Sensors and transducers based on fibre Bragg gratings (FBGs) have numerous significant advantages over more conventional electrical sensor technologies:
Tolerant of Harsh Environments
FBG sensors and transducers are completely passive and use no electronic components. As a result they are capable of operating under extreme operating temperatures from cryogenic to several hundred degrees centigrade and offer long-term survivability in the most extreme environments where electronic sensors and gauges are unable to operate.
Another benefit of the passive nature of FBG sensors is that they are totally immune to interference from electrostatic, electromagnetic or radio frequency sources, allowing them to be installed in locations with severe electrical noise such as power generation and transmission plant. Furthermore, being zero power, they are 100% intrinsically safe and can be used in the most hazardous explosive environments
Optical fiber is a very efficient signal carrier. Because of this, the electrical interrogation unit can be sited many tens on Kms away from the sensing location, whereas conventional electrical strain gauge systems require regular amplification to avoid signal to noise degradation. For monitoring long, remote structures such as wellholes, risers, pipelines or tunnels, this is a unique and invaluable benefit. Optical sensors are immune from down-lead effects and, since the measurand in a FBG sensing system is wavelength which is unaffected by signal attenuation, it is not possible for the value of a remote sensor to be corrupted whilst being transmitted along a long fiber.
Another significant benefit that FBG sensors offer for remote monitoring is their stability over time. Being a passive sensor, a FBG has zero drift and can be used for many years with no need for recalibration. Indeed, it is practical to attach sensors to a structure and return with an instrument to interrogate the sensors every few years to get a true picture of any structural movement since the last reading. This further increases the economic advantage of the technology since one single interrogation unit can service numerous structures
The fiber into which FBGs are recorded is tiny, just 0.15 mm or so in diameter. The means that many sensors can be applied to a structure with very little intrusion. Uniquely, a fiber sensor array can be embedded inside a composite to monitor internal strain, temperature and damage with no effect on the structural performance of the composite.
Many tens of FBGs can be written into one optical fiber, and several hundred can be simultaneously interrogated by one multi-channel instrument. This provides a very low-cost mechanism for densely instrumenting even very large structures, when compared with technologies where every sensor has a dedicated channel. Furthermore, optical fiber is smaller and much lighter than electrical wire and, together with this multiplexing capability, extensive FBG sensor installations can be made that were hitherto impossible in certain applications due to cable mass and volume.
Tests with carbon fiber coupons have shown that embedded fiber sensors show no signs of fatigue or disbonding after one million cycles. Similar tests with glass fiber materials will demonstrate that embedded sensors within wind turbine blades for instance will survive the 25-year service life of the blades themselves. For surface mounted applications, optical fiber sensors are less prone to disbonding and are far more resilient to moisture and chemicals than most electrical gauge technologies.
Ease and Cost of Installation
Consider installing a large number of conventional electrical strain gauges. Each gauge needs to be bonded to the structure under test and then the bond pads associated with each gauge need to be bonded. Solder joints then need to be made in-situ between each gauge and its associated bond pads. Then electrical wires need to be soldered in-situ to all of the bond pads and then routed and secured back to the bank of instruments. Finally, the electrical bridge associated with each gauge needs to be balanced before measurements can commence.
By comparison, the structure can be instrumented by hundreds of FBG strain sensors simply by bonding a few optical fibres to the structure, connecting them to a single FBG interrogator and pressing a single button to take a strain array reference that is valid for all future readings.
Bearing in mind that the installation engineers are skilled labour, and access to certain structures is difficult and expensive, the cost and time savings available through an optical fiber installation are clearly significant.
Smart Fibres have been working with FBGs for many years and have developed some novel solutions for applying them to all manner of structures. Details of these Smart FBG sensors are to be found in our Products pages.