Modeling and control of a hybrid autonomous underwater vehicle -

Abstract

In this work a six degrees of freedom (6DOF) dynamic model of a Hybrid Autonomous Underwater Vehicle (H-AUV) is developed. A new approach for designing a simplified control law is introduced to command the vehicle to track a given three-dimensional trajectory. To get a linear time variant (LTV) state space model, the 6 DOF dynamics and kinematics models of the AUV are linearized about a given desired three-dimensional trajectory. A linear quadratic regulator (LQR) is designed based on the linearized model and is applied to the nonlinear model for validation purposes. The designed control law is robust enough to autonomously switch modes between propulsive and gliding dynamics to ensure minimal tracking error with respect to the desired trajectory. Simulation results show that the linear control law provides satisfactory results when applied to the nonlinear model for tracking basic trajectory maneuvers: helix, saw-tooth trajectory, and 3D Dubin's trajectory. The robustness of the designed controller is investigated in the presence of underwater currents. Simulation shows that the controller is robust enough to command the vehicle to track the desired trajectory in the presence of underwater currents, in the case of thrust mode. However, for the gliding mode, the designed controller gave unsatisfactory tracking performance in the presence of underwater currents. To mitigate this, a disturbance observer is designed based on the linearized model and is applied along with the designed control law which improved the tracking performance.