A comparison of two data acquisition techniques for modal strain identification from sub-microstrain FBG data

Abstract

The identification of changes in the dynamic characteristics which are related to structural damage is the core principle of the vibration-based structural health monitoring (VBSHM). By addressing these changes, engineers can acquire valuable information about the condition of structures. Eigenfrequencies are influenced by environmental factors and this influence can be high enough to completely mask the effect of damage while they are also global structural characteristics and they cannot asses the location of the damage. A promising alternative is the monitoring of modal strains which offers the benefit of high sensitivity to local damage. However, current measurement techniques encounter difficulties in capturing the very small strain (sub-microstrain) levels that occur during ambient or operational excitation with sufficient accuracy and precision. This research aims to accurately identify the modal strains of a steel I-beam from sub-microstrain data using Fiber-optic Bragg Grating (FBG) sensors. For this purpose, two measurement techniques are compared. In the first, an interrogator that measures all sensors simultaneously and saves the wavelength spectra for further processing is used. A novel signal processing algorithm that identifies the Bragg wavelength shift from low-resolution data with high accuracy and precision is used for processing the spectral data into strain values. In the second, an interrogator that scans each sensor individually with a high-speed sweep and therefore offers a high wavelength resolution is used. The peak-shifts from the reflected wavelength are identified with a built-in peak detection algorithm and subsequently converted into strains. The phase shift that is created between the various FBG sensors due to the sweep is resolved by implementing a novel synchronization technique. The accuracy of both techniques in identifying the beam’s dynamic characteristics is investigated through an experimental modal analysis. Two chains of multiplexed FBGs directly glued to the flanges of the beam are used as sensors. The beam is excited at low force amplitudes to explore the ability to identify dynamic characteristics from very low Signal-to-Noise-Ratio (SNR) data. Finally, the accuracy of the identified strain mode shapes from the two techniques is assessed by comparison with a validated finite element model.