Research On On-orbit Condition Monitoring Technology Of Spacecraft Structure Based On Optical Fiber Sensing

With the rapid development of science and technology and the need of national economic construction and national defense modernization,spacecraft is developing in the direction of "high precision,high reliability and long life".Spacecraft has long been confronted with complex space environment including space irradiation,collision between meteoroid and space debris,aerodynamic heating and thermal cycle during launch,orbit change,rendezvous and docking and on-orbit operation,and often accompanied by severe structural vibration.These factors may lead to structural deformation,fatigue,damage and so on,which may bring great hidden dangers to space flight,and even lead to the failure of space missions.Therefore,there is an urgent need for real-time monitoring of structural strain and impact of spacecraft.In recent years,with the rapid development of fiber optic sensor technology,fiber Bragg grating(FBG)sensor is considered as an ideal sensor for structural health monitoring with its merits of small size,high accuracy,strong anti-electromagnetic interference ability,easy wavelength division multiplexing and good compatibility with the structure to be measured.To use FBG sensor to the structural state analysis of spacecraft has obvious advantages over the traditional techniques.However,in the special space environment,there are still many theoretical and technical problems to be solved if the sensor system is to work reliably for a long time.In this paper,the sensoing mechanism of FBG sensor in spacecraft application is analyzed.The sensor suitable for spacecraft,signal demodulation system,signal processing,feature extraction and ultra-high speed impact location identification technology are studied.A spacecraft structural state analysing and monitoring system is established,which realizes real-time sensing and accurate evaluation of spacecraft strain,temperature monitoring and bulkhead ultra-high speed impact.The main research work is as follows:1)Based on the analysis of spacecraft on-orbit operating environment,strain detection requirements and FBG sensing mechanism,the FBG sensor system suitable for space environment is designed.The mechanical simulation analysis of the strain sensor is carried out firstly.In order to solve the problem of cross-sensitivity between strain and temperature,a temperature sensor is designed to compensate for the influence of temperature change on the measurement results of strain sensor.A method of temperature sensor's sensitization is proposed to improve the measurement accuracy and sensitivity.Experiments were carried out to test the accuracy of strain sensors at room temperature,limit temperature and temperature sensors at high and low temperatures.The results show that the designed strain sensors and temperature sensors have good stability,repeatability and high test accuracy.On this basis,FBG sensors are installed and tested on a certain type of spacecraft according to the requirements of spacecraft monitoring.The results show that the developed sensors can meet the requirements of spacecraft on-orbit structural strain measurement.2)The strain detection and demodulation system based on FBG sensors for space environment is designed and developed.A tunable filter demodulation scheme based on the combination of F-P etalon and acetylene chamber is proposed.The problem of inaccurate peak finding of F-P etalon caused by flatness difference of light source is solved by adaptive threshold method.The verification testing experiment of demodulation system designed is tested for temperature stability and accuracy.The experimental results show that the system can effectively overcome the problem of demodulation accuracy decline caused by the transmission wavelength of F-P etalon drifting with temperature,thus realizing the stable demodulation of the central wavelength of FBG sensor with an error of about 1%.3)The development of the ultra high frequency demodulation system and the simulation of the characteristics of the ultra-high-speed impact plate wave signal of single-layer metal plate structure are carried out,and the propagation law of the impact wave in the metal plate is thoroughly understood.Firstly,on the basis of the analysis and Research on the mechanism of hypervelocity impact response of FBG sensor,an FBG hypervelocity impact detection system based on edge filter demodulation principle and uneven broadband light source is constructed by improving the equipment and optical path,and a system performance test platform is built,and the response characteristics of the system are experimentally studied.Then,the simulation model of hypervelocity impact is established by ANSYS software and the numerical simulation method is used to study the propagation process of hypervelocity impact signal,the damage evolution process of metal plate and the modal,dispersion and time-frequency characteristics of hypervelocity impact signal,which provides hardware and data support for source location and damage pattern recognition of projectile hypervelocity impact aluminum alloy plate.4)Based on the research of impact signal characteristics,the impact source location technology based on diamond sensor array and Extreme Learning Machine(ELM)network is proposed.The regional location model based on ELM network and the time difference location model based on diamond sensor array are established.Then the accuracy of the location model is verified by taking part of the experimental data as test samples.Finally,the same signal processing and feature extraction method are applied to the hypervelocity impact signal,and the extracted features are input into the locating model to obtain the impact source position.The results show that both of the two positioning techniques can accurately locate the location of the hypervelocity impact source and solve the problem of accurate positioning of the impact source in complex space environment.FBG sensors for space environment and demodulation system for strain sensors provide practical basis for strain detection of spacecraft on-orbit structures.The built ultra high-frequency detecting system and the analysis of plate wave signal characteristics,as well as the proposed intelligent identification algorithm and time difference location method for hypervelocity impact,provide theoretical and practical basis for on-time sensing,evaluation and location of spacecraft structure in orbit.