Hydrodynamic analysis and motion control of the Coanda-effect jet thruster for underwater robots

Abstract

Underwater robots are usually equipped with vectored propulsion systems for flexible motions. As most mechanical structures of conventional propulsion systems are sophisticated and bulky, it's a challenge to house them inside compact or portable underwater robots. Thus, a Coanda-effect based jet thruster has been developed, which produces propulsion in multi-axis but with only two actuators. To characterize hydrodynamic behaviors of the thruster, not only free jet turbulent theory but also a new numerical calculation method is adopted to analyze its vectored thrust capability. Based on theoretical results, critical geometric parameters concerning the jet thruster are optimized and these analytical results have been validated by fluid simulations. To discuss maneuverability, a jet thruster actuated underwater robot prototype has been fabricated and we've formulated its mathematical models. Specifically, Line-of-Sight navigation is exploited and combined with a multi-variable fuzzy logic controller to guide the robot to follow a planar desired path. Moreover, a PID controller is configured based on decoupled dynamic model of the robot to implement depth control. Finally, experiments regarding vectored thrust, efficiency and robot motion control have been verified, which indicates the jet thruster can steer the robot to achieve good performance in multi-DOF maneuverability.