Study Of Continuously Scanning Laser Doppler Vibrometer Test And Modal Analysis Technique

Aero engine components and complete machine dynamics are one of the key factors for safety and reliable operation.One of the important methods of studying structure dynamics is experimental modal analysis.For the special test conditions such as light structures,thermal structures in high temperature environment,the contact sensors cannot accurately test the structure response due to additional quality or high temperature.Laser Doppler Vibrometer(LDV),as one of non-contact test method,shows a huge advantage in such applications.However,the current Scanning LDV(SLDV)can only test discrete points using point-by-point scanning method and there will be a long test time measuring a large number of points.In order to extend the application scope of SLDV and test structural modal parameters and dense mode shapes efficiently,the Continuous SLDV(CSLDV)and the corresponding modal analysis methods are studied in this paper.The main work and contributions are as follows:The basic theory of CSLDV under sinusoidal excitation and modal analysis based on operation deflection shapes(ODS)is studied.Uniform rate and sinusoidal rate line scan under single frequency excitation and sinusoidal rate line scan under multi-tone excitation test and modal analysis are included.The methods of testing delay removal and optimization,demodulation filter selection,test coordinate transformation in uniform rate line scan and sinusoidal scan test data processing pressure are introduced.Based on the research of theory and method,CSLDV test and mode shape demodulation software under sinusoidal excitation is implemented.CSLDV test under sinusoidal excitation is carried on a cantilever rectangle metal plate and mode shapes of CSLDV testing are compared to those of SLDV test.The results show that CSLDV can test the structure mode shapes with high spatial resolution and the testing efficiency is much higher than that of SLDV.The basic theory of CSLDV under hammer excitation and modal analysis based on CSLDV frequency response function(FRF)is studied.The vibration response based on a CSLDV test under an impact excitation is usually modulated by the deflection shape as the Laser point scanning over the surface introduces multiple exponentially-decaying components beating together to give a modulated transient.A novel FRF of the CSLDV test under the impact excitation is described and the basic approach to identity the natural frequency,modal damping and mode shapes demodulation is also introduced.CSLDV test under impact excitation and mode shape demodulation software under is implemented.Then,simulation of CSLDV test under the impact excitation is taken place to verify the effectiveness of the CSLDV test and modal identification method based on a Bernoulli-Ruler beam model.The influence of structure damping and scanning rate on CSLDV test are studied and research shows the method is applicable to small damping structures.Finally,a practical beam structure is tested with CSLDV under impact excitation and modal parameters of the first 3 modes are identified with the approached method.Comparison of the CSLDV results between SLDV test and theoretical calculation respectively demonstrates that this method is practical and efficient with high spatial resolution of mode shapes.The application of SLDV and CSLDV test on the actual blade with complex surface is studied.The coordinates of each measurement point on the blade surface are obtained from the geometrical model and blade surface is fitted.The angles between test laser beam and normal line of the measurement point are solved based on the fitting surface,which is used to decompose the test mode shapes.SLDV test results have good correlation with the results of finite element method(FEM)after being decomposed,which verifies the mode shape decompose method.Only the first mode shape is well tested by CSLDV and the test of high-order mode still needs further study.It is proved that CSLDV method has the potential of testing modal parameters and compact vibration mode of actual structure.