Embedding sensors for clever composites

Dries Van Thourhout is a Professor at Ghent University and senior researcher at IMEC (a Belgium research institute), which was the co-ordinating partner in the 'SmartFiber' project – a project that intended to use existing fibre optic sensor technology to test the condition of a material. Van Thourhout explained: "The goal was to make a miniature interrogator system including the methods to embed them in composite structures." This is important for composites used, for example, in wind turbines. Van Thourhout said: "The people who design these structures do not know exactly when they will break so they overdesign them. In general, therefore, they are too heavy or perhaps wind turbines are never run at full capacity because they are afraid that they will break. At the moment there is no good way to read out over a long period of time if there is breakage or damage to the blade." Fibre Bragg Gating (FBG) sensors, the actual data collection device in the SmartFiber project, are an established technology. FBGs are created by modifying an optical cable with UV light. When light from a broadband light source is coupled into a FBGs fibre, only a narrow spectrum of frequencies of the input light is reflected: the central wavelength of this spectrum is commonly called the Bragg wavelength. The light that is not reflected by the Bragg grating further propagates through the fibre. A mechanical strain applied to the sensor will result in a change of the period of the grating and also of the refractive index in its proximity. The resulting relative shift in the Bragg wavelength, is proportional to the applied strain and thus can be used as a measure for it. However, while embedding 125µm fibre optics in composites had been done before, even something this small could have a negative impact on strength and durability. To obviate this, FBGS Technologies, one of the partners in the project, developed a draw tower fibre Bragg grating (DTG) with a 'cladded' diameter of just 60µm, which researchers found to be as strong in pull tests as commercially available fibres. While sensors have previously been hand placed in composites before, a robot platform was used as the basis for an automated system in the project. A dedicated optical fibre placement head was designed and built for mounting onto the robot arm, with a view to providing a complete manufacturing solution for a composite part in the future. The main part of the project involved the development of the embedded read-out unit. The core of the interrogator is a photonic integrated circuit (PIC) fabricated in a Silicon Photonics platform. The signal transmitted by the PIC, is picked up by means of an array of photo-detectors, and then the resulting electrical signal is processed before being transmitted to the external read-out unit through the wireless channel. The external read-out unit, in addition, provides the wireless power supply to the embedded interrogator. The 'optical engine' block is the core of the interrogator, where the optical signal reflected by the FBG is transformed into an electrical signal that, after further processing, will allow calculation of the position of the FBG peak. The optical engine includes the pigtailed PIC, the detectors and the read-out circuit (ROIC). The light source (SLED), the detectors and a set of sensors and actuators integrated on board, are controlled by a Complex Programmable Logic Device (CPLD). The interface to the outside world is provided by the wireless sub-module – a double inductive coil – which provides both data and power transmission. The embedded interrogator unit is 10cm in diameter. Van Thourhout said: "It is UFO shaped because it is designed to have little impact on the composite. What is important is not the diameter but the thickness, we had to keep it as thin as possible. The height is now 7mm." Such a size could still have an impact on the structural integrity of the composite admitted Van Thourhout: "An action point would be to further reduce the height. There are some obvious ways to reduce it to about half, but if you want to go really thin, like 1mm, you need to look at unpackaged chip sensors, which are determined more by volume. If you have sufficiently large volumes you can make it smaller, but if it is for limited volumes you can't afford to work with unpackaged chips. That is the trade off we are struggling with right now." Although not designed for a particular material, performance of the whole system is not consistent in both glass fibre based composites and composites based on carbon fibre – in the latter sensing still works but it is much more difficult to get the wireless signals through. Van Thourhout said: "Fraunhofer does have some indication that at lower frequencies it should work, but that is something that needs to be worked on further. Within the glass it certainly works, but even there the curing temperature can vary a lot. The composites we used had a relatively low curing temperature, around 100°C. Some composites use a higher curing temperature and that can be a limitation on the electronics inside." Would such a system ultimately be adopted in all applications that require such monitoring, or would it just be used as a design tool during prototyping and development? "It all depends on the price," said Van Thourhout. "At the moment I think it will be for designing new composite structures. In the project we looked at tidal turbines which is new and at the development stage. People are looking at developing new shapes of blades and ways of monitoring these new blades. So I think in the first place it will be employed to help in the design procedure but ideally in the end we want it in every blade or every large composite structure." The SmartFiber project partners The sensor system was assembled by Optocap on a printed circuit board designed by Xenics. The optical subsystem consists of a silicon photonics integrated circuit developed by IMEC and photodiodes and read-out integrated circuits (ICs) provided by Xenics. Fraunhofer IIS was responsible for the wireless interface. It provides power to the embedded system and at the same time reads out the acquired data at high speed. After connecting the system to an optical fibre sensor chain manufactured by FBGS International, it was casted in an epoxy shape specifically designed by Ghent University to minimise the impact on the composite material. Finally, together with the attached fibre sensor chain it was embedded in the blade of a tidal turbine by Airborne.