Study On Liquid Level Sensor For Polymer Optical Fiber Surface Mechanical Molding

Liquid level detection is widely used in daily life and industrial fields,such as liquid fuel storage,metallurgical industry,flood warning and so on.In recent years,the development of optical fiber sensor has been growing,and there are many categories of optical fiber sensors.Optical fiber liquid level sensor is widely applied in many fields,considering that optical fiber has good electromagnetic interference resistance,flame-proof,small size,corrosion resistance,and is suitable for complex environment.In complex and harsh environment,it has more advantages than traditional liquid level sensor,and the stability of sensing results is not easy to be affected.In this work,a polymer optical fiber surface mechanical forming liquid level sensor is proposed.Its sensing mechanism is based on the influence of filling different media in the micro groove on the transmission power of light wave in the optical fiber to achieve the measurement of liquid level.The cladding material of the polymer optical fibers processed in this experiment is fluorine resin,and the core material is polymethyl methacrylate(PMMA).The refractive indices are 1.417 and1.492,respectively.The micro-groove is carved on the polymer fiber perpendicular to the axis direction by the engraving machine.The core of the grooved fiber loses the cladding protection when the grooved fiber is in the air,so the light transmitted in the fiber will diverge partly to the outside.But when the groove structure on the optical fiber is immersed in the liquid medium,the liquid outside the optical fiber is equivalent to a new cladding,which reduces the light divergence in the optical fiber,thus achieving the purpose of measuring the liquid level height.Firstly,the theoretical analysis of the optical conductivity of the micro-grooved optical fibers is carried out,and the mathematical model of the liquid level sensing mechanism is established.The relationship between the structural parameters of themicro-grooved optical fibers and the power loss of the transmitted light wave is numerically simulated.The influences of the size of the optical fibers,the numerical aperture of the optical fibers,the parameters of the micro-grooves and the refractive index of the environmental medium on the liquid position measurement have studied.Secondly,in order to verify the correctness of the numerical simulation of the influence of the parameters of the micro-groove and the refractive index of the environmental medium on the optical power,the micro-groove is carved on the optical fiber section by the engraving machine.The experimental results are in agreement with the numerical simulation,and the following conclusions can be drawn:(1)the wider the groove structure width,the greater the loss of optical fiber connection;(2)the deeper the groove structure depth,the greater the corresponding optical power loss;(3)the larger the refractive index of liquid medium,the smaller the optical power loss.Next,the liquid level sensor head is made.Considering the engraving technology and measuring range of the engraving machine,the sensor head has 10micro-grooves,and the parameters are the same: the groove spacing is 1.00 mm,the groove width is 1.00 mm,and the groove depth is 0.10 mm.Because the groove width and groove spacing sum is 2.00 mm,the sensor system will respond when the liquid level changes to 2.00 mm,so the sensor resolution is 2.00 mm.Because of the spacing between the groove structures on the sensor head,the sensor head can not realize continuous liquid level measurement.Therefore,we improved the sensor head by changing the groove structure which was originally engraved on the polymer optical fiber and perpendicular to the fiber axis into an inclined groove structure with45 degrees angle to the fiber axis,so that the head and tail of the micro grooves engraved on the polymer optical fiber are in the same position.Therefore,the improved sensor head can continuously measure the liquid level change.In addition,the influence of the temperature of the liquid to be measured on the performance of the sensor is also explored.The experimental results show that the measurement results are not affected by the ambient temperature of the sensor head in thetemperature range of 0 ? — 50 ?.Finally,a liquid level measurement sensor based on Bragg grating(FBG)pair is proposed.The principle is that the distance between the two FBG central wavelengths of 1552.0 nm can be shifted at the same time,and the peak power corresponding to the central wavelength of the Bragg grating can change from the shift of the Bragg wavelength to the change of the peak power,thus realizing the purpose of measuring the change of liquid level.By changing the liquid level height,the buoyancy of buoys connected to FBG pairs will change,resulting in the change of force on two FBG pairs,making the central wavelength position of FBG pairs move in opposite direction,and the peak power change,thus reflecting the change of liquid level height.The measurement signal of this sensor system replaces the change of wavelength by the change of peak power,which makes the measurement result more sensitive,and the power measurement device is simpler and cheaper than the wavelength measurement device.This sensor system can realize continuous and high-precision liquid level measurement,and its performance is not affected in the temperature range of 25 ? — 45 ?.The tool used in this experiment is a sculpture machine.Its processing accuracy can reach 0.01 mm.It has the advantages of small size,cheap price,easy intelligent operation and little damage to polymer fiberis.