Nonlinear station keeping control for underactuated unmanned surface vehicles to resist environmental disturbances

This paper presents a station keeping control method for underactuated unmanned surface vehicles (USVs) to resist environmental disturbances with unknown magnitude and direction. Three control objectives are developed, where two of them are for the surge and yaw motions directly controlled by actuators and the last one is for the sway-offset motion affected by sway environmental disturbance acting on the vehicle body. The nonlinear separation principle is introduced and adopted to stabilize the surge, sway, and yaw cascaded systems. To clearly demonstrate the sway-offset impact induced by environmental disturbances, the sway offset dynamic model affected by sway environmental force is subsequently derived. Finally, a slowly varying adaptive law is proposed to gently alter vehicle heading for the sake of positioning error minimization, and detailed benchmarks are presented to quantitatively determine the gain setting for this adaptive law. When the preset position is well kept, the stable vehicle heading will point against the mean direction of external disturbances such that the yaw control moment is minimized and onboard energy can be saved. Instead of being validated only through simulations, experiments are conducted under real offshore marine conditions to verify the validity and effectiveness of the proposed station keeping method.

Automated summary

This paper proposes a station keeping control method for underactuated unmanned surface vehicles to resist environmental disturbances. It stabilizes surge, sway, and yaw motions using nonlinear separation principle and minimizes positioning error through a slowly varying adaptive law. The validity of this method is verified through experiments under real offshore marine conditions.