System Development
4nm
4nm
1,512
1,516
2nm
2nm
1540
1542
2nm
1,580
2nm
1,582
Figure 3
Each FBG optical sensor in an array must occupy a unique spectral range.
temperature sensors to ensure the FBG inside the package is not coupled to any bending, tension, compression, or torsion forces. FBG strain sensors are somewhat more complex because both temperature and strain inf luence the sensor’s ref lected wavelength. For proper strain measurements, you must compensate for the temperature effects on the FBG, similar to conventional electrical foil strain gages. You can achieve this by installing an FBG temperature sensor in close thermal contact with the FBG strain sensor and subtracting the FBG temperature sensor wavelength shift from the FBG strain sensor wavelength shift. The selection and use of FBG optical sensors is simple, with an installation process almost identical to the established methods originally created for electrical sensors. The installation of these FBG optical sensors is equally simple and often made easier when compared to traditional electrical sensors: one can glue, weld and embed FBG sensors while having fewer cables to manage and no noise, isolation and/ or shielding considerations to address. Also a variety of types of optical cables exist, from simple low-cost to rugged deep sea cables, meeting the most extreme and stringent requirements.
Benefits and Use of FBG Optical Sensing FBG optical sensors are non-conductive, electrically passive, immune to EMI, lightweight and non-corrosive. This translates to the ability to
perform sensor measurements near high voltages, high sources of electromagnetic interference and in explosive environments. Also, contrary to technologies like electrical foil strain gages, the behavior of optical fibers is very stable over time, making it an excellent option for long-term SHM, where correlating measurements over years and decades are required. FBG optical sensing systems can also interrogate sensors over long distances with the use of powerful lasers and low-loss
fibers arrays. With an industry-leading OSI like the NI PXIe-4844, one can achieve a temperature resolution of around 0.1°C, strain resolution of around 0.7 microstrain, and interrogate sensors over 10 km away from the measurement system. In civil infrastructure and geotechnical military applications, sensors are often deployed over extremely long distances, exposed to harsh environmental conditions, and are vulnerable to lightning. The ability to daisy chain multiple sensors on a single 10+ km fiber greatly reduces the complexity of the system. Furthermore, optical fiber does not corrode or conduct like copper wire, which increases longevity and reduces the risk of damage due to lightning. And unlike electrical sensing systems, interrogators like the NI PXIe-4844 (equipped with a NIST-traceable gas cell reference) never require external calibration, a great benefit for long-term system deployments. For vehicles and mechanical structures, external noise and weight
Figure 4
FBG optical sensing makes sense on military aircraft because they’re immune to EMIinduced noise and allow system designers to avoid adding dozens of strain gages with four copper wires per gage—which adds a lot of weight. August 2011 COTS Journal [ 51 ]