This paper focuses on the problem of output tracking with prescribed transient and steady-state performance for strict-feedback systems with unknown nonlinear functions and unmatched disturbances. In lieu of Nussbaum gain techniques, parameter estimation algorithms and switching control strategies, a continuous static low-complexity control solution is provided by means of a novel combination of smooth orientation functions and error transformation functions. The proposed method possesses inherent robustness against model uncertainties, disturbances, and virtual control signal derivatives, thus eliminating the needs to introduce extra robust control schemes and compute analytic derivatives. Comparative simulation results further illustrate the above theoretical findings.

This paper is concerned with the output-feedback tracking control problem for a class of nonlinear systems. The challenge lies in how to drive the tracking error to a prescribed region in a given time globally, under both unknown nonlinear functions and unknown virtual control coefficients. To deal with the unknown control direction, an orientation function is exploited in lieu of the Nussbaum gain technique, the supervisory switching strategy, and the parameter estimation approach. To attain global tracking performance, a tuning function is employed to adjust the tracking error, and a barrier function is used to combat this error. The resulting controller guarantees the prescribed tracking performance and the boundedness of the signals in the closed loop for any initial condition.

This article is concerned with the trajectory tracking control problem for the wheeled mobile robots (WMRs) subject to actuator faults. The challenge lies in the partial loss of effectiveness of the actuated wheels which results in the loss of strong controllability of the WMR, rendering the classical fault-tolerant control methods infeasible. To overcome this obstacle, a novel mixed-gain adaption technique is put forward and is skillfully combined with the robust prescribed performance control method. The resulting control achieves trajectory tracking of the WMR with the predefined settling time and accuracy, against the actuator faults, the model uncertainties, and the disturbances. Moreover, it exhibits significant simplicity, without parameter identification, function approximation, disturbance estimation, or fault estimation. The theoretical findings are illustrated by a comparative simulation.

This article investigates the trajectory tracking control problem for a family of underactuated autonomous surface vessels under the condition of model uncertainties, actuator faults, and environmental disturbances. The emphasis or difficulty is how to ensure the trajectory tracking with specified accuracy in the fixed-time settings. A new-type fault-tolerant tracking control strategy is developed based on the marriage of a novel bounding function and a method of handling constraints. Compared with the existing results it can be noted that: first, the position error enters into a preassigned residual set in a priori designate time and remains there; second, no adaptive mechanisms, disturbance observers, or command filters are involved, leading to a simplicity control attribute. Results from a comparative simulation on the CyberShip II further illustrate the above theoretical findings.

The problem of prescribed performance tracking control for unknown time-delay nonlinear systems subject to output constraints is dealt with in this paper. In contrast with related works, only the most fundamental requirements, i.e., boundedness and the local Lipschitz condition, are assumed for the allowable time delays. Moreover, we focus on the case where the reference is unknown beforehand, which renders the standard prescribed performance control designs under output constraints infeasible. To conquer these challenges, a novel robust prescribed performance control approach is put forward in this paper. Herein, a reverse tuning function is skillfully constructed and automatically generates a performance envelop for the tracking error. In addition, a unified performance analysis framework based on proof by contradiction and the barrier function is established to reveal the inherent robustness of the control system against the time delays. It turns out that the system output tracks the reference with a preassigned settling time and good accuracy, without constraint violations. A comparative simulation on a two-stage chemical reactor is carried out to illustrate the above theoretical findings.

This article is concerned with the fast and accurate trajectory tracking control problem for a sort of underactuated surface vehicle under model uncertainties and environmental disturbances. A novel neural networks (NNs)-based prescribed performance control strategy is proposed to solve the problem. In the control design, a new type of performance function is constructed which provides a way to predefine the settling time and accuracy, straightforward. Then, a pair of barrier functions are employed to combat not only the position error but also the virtual control input. This evades the possible singularity or discontinuity of the control solution. Next, an initialization technique is exploited, removing the requirement for the initial condition of the control system. Finally, two NNs are employed to deal with the unknown ship nonlinearities. The performance analysis not only demonstrates the effectiveness of the proposed approach but also reveals its robustness against disturbances and unknown reference trajectory derivatives. There is, thus, no need to acquire such knowledge or employ specialized tools to handle disturbances. The theoretical findings are illustrated by a simulation study.