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This paper studies the trajectory tracking problem of unmanned surface vehicle subject to unmeasurable velocities and unknown disturbances. By combining a fixed-time extended state observer (FESO) and a fixed-time differentiator, a fixed-time sliding mode control (FTSMC) law is proposed, in which a saturation function is adopted to make the terminal sliding mode surface leave the singularity area. The value of this paper can be described: first, this paper designs a novel guidance law that can converge in a fixed time to reduce the convergence time of the error. Then, unmeasurable velocities and lumped disturbances are estimated by applying a FESO. Meanwhile, a fixed-time differentiator is used to obtain real-time differential signals, thus reducing the difficulty of controller design. Subsequently, a novel auxiliary dynamic system is designed to address actuator saturation. According to Lyapunov's theory, the entire closed-loop control system has uniformly global fixed-time stability (UGFTS). The superiority of the designed controller is demonstrated through numerical simulations. • A novel fixed time guidance law is developed, which makes position errors reach zero within a fixed time. • A FESO is designed to estimate unknown dynamics within a fixed time, which does not require precise model parameters. • A novel auxiliary dynamic system is designed to handle input saturation, which can ensure that the state converges in finite time. • The fixed-time differentiator is introduced to achieve satisfactory filter and differential performance. • A novel FTSMC strategy is proposed for underactuated USV. Finally, the system proved to be uniformly global finite-time stable. [ABSTRACT FROM AUTHOR]

This article proposes an output-feedback fixed-time trajectory tracking control methodology for hypersonic flight vehicles subject to asymmetric output constraints. In contrast to the state of the art, the most distinguishing feature of our control design lies in avoiding using conventional recursive design methods (e.g., backstepping technique, dynamic surface control, etc.) and in not relying on full-state availability. In the velocity control loop, an asymmetric integral barrier Lyapunov function is adopted to confine velocity variable within a well-defined compact set all the time. In the altitude control loop, after utilizing its cascaded property and proposing a novel scaling function, the original constrained system is transformed to an unconstrained one, which facilitates the control design and stability analysis. Moreover, the proposed control algorithm only involves one fuzzy logic approximator as well as one fixed-time differentiator in the transformed system and guarantees that the tracking errors of velocity and altitude converge into the user-defined residual sets within fixed time. Several comparative simulations have been conducted to highlight the superiorities of the developed method. [ABSTRACT FROM AUTHOR]

This paper proposes a fixed-time differentiator running in parallel with a feedback linearisation-based controller, which allows quadrotors to track a given trajectory. The fixed-time differentiator estimates outputs' derivatives in a predefined convergence time, largely compensating for initial condition problems and solving the delay problem. Combined with a state reconstruction step, a whole observer–estimator–controller scheme can be constructed for quadrotors to solve the trajectory tracking problem. Also, an Linear Matrix Inequality (LMI) optimisation-based algorithm to tune parameters of the differentiator is developed here. The high performance of the proposed model is illustrated by numerical simulations. [ABSTRACT FROM AUTHOR]

This paper studies a novel adaptive fixed-time sliding mode attitude tracking control for a submarine-launched missile, which is affected by sea winds, sea waves, ocean currents and other disturbances during the water-exit process. Firstly, the nonlinear water-exit dynamic model of the submarine-launched missile is established, and then it is transformed into a simple second-order attitude tracking system. Subsequently, a novel non-singular fixed-time fast terminal sliding mode surface (NFFTSMS) with fixed-time convergence is presented, and the pre-established settling time is also developed. Moreover, a novel adaptive non-singular fixed-time fast terminal sliding mode control (ANFFTSMC) is presented by employing a fixed-time disturbance observer, a fixed-time differentiator and the proposed NFFTSMS. Closed-loop stability of the proposed controller is proved by utilizing the Lyapunov methodology. Finally, numerical simulations including two typical launch trajectories of the missile are carried out to demonstrate the strong robustness of the proposed control scheme. [ABSTRACT FROM AUTHOR]

The proof of Theorem 1 in Moulay et al., (2022) is flawed; thus, it is unclear whether the proposed controller in that paper is capable of achieving the claimed result. [ABSTRACT FROM AUTHOR]

The aim of this article is to develop new second-order fixed-time sliding mode controllers able to tackle mismatched disturbances. Contrary to what is done in the literature, the proposed sliding-mode control strategy does not use disturbance observers because the sliding surface possesses robustness properties. The results are applied to define a new fixed-time differentiator. [ABSTRACT FROM AUTHOR]