probe filled with the uv-curable adhesive is increased to 11.978nW/mm,and the measurement range is increased to 600 mm.The resolution and accuracy are 0.008 and 1.7mm,respectively.The parallel and continuous liquid leakage probe filled with the uv-curable adhesive has a sensitivity of 10.1nW/mm and a measurement range of 800 mm.The resolution and accuracy are 0.010 and 1.6mm,respectively.The multi-point liquid leakage probe filled with the uv-curable adhesive show a stepwise descending output curve with the liquid level rising.While each curve area is immersed by water,the variation vbasisalue of the output power is 112.36 nW.In order to realize non-contact measurement of liquid leakage,the agarose is filled in the gap of the side coupler.When liquid leakage event occurs,the relative humidity around the coupler changes,and the coupling ratio of the side coupler changes,providing a basis for the design of non-contact liquid leakage probe.The humidity response characteristics of the probe with and without agarose are compared by experiments.The effects of the curvature radius on the sensitivity of the probe are analyzed.The reversible,real-time and temperature-dependent characteristics of the probe are also studied.The experimental results show that the sensitivity and linearity of the probe filled with agarose are 4.23nW/% and 99.30%,respectively,and the detection accuracy is 0.70%.When the relative humidity changed from 80% to 90%,the sensitivity and linearity of the probe filled with agarose increases to 12.49nW/% and 99.8%,and the detection accuracy decreases to 0.55%.Thirdly,a liquid leakage probe based on PFSC is designed and implemented.It is proved by the ray theory that the coupling ratio of side coupler increases with the increase of RI of the coupling medium.The effects of the single-structure defect and multiple-structure defect on side coupling ratio and sensitivity are compared.The effects of different borehole diameters,depth and number of holes on side coupling ratio,sensitivity and attenuation ratio under multiple structural defects are studied.A three-holes structure defect with a aperture of 0.5mm and a depth of 1.0mm is selected for further study.Finally,a liquid leakage sensing system based on the side coupler of POFs is designed and implemented.In the liquid leakage sensing system based on FFSC,the localization method based on address code is adopted,which can control up to 256 liquid leakage probes.The spatial resolution is determined by the arrangement of sensors.In the liquid leakage sensing system based on PFSC,a simpler positioning method based on the scanning timing sequence of flexible LED lamp strip is innovatively proposed.The measurement range and spatial resolution are determined by the number and interval length of the side coupling structures of POFs.Both of these sensing systems can be used in the field of liquid leakage sensing.However,when the number of test points is too high,the latter is simpler in structure and lower in cost.Therefore,the liquid leakage sensing scheme based on PFSC is adopted.The experimental platform was builded,and the liquid leakage sensor with a measuring range of 1m and a spatial resolution of 0.1m was realized.The verification test was completed.As a result,by connecting POFs in series and parallel,a flexible belt leakage sensor with a measuring range of more than 8m and a liquid leakage spatial resolution of less than or equal to 0.05 m can be implemented.