Fibre Optic Sensors
based on Fibre Bragg Gratings
Fibre Bragg Draw Tower Grating Technology for use in a machine 'Nervous System'
As in many current fields of engineering investigation, attempts to replicate attributes of the animal world to better understand or design for the mechanical world includes the development of a measurement 'nervous system'.
Monitoring of signals from deep inside a structure offers many practical advantages to traditional sensing techniques. However once the sensor is sealed or the system deployed, the benefits of this process can only be realised if the measuring system is sufficiently robust to endure a harsh environment for many years. Once strictly the domain of popular science fiction, today fibre optic sensor technology makes this possible.
Fibre optic sensors for strain/vibration and temperature based on Fibre Bragg Gratings (FBG) offer several crucial advantages to traditional electric sensor technologies. Being robust and corrosion-free they remain unaffected by electrical environments and they permit optical transmission of sensor signals over large distances. With these properties, fibre optic sensors are well suited for safety-critical areas from mining through renewable energies, to aerospace and have proved themselves under the most difficult environmental conditions.
Fibre optic sensors of the FBG type transpose temperature, strain and vibration into a spectral selective reflection of the in-coupled infrared light. Due to their hair-thin dimensions they can easily be embedded within modern composite materials. Additionally their ability to multiplex many different sensors on one common fibre, saves a considerable amount of space, but also drives down the cost of traditional complex measurement networks.
The opportunity to deploy sensing earlier in the manufacturing value-chain may offer equipment manufacturers better know how and understanding of their systems or equipment and also greater say in the selection of test & measurement platforms. Making data available sooner will allow certain industries the opportunity to monitor during curing (of composites), construction or commission which may offer insights into the performance of materials and processes.
This technology is very relevant for material science in general and especially for the composite research community. Embedded FBG sensor technology is capable of providing very useful information on residual strains within a material and the structural integrity of components during service. It is no secret that aircraft manufacturers are looking at incorporating structural health monitoring (SHM) technology into new generation airplanes. However it is also suitable for investigation and evaluation of wind turbine blades, oil and gas pipelines and boat hulls etc.
As opportunities to extend the sensing network into our designs' become more common, like the central nervous system they mimic, ultimately they will become intrinsically linked with the machine. This means their ability to survive in their host for many months or years becomes paramount and the emphasis has unsurprisingly moved to the longevity of such a system.
As self-healing properties are currently beyond the scope of even this technology, should there be a problem once sealed-for-life in an inaccessible or remotely deployed structure, any investment in this process would be lost. The performance yardstick might be; transmission of reliable data from within a machine or structure, operating at the extremes of its design parameters for years to come. As such the draw tower technology means that higher quality coated draw tower fibre Bragg gratings (DTG's) can be produced that are less fragile than conventional FBG's. This type of FBG is written in highly GeO2 doped (photosensitive) silica fibre using one single laser shot during the drawing of the optical fibre. Because of the high doping concentration, the bending loss of the resultant DTG fibre is extremely low and is also less sensitive to transversal effects. This offers some unique properties that makes' them the preferred product for this type of embedded application.
The traditional method of manufacturing optical sensors of the Fibre Bragg type usually starts with a fully drawn and coated fibre - the coating of which must first must be removed. This is a manual process that cannot exclude damage to the fibre, or at least diminishing its mechanical stability. Thereafter, at the intended measurement point, the fibre is exposed to ultraviolet laser light. This exposure produces an interference pattern on the fibre, which results in periodic areas of high and low refractive index, acting as a wavelength selective mirror. After this process the fibre is recoated at the exposed locations.
To avoid damage to the fibre but still benefit from an automated process, a draw tower is used to combine the drawing of the optical fibre with the writing of the grating. The input of the process is a glass pre-form which is pulled from beneath and forms the fibre. Further in the production process, the fibre crosses the optical axis of a laser where the grating is written. Using a pulse selector and whilst monitoring the draw speed, FBG's can be accurately positioned in the fibre. When the grating has been written, the fibre is coated by entering a coating reservoir, followed by a curing step of the coating. Finally the location of the FBG is marked automatically and the fibre is coiled onto a reel.
The resultant Draw Tower Grating offers a number of very significant advantages over conventional FBG's, most notably:
Extremely high mechanical strength compared to conventional gratings - more than 5 times higher.
Draw Tower Grating technology permits spliceless grating chains to be produced with a high number of sensor elements.
The ORMOCER® coating material allows them to be used in a wide temperature range of between -180°C and +200°C.
This coating offers excellent adhesion properties with fibre glass, which means they can be applied directly to structures without coating removal.
The coating is uniform along the complete fibre length, even at the FBG position.
As they are fabricated using an automated production process, a very high repeatability and quality is achieved.
This process of simultaneously drawing the fibre and writing the grating results in high strength grating chains. The fibre coating is applied directly after the grating inscription. As such, the commonly used stripping and recoating process of standard FBGs is not necessary and the pristine fibre integrity is maintained during the DTG® manufacturing process.
At the grating inscription point the fibre already has its final diameter; it is also clean and has a mechanically error-free surface. Subsequent polymer coating largely protects the fibre against extrinsic impact and in applications such as strain sensors, a hard Ormocer coating (organic modified ceramic) is used. Thus, inscribing the FBG during the fibre draw process proves a very efficient method for producing FBG sensors of the highest mechanical stability. To distinguish them from traditionally produced FBG's, these sensors are named DTG (draw tower grating); and are trademarked by FBGS.
Whether used to determine strain on a simple machine axis, embedded deep within a composite material or attached to parts that will ultimately be used in deep sea or deep space, the DTG adds long term integrity and therefore increased confidence to this particular measurement process. Unlike the basic FBG device, the DTG chain/array can be engineered for many different sensor elements within a common fibre. As these are written into the fibre during the manufacturing process, they are each homogeneously coated and remain pristine even after mounting onto the surface of a structure.
If fibre optic sensors are to continue to carve out a niche for themselves amongst the very highest of the high-tech applications, where the sensing element is sealed for life and is to become an integral part of a machine, then only DTG's can imbue the confidence necessary to truly fit and forget.
Powering of an imbedded system is not an issue, as remotely powered supplies or energy harvesting techniques are already well established. The fibre optic sensors themselves are monitored using a variety of interrogator units depending on the application. Single or multichannel measurements are possible in either static or dynamic conditions. Within some of the interrogator units an embedded PC is supplied, making it possible to process the signals onboard the device. Via the software supplied (Illumisense Pro/Wave), further data acquisition and analysis can be achieved.
More information about FBGS International
FBGS is a Belgium based developer and manufacturer of high strength FBG sensors using draw tower technology, where the grating array is written during the drawing process before the fiber is coated. This automated process results in high quality low cost Fibre Bragg Gratings with unique characteristics. These are suitable for both standard and bespoke sensing solutions including applications within aerospace, oil & gas, renewable energy, process, composite & medical industries.
Author - David Johnson - Sales & Marketing Manager Europe
Mobile: +44(0)7918 744718
Bell Telephonelaan 2H - 2440 Geel - Belgium
Contact Belgium: +32 14 58 11 91
Fax: +32 14 59 15 14
Website : www.fbgs.com
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