| dc.description.abstract | Energy efficiency is crucial for the operational time and reach of autonomous underwater vehicles (AUVs). A new class of AUVs, underwater snake robots (USRs), has an articulated body that may be utilized to enhance propulsion efficiency and achieve energy autonomy. Naturally occurring phenomena such as underwater currents, waves and wakes that form behind bluff bodies, are energy-rich and are capable of inducing motion in a USR.
This thesis is concerned with the theoretical development of control methods that allow an USR to operate in the wake downstream from a bluff body when exposed to a flow. The vortices shed from the bluff body induce a sinusoidal motion in fish that swim in the wake, allowing them to swim in the wake with minimal energy expenditure. The proposed methods aim to either stabilize the motion of the USR, achieve efficient propulsion, or harvest energy from the induced motion while it operates in the wake.
The USR is exposed to highly nonlinear and time-varying disturbances when it operates in the wake of a bluff body. Moreover, the motion of the USR disturbs the formation of the wake. Therefore, to develop control methods for USRs that operate in wakes of bluff bodies, it is necessary to use simulation models that capture the fluid-structure interaction between the USR and the surrounding fluid to validate the control design. The implementation of such a solver is presented along with two models of USRs.
The USR is exposed to highly time-varying forces and moments dependent on time and placement while it operates in the wake. In the presence of these non-vanishing perturbations, asymptotic stability may not be attainable. Instead, the system converges to a neighborhood of the origin, a notion known as practical stability. This thesis presents Lyapunov conditions for the origin of a time-varying nonlinear system to be uniformly practically asymptotically stable (UPAS). Moreover, a theorem for cascaded systems consisting of sub-systems with UPAS origins is presented, showing that the UPAS property is retained. In the subsequent chapters, control methods to stabilize the USR while operating in the wake of a bluff body are developed. The USR is controlled to follow a sinusoidal gait pattern to achieve forward propulsion and turning motion. First, a control method for position control of an USR swimming towards a current is developed. Moreover, a control method for automatic alignment with a wake that slowly changes directions is proposed. Analysis shows that both the closed-loop systems are UPAS, using theorems for UPAS of cascaded systems developed in this thesis. The control design is validated through simulation studies in both the ideal case and high-fidelity simulations, showing that the performance is in accordance with the theoretical results.
Then, a Guiding Vector Field (GVF) approach is applied to achieve path-following of a static sine path. The path is an approximation of the path between the vortices that are shed from a bluff body. The resulting closed-loop system is analyzed and shown to be uniform global asymptotic stability (UGAS). The control design is validated through simulations with a complex model of the USR in constant and irrotational currents.
To achieve more efficient propulsion, this thesis investigates how nonlinear model predictive control (NMPC) can be applied to USRs to achieve a sinusoidal gait. The prediction model used does not account for the hydrodynamical forces and torques that the USR will experience when operating in a fluid. Moreover, the control method is studied through high-fidelity simulations, where the tuning of the cost function is varied to achieve more efficient usage of the actuators. The simulation study considers still water and a time-varying current, showing that tracking is achieved despite unmodelled disturbances. Additionally, it is shown that the energy usage of the actuators can be significantly reduced, however, at the cost of performance.
Finally, it is investigated how the distance from the USR to a bluff body affects the energy harvesting capabilities of the swimmer. The energy dissipated in the rotational dampers between the links is used as a proxy for the energy harvested. Several simulations are performed with high-fidelity simulations at different distances, and the results indicate that there is an optimal distance to the bluff body. An extremum-seeking controller is implemented to achieve the optimal position. The control method is studied through a simulation study, which shows that the optimal configuration is achieved. Moreover, the numerical results are studied through experiments where two swimmer configurations are tested at several distances to a cylinder in a turbulent flow. The results indicate that the energy harvested can be significantly increased by adjusting the distance from the swimmer to the bluff body. | en_US |
