Study On The Structural Dynamic Testing And Modal Parameters Estimation Based On Fiber Bragg Grating Sensors

With the development of aerospace technology, the urgent demand for large spacecraft brings out structures of light quality and high flexibility because of large-scale tendency. It brings huge challenges to the design and application of spacecraft. Therefore, it is very important to realize online modal identification, real-time health monitoring and on-orbit control to large complex spacecraft structures. Traditional measuring method of structural dynamics is mainly based on the acceleration sensor, but it couldn’t satisfy the real-time distributed measurement of spacecraft structures very well due to its limitation of quality,size, line layout, signal anti-jamming and so on. However, the fiber Bragg grating sensor is able to adapt to the online measuring and monitoring of spacecraft structures because of its characteristics of multiplexing, lightweight, immunity to electromagnetic interference, and etc.This thesis studies on the structural dynamic test and modal identification methods based on the fiber Bragg grating sensors. It provides theoretical support and technical approach to the modal identification, online health monitoring and dynamic design of spacecraft structures. The main contents of this thesis are followed by:The research background and significance of this thesis is introduced. Research and application status of fiber Bragg grating sensors, strain modal analysis, modal identification and finite element model updating is summarized to supply the primary support for the further research on the structural dynamic test and modal identification based upon fiber Bragg grating sensors.The basic theory involved in strain modal analysis are summarized. Especially, the introduction and discussion about the expression of strain modal derived in detail on the basis of displacement modal, the orthogonality between strain modal shapes and characteristics of strain frequency response function supplies the theoretical basis and assumptions for structural strain modal test and strain modal identification.Sensor technology of fiber Bragg grating is studied, containing the basic principle of structural dynamic strain response measurement using fiber Bragg grating sensors, basictechnical indexes of fiber Bragg grating sensors, dynamic response characteristics of fiber Bragg grating sensors, fiber grating interrogation technique and fiber Bragg grating sensor multiplexing technique. Based on the above study, the structural dynamic measurement method using fiber Bragg grating sensors is put forward, and a test system which could measure strain dynamic response and acceleration response at the same time is established.According to the relationship between strain and displacement, a strain modal identification method for structures based on the least square complex frequency method is put forward. Using the data from structural dynamic experiment established above, strain modal parameters and displacement modal parameters of the singleinput-multioutput structural dynamic system under different burst random excitation are identified respectively. In addition, the modal parameters of output-only system are identified to enable a comparison and analysis.A novel estimation method for displacement modal shape based on finite element model updating is put forward. More accurate displacement modal shapes are obtained by the finite element model which is updated based on the identified strain modal parameters,thus establishing the transformation relationship from strain modal to displacement modal.This method solves the problem that fiber Bragg grating sensors couldn’t directly measure structural displacement parameters. In this thesis, the structural dynamic response of a beam structure with an additional mass block is measured, and the finite element model of this structure is updated based on the Sequential Radial Basis Function Surrogate-based Efficient Optimization algorithm. Then, the displacement modal shapes of this structure are obtained by updated finite element model. The result shows that the accuracy of finite element model could be improved effectively by model updating based on strain modal parameters identification, therefore, more accurate displacement modal shapes could be obtained updated finite element model.