High-sensitivity bend angle measurements using optical fiber gratings

Load monitoring of a cantilever plate by a novel multimodal fibre optic sensing configuration

Abstract

Optical fibre sensors and in particular fibre Bragg gratings (FBG) have received a lot of interest for Structural Health Monitoring in different application fields, such as aerospace, pipeline and civil engineering. FBGs are conventionally used to monitor strain and sometimes temperature. In this paper, we propose a new method for load monitoring of a cantilever plate subjected to point loading. The bending of plate is complex due to the interaction between the axial and transverse bending stiffnesses of the material. We use a novel algorithm for interrogating fibre Bragg grating sensors based on both hybrid interferometry and FBG spectral sensing. The method is demonstrated in this paper using a single-mode optical fibre containing four FBG sensors to estimate both the point loading position and the loading magnitude at an arbitrary location on a 1 m$$^{2}$$ 2 cantilever plate. The algorithm first utilizes point strain information through spectral sensing as well as strain from interferometric sensing over a long path. The gratings are interrogated using Wavelength Division Multiplexing (WDM). We calibrated the system using an experimental model. This model was then verified by using single point static loading tests and comparing the calculated sensing position with the actual position. The method achieved a good estimation of loading position achieving a measurement error of about 9% in a 2D plane. The analysis discusses the possible sources of inaccuracies. This study forms the basis of our future work involving morphing smart-wing sections for the purpose of load monitoring.

Article highlights

Quantitative phase imaging of fiber Bragg gratings in multicore fibers

Fiber Bragg gratings (FBGs) and multi-core fibers (MCFs) have independently demonstrated high levels of performance in numerous diverse applications. When integrated together, the devices can offer enhanced performance as well as open more applications. In all of these cases, the refractive index (RI) characterization of the FBGs is crucial in monitoring and validating the fabricated devices. To accomplish this, quantitative phase imaging (QPI) is a promising RI characterization candidate satisfying all of the requirements: noninvasive measurement, quantitative characterization, sub-micrometer resolution, no a priori knowledge, and 3D reconstruction. In this paper, we propose a new QPI method for characterization of the RI distribution of multiple FBGs in a single MCF. We have identified the key challenges associated with this approach: the pixel integration effect, aliasing effect, numerical aperture requirement, and characteristic functions recovery. We have further identified approaches for overcoming each of these challenges that have previously impeded this direction of research. The proposed method is supported by simulations of 2D and 3D gratings.