IoT Node Interrogation System for Fiber Bragg Grating Sensors: Design, Characterization, and On-Field Test

This article describes the design, characterization, development, and preindustrialization of a novel interrogation system for fiber Bragg grating (FBG) sensors capable of taking advantage of the Internet-of-Things (IoT) technology. Due to its complexity and cost, classical interrogation systems limit the design and use of a sensor network based on fiber-optic technology that is often replaced with less performant electronic sensor systems. The proposed system has the potential to overcome the limits of today's commercial interrogators, converting the classic FBG interrogator into an IoT node, where each circuit is represented by a sensory node that continuously exchanges and publishes data on the cloud. Moreover, the proposed concept may be applied in harsh environments, still taking advantage of the fiber-optic technology. The interrogator is composed of a passive optical device section and a four-layer electronic board section (one board for each connected FBG in the external array), which conditions the optical signal to a compliant voltage at the downstream analog-to-digital converter (ADC). In the subsequent digital processing section, created inside an ARM-based board, the conversions are then carried out through dedicated algorithms to match the voltage variation to the Bragg wavelength shift and the subsequent conversion into the relative physical quantity (temperature, deformation, or humidity) via a subsequent conversion algorithm. Finally, physical values, in real-time, are published on a remotely accessible server. The proposed system was tested and validated by comparing data produced by commercial interrogations. The tests were performed using an array composed of six independent FBG sensors. It was tested through stability and on-field measurements, detecting one temperature-compensated strain point and four temperature-monitoring points.