Research On Optical Fiber Vortex FLow Measurement Technology

Vortex-shedding flowmeter has become one of the most indispensable members inthe kaleidoscopic family of flowmeter because of the unique measuring features. Theprinciple of vortex shedding flow is based on the corresponding relationship betweenvibration frequency and velocity of the fluid. Vortex shedding flow is very popular in theindustrial marketing because of the wide measuring range, high precision, no moving parts,and simple structures. Optical vortex shedding flow system exerts the advantages ofanti-electromagnetic interference properties and high precision to the most, which is moresuitable for measuring flammable and explosive gases.However,due to the wide varity offluids and the complex of working conditions,although numbers of methods andinstruments for fluid paprameter measurement have been devised,there still exist manyurgent practical problemsto be solved.The thesis aims at designing of a brand new optical-fiber vortex-shedding flowmeasuring system, which can be applied in flammable and explosive detection, meantimereaches certain demand of precision. First, this thesis illuminates the features of theoptical-fiber vortex-shedding flow measuring and the principle of FBG vortex sheddingflowmeter, and then analysis the vortex signal's composition and characteristics, and thenitmeasures and calculates the main signal characteristics quantities.It proposed themathematical model. This model plays a guiding role for choose of signal processingmethods. Then, a comprehensive structural of the optical fiber vortex flow measuringsystem is designed in this thesis,including the design of the optical fiber probe,signalprocessing circuit and so on,and it proceses the system's optimization scheme.The main works in this thesis are shoewd as follows.1. The optical fiber sensor was introduced into the vortex flow measurement. Thesystem uses the optical-fiber as the sensor. Meanwhile the researcher carries out theresearch on the liquid field around a column, liquid force on the optical fiber sensor andthe transmission loss along the optical fiber sensor respectively from such aspects ashydrodynamics, crystal physics and wave optics theory. Through the hydrodynamicanalysis of the flow field and the analysis of the characteristics of vortex signal analysis,it describes optical vortex flow measuring system theory. It proposed the mathematicalmodel of the optical fiber vortex flow signal under the invariant turbulence state, whichhas prepared for the signal measuring and processing in the latter. The establishment of themathematical model played a guiding role for the latter design of the fiber optic probedesign and the choice of signal processing methods.2.The author measure and calculate the round range of frequency to be measured andthe amplitude of the AC signal which was transformed from light signal to electrical signal,analyses the security of the deformation on optical-fiber. This work can ensure the opticalfiber will not be broken when the flow is high and the fluid impact is strong.3. A comprehensive structural of the optical fiber vortex flow measuring system isdesigned in this thesis, including the crust design, choose of the optical fiber ,the lightsource and photodetector, the location of the optical fiber probe ,the pressing circuit andother important structures.It also analysis the different vortex shedding bodiescharacteristics,then designed a trapezoid acute angle shedding body which has a betterperformance.It proposed a dual-processor computing system which is used the DSP as theprocessing center and the SCM as the control center. Then it proposed a optimize plan ofthe system on the structure.4. Based on the analysis of the vortex signal's composition, this thesis used somedifferent methods to do the signal processing, such as the spectral analysis, the adaptivefilter and the wavelet analysis which focus on the denoising to the low frequencysignal.The detailed steps to realize the concrete algorithm are given in the paper.Thecharacters of various methods and its applying condition in the vortex flow measuringsystem detection are discussed.In view of the character of vortex flowmeter singnal ,thepaper uses spectrum correction to improve the measurement precision. Then it analysisand processes the data which is collected from the laboratory,which proves the accuracy ofthe system.