The measurement accuracy of dynamic fiber-optic sensing interrogators, which use frequency scanning to determine the value of the measured, err as either the event bandwidth approaches half the instrument sampling frequency or when the event dynamic range comes close to the instrument designed value. One main source of error is the common practice of assigning sampling at a non-uniform grid to a uniform one. Harmonics higher than -20 dB are observed for signal frequencies exceeding 25% of the sampling rate and/or for signal amplitudes higher than 15% of the instrument dynamic range. These findings have applications to fiber-Bragg-grating and Brillouin interrogators.
The ability of Brillouin-based fiber-optic sensing to detect damage in a moving cantilever beam is demonstrated. A fully
computerized, distributed and high spatial resolution (10cm) Fast-BOTDA interrogator (50 full-beam Brillouin-gain-spectra
per second) successfully directly detected an abnormally stiffened (i.e., ‘damaged’) 20cm long segment in a 6m
Aluminum beam, while the beam was in motion. Damage detection was based on monitoring deviations of the measured
strain distribution along the beam from that expected in the undamaged case.
In traditional OFDR systems, the backscattered profile of a sensing fiber is inefficiently duplicated to the negative band of spectrum. In this work, we present a new OFDR design and algorithm that remove this redundancy and make use of negative beat frequencies. In contrary to conventional OFDR designs, it facilitates efficient use of the available system bandwidth and enables distributed sensing with the maximum allowable interrogation update-rate for a given fiber length. To enable the reconstruction of negative beat frequencies an I/Q type receiver is used. In this receiver, both the in-phase (I) and quadrature (Q) components of the backscatter field are detected. Following detection, both components are digitally combined to produce a complex backscatter signal. Accordingly, due to its asymmetric nature, the produced spectrum will not be corrupted by the appearance of negative beat-frequencies. Here, via a comprehensive computer simulation, we show that in contrast to conventional OFDR systems, I/Q OFDR can be operated at maximum interrogation update-rate for a given fiber length. In addition, we experimentally demonstrate, for the first time, the ability of I/Q OFDR to utilize negative beat-frequencies for long-range distributed sensing.
An airborne, high resolution, load tracking and structural health monitoring system for unmanned aerial vehicles is
presented. The system is based on embedded optical fiber Bragg sensors interrogated in real time during flight at 2.5
kHz. By analyzing the recorded vibration signature it is now possible to identify and trace the dynamic response of an
airborne structure and track its loads.
G. Tsamasphyros, K. Kalkanis, G. Kanderakis, N. Pantelelis, M. Tur, Y. Botsev, N. Gorbatov, G. Maistros, M. Kazilas, S. Ronen, U. Ben-Simon, I. Kressel, A. Green, R. Chemama, E. Inglisian, F. Voelkermeyer, F. Fischer
The combined use of Bragg grating and dielectric sensors has been studied for the monitoring of bonded composite
repairs to metallic parts, using standard composites field repair equipment and specially built "sensing mats"
Measurements retrieved via a variety of sensors were correlated, to analyze the interactions of the chemical and physical
changes occurring during the bonding process. It was concluded that the combined use of Bragg gratings and dielectric
sensors can provide important information for the cure monitoring of bonded composite repairs.
The potential for using fiber Bragg grating rosettes for the location of ultrasound sources in anisotropic structures is
discussed. Anisotropic propagation of Lamb waves in a carbon fiber composite plate has been investigated using two
approaches. Firstly, a finite element model of the displacements/strains produced by the wave was developed and,
secondly, in-plane ultrasonic strain mapping was carried out, utilizing the directional properties of FBG sensors. The
possibility of designing FBG rosette configurations for use on anisotropic structures is discussed in light of these results.
In previous work we have described the detection and location of damage in isotropic materials using fibre Bragg
gratings rosettes to directionally detect Lamb waves. To extend this technique to composite materials it is necessary to
understand the propagation characteristics of ultrasound in these materials as a function of their orientation with respect
to the ply, and also the directional response of fibre Bragg gratings to them. Finite element modeling of Lamb wave
propagation in a 0°, 90° carbon fibre plate is described, as are experiments to detect these waves for various orientations
of the source and alignments of the FBG transducers. Results of the experiments are interpreted with respect to
predictions from the FE modeling and are shown to give good qualitative agreement.
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