Time-varying formation trajectory tracking scheme for underactuated USVs based on adaptive sliding mode control with improved guidance strategy

An improved guidance strategy-based sliding mode control (SMC) scheme with Frenet trajectory planning is proposed in this paper, in order to achieve the time-varying formation trajectory tracking control of underactuated unmanned surface vehicles (USVs) in the presence of unknown marine disturbances. Firstly, by introducing the Frenet coordinate system, a time-varying formation strategy is implemented based on the leader-follower framework to obtain a smooth transformation in the desired trajectory. Meanwhile, to address the oscillation of the heading angle in traditional guidance strategy, an improved guidance strategy is proposed by designing a virtual heading rate to stabilize the heading angle errors. Furthermore, considering the complexity of designed virtual velocity, a first-order low-pass filter is designed to overcome the “differential explosion” issue and ensure a smooth transition effect. An adaptive sliding mode reaching law is proposed to reduce convergence time and eliminate the chattering of control inputs. In addition, a nonlinear disturbance observer is developed to estimate and compensate for complex ocean disturbances in real-time. Finally, the effectiveness and superiority of the proposed control scheme are verified by several simulation experiments.