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Monitoring loadbearing structures of large scale public facilities with fiber optic sensors

There are plenty of different fiber optic applications of FBG sensors; it can monitor a variety of structures, buildings, and other facilities. This technology is often used in construction work and to track different parameters in industrial production and other areas.

Fiber optic applications include effective monitoring of loadbearing structures in different public facilities. Most of the modern construction sites are technically sophisticated and uniquely designed. In the process of project development, certain safety measures are planned, however, it may change in time after the building utilization has started. Regular monitoring becomes crucial in this context and it may prevent different accidents. The automated monitoring fiber optic system allows tracking the technical condition of the object in real-time with minimal human involvement.

Fiber optic Bragg gratings (FBGs) are used as a sensing element in this monitoring fiber optic system. This kind of sensors doesn’t have electronic components and it makes them resistant to electromagnetic waves and fire safe. Fiber optic Bragg gratings are inscribed in the fiber with a UV laser. FBG in each sensor reflects the specific fiber optic wavelength with a spectrum width of approximately 1 nm. The grating is affected by mechanical and temperature impact and its physical change cause the shift in the fiber optic wavelength of the reflected light. The fiber optic wavelength shift is measured, which allows measuring the deformation and temperature change. Two gratings are used simultaneously to separate these parameters, where one of them is isolated from the mechanical impact. This allows tracking the temperature impact on the second grating and measuring the deformation. One optic fiber may have multiple gratings, each one of them provides a response at its own length, and the distance between gratings vary from 10 mm to several kilometers.

One of the most common methods to investigate sensors is to scan the FBG aggregate with a tunable source. The tunability range is usually around 100 nm (1500 – 1600 nm), the investigation frequency is from 1 to 500 Hz, and the number of parallel channels can be up to 8 channels.

Usually, it is enough to interrogate sensors once per 6 hours, when they are installed on a large public facility. Each sensor has 2 critical measurement levels (yellow and red), when this level is reached the optic fiber system sends out an alarm signal. The frequency of interrogation increases to 1 time per 30 minutes. Moreover, the notifications are sent out in case of malfunctioning. The software allows viewing the data collected from all the sensors as the graphs of relative deformation and temperature.

This fiber optic application of FBG sensors works perfectly well for large constructions, such as stadiums.