Multi-Axial Fiber Optic Reference Strain Sensor for Flexible Dynamics of Structures
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Launch vehicles are in an ever-decreasing weight trend, with bodies becoming more slender to reduce aerodynamic loading. Mass and stiffness tend to be opposing concepts when designing launch vehicles: a stiffer structure means it is usually heavier. Lower stiffness on launch vehicles may produce excessive vibrations during flight, reducing the performance, operating characteristics and safety margins of the vehicle. It also impacts flight dynamics: as the vehicle flies, the orientation of its thrusters and actuators are impacted by the vibrations, creating potential instabilities in the control schemes. This is currently accounted for by using single-point inertial measurements and feeding the results into the main control loop via the use of notch filters, introducing phase lag. This Dissertation presents the development of a multi-axial sensor along with shape determination methods on multiple degrees of freedom of launch vehicles, relying on fiber optic strain sensors. The main advantage of this system with respect to traditional Inertial Measurement Units (IMU) is that it can resolve the entire shape of the vehicle and extract the amplitudes of each mode shape in real time, without relying on notch filters or other lag-inducing devices that may impact controller performance. Two shape determination methods are presented, one based purely on the assimilation of the sensor as an Euler-Bernoulli beam, whereas the second method is based on modal decomposition of the strain measurements. Results from a scaled down, laboratory sensor acquiring data at a frequency of 1kHz, successfully extracts the shape of a dynamic system up to the third mode shape in bending, as well as extracting coupled structural modes including bending in two directions and torsion at the same time. Prospective applications of these methods and system are also discussed in this Dissertation.