Attitude constraint-based recovery for under-actuated AUVs under vertical plane control during the capture stage

Abstract

The landed docking method is one of the most widely used underwater recovery methods because it imposes fewer restrictions on AUV configuration and applies to a wide range of applications. During the capture stage, the AUV and the recovery device are in close proximity, and if the attitude of the AUV fluctuates significantly, a collision may occur and cause damage. Existing studies for the common underactuated AUV do not effectively constrain the non-directly controlled degrees of freedom, posing a significant safety risk to the underwater recovery in the capture stage. To address this issue, a vertical plane control strategy that considers the safety interval of non-direct controlled degrees of freedom is proposed to keep the attitude angle within the safety interval during the docking process. To begin, the non-directly controlled AUV's pitching motion is analyzed and simplified to demonstrate that it has input-to-state stability during the capture stage, and an estimate of its uniform ultimate bound is provided so that the motion boundary can be estimated. Then, based on the backstepping method, a thrust-gain-constrained vertical surface control strategy is proposed: for the directly controlled heaving motion and rolling motion, precise control is achieved through a reasonable configuration of control parameters; for the non-directly controlled pitching motion, the thrust-constrained gain is calculated through the ultimate bound so that the force applied to the pitch degrees of freedom of the thruster is constrained and the purpose of indirect control is achieved. Using this strategy, the pitch motion is guaranteed to be inside the safe interval, resulting in accurate, safe, and quick recovery control during the capture stage.