Operating Modal Parameters Identification Of Propeller Based On FBG Sensor

During the operation of the ship,the vibration of the propeller brings many harmful effects to the ship.In order to dampen and reduce noise,it is of great significance to identify the modal parameters of the propeller.Since it is difficult to measure the propeller’s load when it works underwater,the traditional modal parameter identification method is no longer applicable.Therefore,this dissertation adopts the method of operating modal parameter identification to identify the modal parameters of the propeller.Compared with traditional sensors,Fiber Bragg Grating(FBG)sensors have unique advantages such as small size,light weight,corrosion resistance,and easy formation of sensor networks,and are suitable for underwater strain information measurement of propellers.This dissertation takes the single-blade propeller as the research object,and uses the Stochastic Subspace Identification(SSI)method to identify its modal parameters under white noise excitation;The modal parameters were identified by combining the SSI method with the detection and elimination methods of harmonic modal.The main research contents of this dissertation are as follows:(1)The SSI method is detailedly deduced in principle and programmed in MATLAB.Aiming at the harmonic modal problems of rotating machinery such as propellers,the harmonic modal of the propellers are detected and eliminated from the perspectives of the time domain and the frequency domain,so as to obtain more accurate operating modal parameters of the propeller structure.(2)The modal parameters identification of propeller under white noise excitation is carried out.First,the modal simulation analysis of the single-blade propeller is carried out in the simulation software ANSYS Workbench.Secondly,the excitation test of the single-blade propeller is carried out.Finally,the modal parameters identified in the 0° direction(parallel to the leading edge)and the 90° direction(perpendicular to the leading edge)are compared with the modal parameters obtained from the simulation.In the natural frequency,the maximum error in the 0° direction is-4.33%,and in the 90° direction is-5.73%;in the damping ratio,the maximum error in the 0° direction is 14.17%,and in the 90° direction is 12.97%;in the mode of vibration,the maximum error in the 0° direction is-15.45%,and in the 90° direction is-15.86%.(3)The modal parameters identification of propeller under non-white noise excitation is carried out.First,the wet modal simulation analysis of the propeller is carried out in the simulation software ANSYS Workbench.The propeller open-water test platform was built,the strain data of the propeller under three working conditions were obtained,and the harmonic modal were detected and eliminated.Modal parameters before and after eliminating harmonic modal were identified by the SSI method and compared with simulation results.After the harmonic modal are eliminated,the modal parameters of propeller are identified more accurate.And in the natural frequency,the maximum error in the 0° direction is 8.50%,and in the 90° direction is13.49%;in the damping ratio,the maximum error in the 0° direction is-14.81%,and in the 90° direction is 9.26%;in the mode of vibration,the maximum error in the 0°direction is 12.78%,and in the 90° direction is 15.84%.