Study On Modal Testing Of Time-varying Dynamic Structures Based On FBG Sensors And FSO

With the development of research on structural dynamics and the expansion of applications on engineering structures.Whether it is used for the simulation of the real dynamic characteristics of complex time-varying structures or the verification of time-varying structural identification methods,the study on modal testing of time-varying dynamic structures is of great significance.The advantages of Fiber Bragg Grating(FBG)sensors over traditional accelerometers make them very suitable for response measurements in modal testing of time-varying dynamic structures.To solve the problem of physical wiring that cause difficulty for the application of FBG on response measurements in complex dynamic structure,this paper proposes a solution based on Free Space Optics(FSO).This thesis studies on the modal testing of time-varying dynamic structure based on fiber Bragg grating sensors and FSO.The main work of this thesis is as follows.(1)Research and application status of fiber Bragg grating sensors,strain modal analysis,modal testing of time-varying dynamic structures and FSO is summarized to supply the primary support for the further research on modal testing of time-varying dynamic structure based on FBG sensors and FSO.(2)The basic concept of strain mode is derived from displacement mode.The modal of strain mode is deduced.The orthogonality between strain modal shapes,the strain frequency response function and its characteristics are discussed.Those contests provide a theoretical basis for experimental verification of time-varying structures based on strain response.(3)A method for FSO based on GRIN collimators is proposed.Based on gaussian field approximations mathematical models of the way a Gaussian beam is transferred by a single collimator and the loss of the overall FSO system are established.Based the established mathematical model the analysis of the working characteristics of the GRIN collimator and the loss characteristics of the entire FSO system is completed.And the applicability of FSO on modal testing of time-varying dynamic structure based on FBG sensors is demonstrated.(4)Based on the strain response’s measurement of fiber Bragg grating sensors,a FSO method based on GRIN collimator is introduced,and a modal testing method for time-varying liquid-filled barrel structure based on fiber grating sensing and FSO is proposed.A time-varying liquid-filled barrel modal testing system based on fiber grating and FSO is set up and a method of adjusting the GRIN collimator in FSO system is proposed to solving the problem appearing during the testing of this system.(5)The experimental data of time-varying experiment and reference experiments are obtained by time-varying experiments and time-frozen reference experiments.Firstly the results of the reference experiments are obtained.The modal identification of the time-varying experimental data collected by the three groups of sensors(traditional piezoelectric accelerometers,FBG sensors,FBG sensors with FSO)is completed by using the functional series vector time-dependent autoregressive moving average(FS-VTARMA)model.By comparing the modal identification results of the reference experiments based on the data of accelerometers and the modal identification results of time-varying experiment based on the data of accelerometers and FBG sensors,the feasibility of the FSO method using GRIN collimator in the modal testing of time-varying dynamic structure and the effectiveness of the entire modal testing method is verified.