Research On Technology Of Ultra Long-range Doppler Laser Vibrometry Based On Continuous Scanning Methods

The structure dynamic experiments and modal analysis technology have a great significance for structural safety check,design optimization and damage detection.If we choose to use traditional contact-type measurement methods to conduct the experiments for large structures such as bridges,buildings,wind power towers,pylons and rockets,a large amount of length of high-fidelity and electromagnetic shielded data lines with multi-channel instruments are needed.The transducers,have to be fixed on the test structure.Therefore,the non-contact test technology has great advantages for the modal test of these large and lightness structures.The Laser Doppler Vibrometry(LDV)is one of the representative methods.However,the Scanning Laser Doppler Vibrometry(SLDV),which works by switching and testing each point designated by the operator or programs,is the only applicable methods that can be used in the actual industry circumstance now.The SLDV adopts the method of point-by-point testing,and the collection of the spatial information of the entire test piece is insufficient.The test time is very long when the number of test points is large.To overcome these shortcomings,a novel method,called Continuous Scanning Laser Doppler Vibrometry(CSLDV),was developed in 1980 s.In order to adapt to the actual engineering test environment,this paper designs and builds an integrated portable ultra-long-range laser Doppler continuous scanning system on the existing basis,including the construction of the hardware platform and the writing of software.Two types of continuous scanning laser Doppler vibration measurement technology based on working deformation under sinusoidal excitation were studied,namely constant speed linear scanning and sinusoidal speed linear scanning.Based on different scanning methods,the demodulation method and the polynomial method were proposed,and the cantilever beam was actually tested in the laboratory.The polynomial method was successfully used to obtain the relevant modal parameters.The working modal analysis of the structure of CSLDV system under random excitation is studied.The CSLDV system is established as a linear time period model,and the concept of harmonic transfer function is introduced.For random excitation,two output-only modal analysis methods are proposed: harmonic power spectrum method and Lifting method.Finally,the feasibility of the Lifting method is verified by experimental simulation,which lays a theoretical foundation for subsequent outdoor experiments.