INVESTIGATION OF THE EFFECT OF BONDING POINTS ON METAL SURFACE-MOUNTED FBG SENSORS FOR ELECTRIC MACHINES

Fibre Bragg Gratings (FBGs) offer several advantages including their immunity to electromagnetic fields making them excellent in situ sensors for feature extraction in electrical machines condition monitoring. However, the pre-requisite of bonding FBGs circumferentially on either the machine cast frame or stator windings can introduce undesired sensing characteristics. This is because the FBG relies on adhesives as the transfer medium for any sensed parameter between the machine and sensor. Whilst FBG sensors rely mainly on wavelength shift, an intolerably low signal-to-noise ratio will result in difficulty in measuring such shifts. As a complementary signature, differential optical power can be combined with wavelength shift to broaden the feature extraction capability of FBG sensors. This makes power level (dBm) an important sensing parameter for FBG sensors. The effect of varying number of bonding points on transmitted optical power is investigated using unstripped and stripped bare fibres as well as an actual FBG sensor. Increasing the number of bonding points beyond an optimum number has been observed to significantly attenuate the optical signal power level and quality for a given dynamic range. Hence, as the number of bonding points is increased, the level of attenuation should be closely monitored to ensure that the optimum number is not exceeded if excellent and accurate FBG sensing characteristics are to be realised.