A Research On Theory And Technique Of Demodulation Of Optical Fiber Sensors Used For Structure Health Monitoring

Structure health monitoring system is an important application field of smartstructure and gets more and more concern. Fiber Bragg grating (FBG) sensor andextrinsic Fabry-Perot interferometer (EFPI) sensor are two kinds of optical fibersensors suit for structure health monitoring. In this paper, a research of measuringstrain and temperature simultaneously is carried out with combination of FBG andEFPI sensors from the view of demodulation system. A parallel demodulation systemis designed, which combines the Fourier transform demodulation method of FBG andlow coherence demodulation method of EFPI. Then an experiment system isestablished and a set of experiments are carried out. In the engineering application, anapplication research of FBG on stress measurement of prestressed steel strand iscarried out and a fiber sensor system used for large stress range is developed. 7. The theory of EFPI sensing and demodulation is studied. A sensing model ofEFPI is established. Based on partial coherence theory, the output of interferometerunder different finesse EFPI acting is deduced with Michelson interferometer asreceiving interferometer, and the results of experiment and computation are compared.From the view of system design, the synthesis of light source used for enhancingprocess of low coherence interference signal is studied and an additive signal to noiseratio (SNR) is defined, which helps obtain the actual demand SNR. The experiment ofsynthesis of light source is carried out. The application of Morlet wavelet to retrievethe profile of low coherence interference is discussed and a formula for deciding scaleof Morlet wavelet is provided. Simulation and experiment of retrieving profile arecarried out. 8. The sensing model and Fourier transform demodulation method of uniformperiod fiber grating are discussed in detail. The spectrum response of uniform periodfiber grating is deduced with couple mode theory. The strain and temperature responseof it is studied. The effect of strain induced by heat and cross-sensitivity betweenstrain and temperature are analyzed. The error sources of Fourier transformdemodulation method are discussed and three main noises are analyzed in detail. Theposition and amplitude of main fake spectrums are provided. iii9. The combinations of sensors for simultaneous measurement of strain andtemperature, which are used for structure health monitoring, are analyzed with thecondition numbers of sensing coefficient matrixes. With combination of EFPI andFBG sensors, three parallel demodulation schemes for simultaneous measurement ofstrain and temperature are proposed and designed, based on the different methods ofsignal separating. ASE light source is adopted. Motion stage is made up of crossedroller bearing, piezo linear motor, and linear encoder. The close-loop motion control isrealized with Mini-PMAC card. Through the adjust experiment of PID, velocityforward feedback and acceleration forward feedback, high motion resolution andgood stability of stage are obtained. The interference acquisition is realized withInGaAs detector and AD acquisition card. 10. The software for parallel demodulation is developed and method of dataprocess is studied. With object-oriented software development method,the wholestructure of software is designed according to the function demand, then the maincontrol module and five main C++ classes are developed. Several spectrum evaluationmethods for FBG are studied in detail and a data fusion model is proposed. 11. The experiment system of parallel demodulation for simultaneousmeasurement of strain and temperature is established. The reconstructionexperimental research of Fourier transform spectrum is carried out, the effect of noiseis discussed. With this system, strain experiment of FBG and EFPI is carried out, andprecision of 16.5 με and 13.49 με are obtained respectively. Then the experiment ofsimultaneous measurement...