Optical Fiber Testing Research Of Metal Surface Based On Optical Blade Tip Timing System

As key components of the large rotating machine,blades normally work in the state of high speed and high load,as well as the harsh environment such as high temperature,high pressure,flue gas,salty water immersion and industrial gas corrosion.Therefore,severe vibrations and breakage of blades often occur,seriously affecting the operation safety of large rotating machines.For characters such as small size,high accuracy,anti-EMI and non-contact measurement,optical blade tip-timing(BTT)system has attracted wide attention.However,signals of the optical BTT system is easily affected by factors like tip clearance,tip surface roughness,tip section shape change and so on,resulting in the fluctuations of the receiving light intensity and timing identification errors of the optical BTT system which can affect the measurement accuracy of the whole system.In order to solve above problems,a new timing identification method based on micro-structured surface of the blade tip is introduced in this thesis and main research contents are shown as follows:Firstly,the sensing principle and the structure of the reflective intensity modulation sensor used in the vibration measurement system are introduced.Taken the three-fiber probe as the basic model of the sensor,the simulation model of light scattering from the metal surface of the rotating blade tip with periodic structure was built by using multi-physical field simulation software.Influence to the scattering light of characteristic parameters of the periodic metal surface such as surface period,modulation depth and surface shape were studied in detail by simulations.Then the optical fiber testing system was established and optical fiber tests were carried out on the metal surface processed by different processing methods.The testing results showed that processing methods of flat milling,end milling and planer were more likely to form the periodic surface structure.Also,principles shown by testing results of the influence to the scattering light caused by periodic surface characteristic parameters are in agreement with simulation results.Based on results of metal surface simulations and optical fiber tests,a new timing identification method based on micro-structured tip surface was proposed.Some processed blades can form micro-structured surface with characteristic period.By extracting characteristic period frequency,the characteristic signal can be established and phase information of the characteristic signal can be obtained by demodulation of the filtering algorithm or phase-locked algorithm.Here,the phase information equals to the time information which can be used as a fixed timing position on the blade surface for timing identification.The principle of this timing identification method and corresponding time modulation algorithm are explained in detail.in this thesis.Two kinds of micro-machined surfaces,i.e.end-milling Ra=0.8 um and end-milling Ra=3.2um,were tested by a low-speed test system built in the laboratory when tip clearance,installation angle and blade cross-section symmetry changed.Compared with the single threshold frontier time identification method(traditional timing identification method in optical BTT system)under the same condition,it showed that timing errors can be effectively reduced by using the timing identification method based on micro-structured surface.A three-blade rotating vibration measurement system including photoelectric conversion circuit,acquisition card and LabVIEW signal processing program was built.Two of the three blades were machined with micro-structured surfaces,and the surface characteristic period was about 400?m.Timing errors of the blade with micro-structured surface and the blade with no process were compared at 1000 rpm speed.Results showed that the former error is significantly smaller than the latter,proving the feasibility and superiority of the timing identification method based on micro-structured blade surface in high-speed rotating system.Compared with the analog signal processing circuit commonly used in the traditional vibration measurement system,the system described in this thesis mainly adopts digital processing system,which has the advantages of easy debugging,good maintenance and high accuracy.Finally,the bandwidth requirement for the digital processing system was also discussed.