Fibre Bragg grating sensors (FBG) for structural health monitoring has gained increasing importance in aerospace applications, since it enables large-scale measurement of most relevant structural parameters while mitigating well-known technical constrains of conventional sensors.
The main drawback of a fibre Bragg grating strain sensor is its thermal cross-sensitivity. Currently such a single parameter measurement is difficult to implement, since cross-sensitivity to temperature compels the use of an additional temperature reference.
In this project a passive athermal FBG strain gage that renders optional the measurement of temperature was proposed. Such a design benefits large scale system design and performance. The innovative design ensured a thermal operation of the strain gage by cancelling the intrinsic fibre optic thermal sensitivity. Moreover, the passive athermal design may be adjusted to further compensate for structural thermal expansion, thus enabling stress and load-induced strain-components to be measured.
Special care was taken on the design of the sensor enclosure to enable multiplexing of several sensors over a single optical fibre and ease installation procedures in aerospace applications. Qualified space fibre optic cables for sensing network deployment were employed.
Commercially available industrial interrogation unit equipment was taken as a base to evaluate the optoelectronic hardware adaptation that would be required in order to fulfil aerospace specifications. The design requirements were assessed in terms of mechanical (mass, volume, vibration and shock), thermal (heat dissipation and operation temperature range) and electrical parameters (power consumption and communication interface).
Embedded software allowed for data conversion from wavelength measurements to engineering parameters (strain, temperature, load) that afterwards was processed considering SHM requirements to provide automatic alarm generation.