Optical fibre strain sensors

Optical fibre-based sensors have drawn most researchers attention, due to their versatility and compatibility with structural composites. They can detect a number of parameters such as temperature, strain and pressure and can be embedded into composites without major changes in the current manufacturing processes. 

Those based on the fluorescence lifetime, rather than intensity (e.g. the Ipitek system), allow one to avoid problems of light loss and other factors that could affect calibration, as discussed above. Assessment of tilted Bragg gratings and long-period gratings on optical fibres has shown them to be a probe of cure of the resin as well as being both temperature- and strain-sensitive (Buggy et al, 2007). The complexity of the response of these and fibre-optics based Fabry-Perot interferometers to strain, temperature and refractive index makes it necessary to employ combinations of sensors if measurements of all of these properties are required separately. 

As mentioned earlier, optical fibres with Bragg gratings can also be applied as pressure and strain sensors. When these fibres are integrated into a mattress or bed sheet, they will be deformed as soon as pressure is exerted on them. The deformation will cause proportional changes in the Bragg wavelength and, hence, the movement can be tracked (Dziuda and Skibniewski, 2014). 

Body movement, posture Piezoresistive strain/ pressure sensors, accelerometers, gyroscopes, optical fibre sensors Body kinematics Dependent on motion to be analysed... 

Kuang KSC, Kenny R, Whelan MP, Cantwell WJ, ChaUcer PR. Residual strain measurement and impact response of optical fibre Bragg grating sensors in fibre metal laminates. Smart Mater Struct 2001 10(2) 338—46. http //dx.doi.org/10.1088/0964-1726/10/2/321. 

Wehrle, G., Nohama, P., Jose Kalinowski, H., Ignacio Torres, P., Valente, L.C.G., 2001. A fibre optic Bragg grating strain sensor for monitoring ventilatory movements. Measurement Science and Technology 12 (7), 805. 

Lau K T, Chan C C, Zhou L M and Jin W (2001a), Strain monitoring in composite-strengthened concrete structures using optical fibre sensors . Compos Part B - Eng, 32(1), 33-45. 

James, S.W. and Tatam, R.P. Optical fibre long-period grating sensors characteristics and application. Meas. Sci. Technol. 14(2003), pp. R49-R61 Farahi, F. Simultaneous Measurement of Strain and Temperature Using Fiber Grating Sensors. Proc. 11th Eng. Mechanics Conf. Fort Lauderdale, Conf. vol. 1 (1996), 351-354... 

Lloyd, G.D. et al. Re-configurable, multi-channel, high-speed FBG strain sensing system for vibration analysis in oil risers. 17th Intern. Conf. on Optical Fibre Sensors (OFS-17), Bruges, Belgium, SPIE-Vol. 5855 (2005) pp. 218-221... 

Therefore, if an FBGS is used as an embedded sensor, it is necessary to make a correction of the sensitivity factor with respect to the transverse principal strain. Otherwise, only when the transverse principal strain of the optic fibre is not sensitive to the host s strain field can the sensitivity factor be regarded as a constant. 

For the FBGS to be an ideal embedded strain sensor in a textile composite, the following conditions will have to be met (1) the integrated optic fibre has little effect on the host strain field (2) the axial strain of optic fibre can represent the nearby host strain in the optic fibre direction and (3) the effective Poisson s ratio is constant during the measurement period. The embedded-... 

Figure 10.6(h) shows the reverse trends of strain distribution in the host. The strain level of the middle host is lower than that of the host near the boundary, which is because the middle host is restricted more by the fibre and the host near the boundary can expand more freely under a thermal load. The normahzed strain values of the host are very close to 1 (0.995 < e < 1), which imphes that the host with the embedded fibre sensor has a strain field very close to that without the embedded optic fibre. 

Location and direction of sensors in composite hosts usually the FBG sensor is used to measure certain positional and directional normal strain in the host. However, in the procedure of embedding the FBG sensor into the composite host, some deviation of the optic fibre sensor may occur from the designed position and direction. To reduce the measurement error, determination of the real location of the grating of FBG sensors inside the composite may be a key issue, especially when the sensors are... 

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