Strain sensor calibration is investigated using extrinsic Fabry-Pérot interferometric (EFPI) fiber optic sensors. The approach is based on the Bessel harmonic components from the nonlinear EFPI output signal under sinusoidal excitation. For known sensor parameters of optical wavelength and gauge length, the local maxima and minima of each Bessel harmonic profile occur for specific strain levels. The theoretical Bessel harmonic profiles are shown and a calibration approach is proposed using local maxima and minima in selected Bessel harmonics. The signal from a colocated companion strain sensor can be calibrated by comparison to salient EFPI reference strains. An experimental implementation is demonstrated for a polyvinylidene fluoride (PVDF) piezoelectric strain sensor that is colocated with an EFPI reference sensor on a cantilever beam apparatus. The experimental strains at the local maxima and minima closely match the strains determined theoretically.
A demodulation system employing neural networks is used to process the non-linear signal from an extrinsic Fabry-Perot interferometric (EFPI) sensor. A sinusoidal strain is theoretically shown to produce well-defined Bessel harmonics in the EFPI signal. The neural network demodulator (NND) uses a Fourier Series Neural Network to separate the Bessel harmonic components of the EFPI signal and a Back-Propagation Neural Network is used to predict the strain levels through the analysis of the Bessel harmonics. The NND is first simulated in a computer program and then actually employed in an experimental setting to determine the frequency response of a 25 cm composite cantilever beam. A function generator was used to drive a PZT actuator attached to the composite beam and resulting periodic strain was measured by the EFPI; the frequency of the composite beam was varied between 10 Hz and 900 Hz. The NND demodulated the EFPI signal and determined the frequency response of the composite beam. The results show that the NND accurately reproduced the natural frequencies and mode shapes of the cantilever beam.
A three element, 15.3 cm, fiber Bragg grating array (FBGA) operating at 1550 nm wavelength is fabricated using a single mode photosensitive fiber. The FBGA is initially simulated using in-house developed software based on the Transfer Matrix Method, then fabricated using a double frequency Argon laser and a phase mask technique, and interrogated using Optical Frequency Domain Reflectometry. A single fiber Bragg grating (FBG) is accurately strain calibrated using a Fabry-Perot interferometer and piezoelectric actuation. The piezoelectric is linearly ramped, and the shifts in the Bragg wavelength along with the fringe count from the Fabry-Perot interferometer are recorded. The fringe count is then used to determine the strain on the FBG and compared to changes in the Bragg wavelength in-order to calculate the strain gage factor. This result is used to calibrate the FBGA for strain measurements. The FBGA is then bonded to a cantilever beam with three electric strain gages attached next to each FBG in the array. The axial strain results obtained from the electric strain gages and FBGA are compared for various displacements of the cantilever beam. The Fabry-Perot interferometer and piezoelectric calibration method is a non-destructive process that eliminates the need to bond the FBG to an external support during the calibration process, and can also be used to calibrate electric strain gages.
An overall strategy in infrastructure health monitoring systems is given through the concept of Infrastructure Optics. The focus is to design and build optical devices and systems, primarily fiber optic communication technology, for health monitoring of infrastructure. Recent developments in the use of Optical Frequency Domain Reflectometry (OFDR) to demodulate Fiber Bragg Grating Arrays (FBGA) have shown promise in its use in infrastructure health monitoring systems. However, the number of papers on the simulation and characteristics of FBGA using OFDR demodulation for health monitoring purposes is not great. In this paper, a FBGA is simulated using OFDR demodulation technique to extract strain information from a simulated cantilever beam host. The structure is first simulated using a Finite Element Model (FEM) to determine displacement and strain response, and the results are used as inputs to the FBGA. An OFDR program then demodulates the array to extract the strain response of the cantilever beam. The characteristics of OFDR and FBGA system is analyzed and compared to actual FEM results.
The performance of sensor systems in smart structures is subject to orientation, installation, and environmental effects. The capability for initial and maintenance calibrations can provide added confidence in sensor information and facilitate investigations of long-term sensor behavior. The use of Extrinsic Fabry-Perot Interferometric (EFPI) fiber-optic sensors was examined for calibration of companion strain sensors. The output of the EFPI sensor is periodic with strain and the signal behavior with periodic strain displays well-defined harmonic content. In particular, the strains giving maximums and minimums in the harmonics can be calculated from the excitation wavelength and the EFPI gage length. A Polyvinylidene Fluoride (PVDF) piezoelectric strain sensor was surface mounted on a cantilever beam and its voltage-to-strain ratio was precisely calibrated using the accuracy of a co-located EFPI sensor. The experimental responses of both sensors were obtained for a periodic actuation. The PVDF output was calibrated using a linear-fit of the strains obtained from four points in the harmonic response of the reference EFPI sensor. The selected maximum and minimum points of the EFPI harmonics were directly observed with a spectrum analyzer. This fast, efficient approach was performed under resonant conditions with relatively inexpensive demodulation requirements. The results of the calibration compared well with the expected PVDF response.
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