Interview by Susan Kraemer

Q: Could you tell readers the objectives of the business for the next 12 – 18 months and your role in the company?

DJ: As an OEM, we supply optical sensing fibres to companies developing measurement solutions in a wide variety of industries. In the case of the wind industry, our customers are companies that typically develop and offer complete monitoring solutions to the wind industry either for pitch control or Structural Health Monitoring (SHM) of the blade structures.

We have customers who have developed sensors for retrofit blade monitoring (mounted on the inner surface of the blade), as well as those who use the fibre to measure the intrinsic strain (embedded into the composite blade structure). They have developed solutions using our FO sensors, in preference to traditional electrical technologies for a combination of reasons. Some of these are high strain level measurements, high fatigue resistance, immunity to lightning, etc. Often standard products are used, but sometimes special sensing fibres or bespoke interrogators are developed for customers.

Whilst the application of this technology in the wind industry is relatively recent, it has been used for strain monitoring for many years. We fully expect that the lessons gained from these applications will make its introduction into the wind industry relatively smooth. The motivation to gather more intelligence from the turbine structure comes from different sources and there exists a fine balance between those wanting to remove cost from the structure and others wanting to improve performance.

For example, insurance companies would like to better understand the measure of risk and perhaps cap the applied loads on the blades or impose restrictions on other operating conditions. Operators would like to increase the effectiveness of units in low wind areas, necessitating longer blades, which asks even more of their structural performance.

Over the next 12 – 18 months customers will continue to conduct trials of their products that incorporate our unique fibre sensors in very inhospitable environments around the world, to prove and validate their solutions. We anticipate further design and development of our own bespoke systems, as well as a general increase in the number of companies offering this type of solution to the wind industry, as the take-up of this technology increases.

As marketing and sales manager of FBGS, I am responsible for supporting customers and distributors and identifying new opportunities for our products. The wind industry is very exciting for us, as the particular monitoring requirement in this industry is a perfect fit with the capabilities of fibre optic sensing in general and our unique sensing fibres specifically.

Q: What are the advantages of using fibre optics over the current hydraulic or electrical sensors in the wind industry?

DJ: Fibre optical sensors offer several significant advantages over conventional sensors, the most important of which are:

• They show no interference with electromagnetic radiation, so they can function in harsh environments where conventional sensors would fail and are also less prone to the effects of lightning.

• Fibre optic strain sensors can measure much higher strain values (> 1% strain) for millions of cycles and also exhibit less drift compared to electrical strain sensors.

• Due to the small diameter of the optical fibre, FO sensors can be integrated into the composite material for structural monitoring purposes.

• There is a possibility to multiplex many sensors in one optical fibre, reducing the mass and complexity of sophisticated sensor networks and as such driving down the cost.

• Furthermore, the optical fibre can transmit the measurement signal over very large distances (kilometres) without the need of any amplification and achieves this without a significant decay in the measurement performance.

Q: How are fibre optic sensors for the wind industry typically created, currently?

DJ: The process steps for the standard FBG inscription method are shown in Figure 1 1. As input for the process, a coated fibre with a photosensitive core is needed.

Two different possibilities exist. The first is to use a high-doped fibre. The second is to use a standard telecom fibre such as ‘Corning SMF-28’. This fibre has very low photo sensitivity, but can be increased by hydrogenating the fibre prior to FBG writing. This is achieved by placing the fibre in a hydrogen atmosphere for several hours at 150-200 bar. After the annealing (later in the process) the hydrogen is diffused out and the induced defects (refraction index changes) remain.

Figure 1 1: Process steps standard FBG inscription.

In a first step, the coating of the fibre is removed, see Figure 1 2. This is necessary in order not to shield the UV light during the writing of the FBG in the core. Different process technologies are possible: mechanical, chemical, heating or laser removal. It should be noted that the stripping process reduces the mechanical strength of the fibre.

Figure 1 2: Step 1-Coating removal.

Once the coating has been removed, a UV exposure will be performed in order to write the FBG in the core of the fibre, see Figure 1 3. The UV pattern can be applied using an interferometric set-up.

Figure 1 3: Step 3 – UV exposure.

The next step is annealing. Due to thermal depopulation of trapped states the reflectivity of the grating initially shows a reduction. In order to stabilise the grating reflectivity, the FBG is annealed by heating and holding it at an elevated temperature for several hours.

Finally, the fibre will be recoated to protect it, see Figure 1 4. Different materials can be used as recoating material: Acrylate, Polyimde. The coating can be applied using a thermal or UV-curing process.

Figure 1 4: Step 5 – Recoating.

Q: How does the draw tower technique differ, and what are the advantages for the wind industry?

DJ: Draw Tower Gratings (DTG®s) are produced using a process that combines the drawing of the optical fibre with the writing of the grating (see fig below)

The input of the process is a glass pre-form. After heating the pre-form, the pulling and formation of the fibre is initiated.

In order to write the grating, further in the production process the fibre crosses an optical axis of a laser and interferometer. These are used to create a periodical UV-light interference pattern. Using a pulse selector and taking into account the draw speed, FBGs 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 reeled onto a spool. This process of simultaneously drawing the fibre, writing the grating and coating the fiber directly after the grating inscription, results in high strength grating chains.

As such, the commonly used stripping and recoating process of standard FBGs is not necessary and the pristine fibre strength is maintained during the DTG® manufacturing process, which results in increased longevity of the sensor under loading conditions.

Q: What are the common concerns or hesitations, if any, about integrating the new fibre optic system by wind energy OEMs, farm operators or developers on existing or future projects? And how, over time, do you believe they will be more willing to adapt to the technology?

DJ: As with the introduction of any new technology, there are often concerns about its reliability over the long term. However, in the case of FO sensing solutions in the wind industry, extended laboratory and field validation tests have already been successfully executed by many of our customers.

The uptake of the technology is therefore more about convincing the wind farm operators of the savings that FO monitoring systems can bring them. However, as there is already a strong growing confidence in the market for fibre optic sensing in general, we believe that a rapid take-up of the technology will take place over the coming years.

Q: What type of product and/or service warranty will the product (s) have (typical benefits, lifecycle, replacement costs)?

DJ: Due to the unique production process of FBGS, our fibre sensors show an extreme high mechanical stability, which guarantees a lifetime greater than the life cycle of a wind turbine (> 20 years). As the fibre can be embedded within the composite material itself, this could serve to provide even greater protection to the glass of the sensors.

Our optical sensing fibres are hence not considered to be a critical element in the monitoring chain and require no maintenance nor replacement when installed properly.

Of course, the total monitoring solution as designed and offered by our customers (containing the measurement device, sensor packaging, data processing unit, etc.,) might of course be subject to certain maintenance activities, which are customer/solution specific.

Q: What feedback have you had to date by industry players?

DJ: Our customers have achieved the results that they have partly through good working relationships with us, but mostly due to the attributes of our draw tower fibre. Because these applications require high repetitive strain levels, often a small bending radius and high mechanical strength, it is highly unlikely that this performance can be matched by any other (FBG) technology.

The feedback has been overwhelmingly positive and in many cases customers have been surprised by the performance of our fibre. This is of course very encouraging for us and we are keen to continue to promote the attributes of draw tower fibre into this industry.

David Johnson is the Sales and Marketing Manager (Europe) for FBGS, a developer of fibre optic sensors and sensing systems. He has more than 20 years’ experience in the sales and marketing of measurement sensors and instrumentation in South Africa and Europe. He has worked with many industries involved in material testing, product development, manufacturing and quality assurance.