Aiming at the low-speed impact load localization and vibration testing requirements of aerospace composite flexible structure, The Fiber Bragg Grating sensor network monitoring system is established. The increasing sampling technique is used to process the impact response signal, then the generalized cross-correlation delay estimation method is used to extract the time difference relationship between the sensor response signals. On this basis, the rapid identification of the impact position of the composite flexible structure is realized by considering the position information of the sensor comprehensively. The experimental results show that the area of six impact sample points and the specific coordinates of the location can be accurately identify respectively by this method. The average error of the coordinate positioning is about 3cm. The self-vibration mode and the natural frequency of the composite structure are achieved by using Fiber Bragg Grating sensor network monitoring system simultaneously. The finite element model of the composite structure with triangular section is established by using Patran. The natural frequency of the simulation is in good agreement with the experimental result, which proves that the Fiber Bragg Grating sensor has good vibration test performance.
Aiming at the impact damage monitoring requirements of aircraft fuselage and wing structure, in this paper, a fast loading identification method based on fiber Bragg grating (FBG) sensor honeycomb layout form is proposed. Firstly, the FBG honeycomb topology network is constructed, which has good scalability and high monitoring efficiency. Secondly, the impact response signal of fiber grating is processed by fractal filter method, then the energy amplitude of the fifth order wavelet transform is chosen as the characteristic parameter of the impact response. Thirdly, a FBG impact response model is established by using a few prior samples. On that basis, according to the location feature of the fiber sensors, a cell location and coordinate location method based on honeycomb cell is proposed to realize fast impact location for four edges clamped plate structures. The research shows that in a random honeycomb monitoring unit which side length is 250mm, 9 randomly chosen simulated impact points are identified with an average error of about 20mm. This method does not require a large number of prior samples, and is suitable for the conventional low speed fiber grating sampling mode. It has excellent environment adaptability and fast response ability.
Long period grating (LPG) is a kind of optical fiber sensing element which is sensitive to the change of refractive index in external environment. Magnetic fluid has the characteristic of changing its refractive index under different magnetic field. In this paper, a novel method for measuring magnetic field of optical fiber is proposed by combining magnetic fluid with LPG and fiber Bragg grating (FBG). Firstly, the LPG is encapsulated in a capillary glass tube filled with magnetic fluid, the shift of the LPG transmission spectrum under different magnetic field is obtained by a optical spectrum analyzer(OSA), the transmission spectrum is shifted by 2.194nm when the range of magnetic field is 0~30mT. On this basis, in order to realize the rapid monitoring of magnetic field, a composite structure of LPG and FBG in series is proposed. The structure is immersed in capillary glass tube filled with magnetic fluid, the external magnetic field information can be detected according to the change of the amplitude of FBG reflection spectrum. When the magnetic field varies, the refractive index of the magnetic fluid will change. Although the FBG is insensitive to the refractive index, its reflection intensity can be modulated due to the shift of the LPG transmission spectrum with the magnetic field. The results show that the maximum sensitivity can reach 0.042dB/mT when the magnetic field range is 0~30mT. This novel fiber grating composite sensing structure can provide a basis for further development of magnetic field monitoring based on intensity modulation.
The fibers aligning is very important in fusion splicing process. The core of polarization maintaining photonic crystal fiber(PM-PCF) can not be seen in the splicer due to microhole structure of its cross-section. So it is difficult to align precisely PM-PCF and conventional single-mode fiber(SMF).We demonstrate a novel method for aligning precisely PM-PCF and conventional SMF by online spectrum monitoring. Firstly, the light source of halogen lamp is connected to one end face of conventional SMF.Then align roughly one end face of PM-PCF and the other end face of conventional SMF by observing visible light in the other end face of PM-PCF. If there exists visible light, they are believed to align roughly. The other end face of PM-PCF and one end face of the other conventional SMF are aligned precisely in the other splicer by online spectrum monitoring. Now the light source of halogen lamp is changed into a broadband light source with 52nm wavelength range.The other end face of the other conventional SMF is connected to an optical spectrum analyzer.They are translationally and rotationally adjusted in the splicer by monitoring spectrum. When the transmission spectrum power is maximum, the aligning is precise.
A novel optical fiber sensor based on surface plasmon resonance phenomenon that can be used for liquid temperature testing around the sensing probe is presented. According to the attribute of liquid refractive index decreasing with the rise of liquid temperature, a series of SPR spectrum curves, corresponding to temperature range from 10 °C to 90 °C, can be obtained from spectrum signal processing. The range of temperature testing can be adjusted by choosing liquids with different boil and refractive index as environmental medium. This paper analyzes theoretically the influence of a variety of liquid temperature on the metal film coated onto the sensing probe surface and the refractive index of fiber core. A real-time double-parameter testing involving resonance wavelength and the lowest light intensity reflectivity is also proposed. Moreover, the proportion concentration of mixed solutions with different refractive indices ranging from 1.3675 to 1.4281 refractive index units, including ethanol and glycol, is investigated utilizing relative spectrum measurement technology. The preliminary experiments confirm the feasibility of the optical fiber SPR sensor based on wavelength-interrogation for temperature and refractive index measurement.