Blade Modal Analysis by Means of Continuous Optical Fiber Sensors

A new method for blade modal analysis is introduced in this paper by using continuous optical fiber sensors and optical backscatter reflectometer technology. The main advantage is that the sensor is few invasive and does not affect substantially system parameters. Moreover, the optical fiber sensor can be embedded in composite blades, for instance directly woven in carbon fiber fabric. This allows the sensor to be always installed and ready to use for continuous condition monitoring of the blade. Differently from classical sensors, which can be placed independently from the others, in this case, all the measurement points are placed on the same wire (the fiber itself), characterized by a finite length. Furthermore, due to the physical characteristics of the fiber, some constraints on how the fiber is placed, such as maximum fiber curvature, must be considered. Moreover, strain measurements are collected and precise positioning is required to reconstruct correctly the displacement modal shapes from the strains. In the literature, many optimal placement methods for sensors are proposed, but they are all referred to independent sensors. An optimal method for optical sensor placing on the blade for modal analysis is first introduced in the paper. Then, numerical and experimental tests performed on some blades are shown.