Abstract
Strain measurements using fibre Bragg grating (FBG) optical sensors are becoming ever more commonplace. However, in some cases, these measurements can become corrupted by sudden jumps in the signal, which manifest as spikes or step-like offsets in the data. These jumps are caused by a defect in the FBG itself, which is referred to as peak-splitting. The effects of peak splitting artefacts on FBG strain measurements show similarities with an additive multi-level telegraph noise process, in which the amplitudes and occurrences of the jumps are related to fibre deformation states. Whenever it is not possible to re-assess the raw spectral data with advanced peak tracking software, other means for removing the jumps from the data have to be found. The two methods presented in this article are aimed at removing additive multi-level random telegraph noise (RTN) from the raw data. Both methods are based on denoising the sample wise difference signal using a combination of an outlier detection scheme followed by an outlier replacement step. Once the difference signal has been denoised, the cumulative sum is used to arrive back at a strain time series. Two methods will be demonstrated for reconstructing severely corrupted strain time series; the data for this verification has been collected from sub-soil strain measurements obtained from an operational offshore wind-turbine. The results show that the proposed methods can be used effectively to reconstruct the dynamic content of the corrupted strain time series. It has been illustrated that errors in the outlier replacements accumulate and can cause a quasi-static drift. A representative mean value and drift correction are proposed in terms of an optimization problem, which maximizes the overlap between the reconstruction and a subset of the raw data; whereas a high-pass filter is suggested to remove the quasi static drift if only the dynamic band of the signal is of interest.
Highlights
Introduction published maps and institutional affilStrain measurements using optical fibre Bragg grating (FBG) sensors are based on interpreting the reflected spectrum of a light pulse which is sent through an optical fibre; the reflection is caused whenever a light pulse encounters a periodic modulation of the refractive index etched into the optical fibre
It is chosen to perform a qualitative assessment of the reconstruction accuracy, since no peak splitting free counterparts to the presented signals are available—the latter would allow to perform a straightforward deterministic assessment of the reconstruction results
The results show that the reconstructed strain time series appear to show less peak splitting artefacts, and that outlying difference samples are detected
Summary
Strain measurements using optical fibre Bragg grating (FBG) sensors are based on interpreting the reflected spectrum of a light pulse which is sent through an optical fibre; the reflection is caused whenever a light pulse encounters a periodic modulation of the refractive index (grating) etched into the optical fibre. The reflected spectrum is narrowbanded and characterized by a single peak centred at the Bragg-wavelength, which is proportional to the grating period. Deforming the underlying specimen results in an elongation or compression of the grating and therewith in a shift of the wavelength of the reflected spectrum. The time history of the wavelength will resemble the strain loading of the underlying specimen [1]. Due to several failure modes, the reflected spectrum can be deformed, potentially resulting in two or more peaks of comparable amplitudes, which is referred to as peak-splitting. If reflected spectra containing peak splitting are interpreted using iations
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