The Research On High Precision Optical Fiber Liquid Level Sensing Technology And System

In the fields of daily life,geological monitoring,industrial production and environmental protection,the accurate measurement of liquid level is of great importance.The liquid level sensing technology based on optical fiber has become a research hotspot because of its high measurement sensitivity,small size,easy integration,anti-electromagnetic interference,good stability,high precision,suitable for extreme environment,safe and reliable,and easy to reuse.However,there are some shortcomings in the current fiber-optic liquid level sensing system,such as difficult fiber fault location,susceptible to power variation,complex fiber structure,high cost of demodulation system,small liquid level measurement range and low measurement accuracy.Some corresponding methods are proposed to solve these problems.And the theoretical derivation and experimental verification are carried out in this thesis.The main work of this thesis includes:1.The liquid level sensing system based on broadband optical chaos and differential demodulation algorithm is proposed and experimentally demonstrated.In the meanwhile,the liquid level sensing principle based on float-FBG structure is studied.And by applying optical chaos and differential algorithm to liquid level sensing,the fiber fault location and strong power disturbance resistance are realized simultaneously.When the diameter of the bottom surface of the float is 45 mm,the liquid level measurement range is 54 mm,the liquid level measurement accuracy is 2.8 mm,the spatial resolution of the fiber breakpoint is 0.06 m.2.The liquid level sensing system based on wavelength sampling and reconstruction is proposed and experimentally demonstrated.Firstly,the liquid level sensing principle based on single-no-core-single-mode structure is studied.It is the first time to apply sampling and reconstructed method to the spectrum of the single-no-core-single mode fiber in the field of liquid level sensing,and the minimum variance algorithm is proposed to demodulate wavelength drift.In the meanwhile,a tunable laser and an optical power meter are used instead of the spectrometer,which saves cost and improves liquid level measurement accuracy.Finally,the system achieves a liquid level measurement range of 500 mm and a liquid level measurement accuracy of 0.5 mm.When the liquid refractive indices were 1.3333,1.3401 and 1.3505,the liquid level measurement sensitivities were 20.72 pm/mm,21.85 pm/mm and 23.38 pm/mm,respectively.Therefore,the system also has the ability to resolve the refractive index of the different liquid.The single measurement time of this system is 800 ms.3.Based on the liquid level measurement system of wavelength sampling and reconstruction,an automated,miniaturized liquid level measurement system was designed.Furthermore,related hardware system selection and software programming are completed.When the liquid level measurement range is 130 mm,the liquid level measurement error is within 1 mm,the liquid level fluctuation is within 0.5 mm,and the minimum distinguishable liquid level is at least 1.67 mm.The system has good stability and is easy to integrate,which is very suitable for the actual liquid level measurement environment.