Embedded fibre-optic monitoring system makes a “smart composite”
This achievement, say the researchers, paves the way toward smart composites that enable continued and automatic monitoring of the structural health of the composite material in – for example – tidal blades, wind turbines, aircraft (for example, wing spars or stressed skins) or marine structures (masts, antennae, hulls of yachts, navy ships, propellers).
The sensor system was assembled by Optocap on an electronic board designed by Xenics. The optical subsystem consists of a silicon photonics integrated circuit developed by imec and photodiodes and read-out 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 cast in an epoxy mould 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.
The silicon photonic integrated circuit, featuring an Arrayed Waveguide Grating (AWG) acting as a spectrometer, forms the core of the sensor system and allows interrogating the Fibre Bragg Grating (FBG) sensors connected to it with high accuracy. The advantages of these FBG sensors compared to other strain monitoring techniques (e.g. electrical strain gauges) are their compactness, light weight, immunity to electromagnetic interferences (EMI), high resistance to corrosion, high temperature operation, and multiplexing capability. The FBG sensors, which exhibit a record small diameter and show unprecedented elongation at breakage, were specifically designed to minimally impact the strength of the composite material. Automatic techniques were developed to embed them in the composite. This, together with the use of silicon-based microfabrication processes on a well-established industrial infrastructure, will enable lowering the cost of embedded strain sensors substantially.
The image shows the Interrogator (diameter 10cm) after embedding in protective epoxy shell with shape optimized for minimal impact on composite structural strength.
The photonics research group of UGent (and associated with imec) is developing novel photonic integrated circuits, mostly using the silicon photonics platform. The focus is on the development of new devices and their application in telecom, optical interconnects and sensors.