This study introduces a new robust nonlinear control scheme based on the theory of nonsingular terminal sliding mode control (NTSMC). Since conventional NTSMC utilizes a discontinuous switching function, a significant flaw called chattering can occur. The main purpose of this study is to design a new switching function based upon Lyapunov stability in order to alleviate this drawback over time. There are many approaches to mitigate the chattering drawback in SMC such as utilizing a smooth approximation of the switching element, or employing higher order sliding mode control (HOSMC) strategy. However, the use of a continuous approximation affects the system's performance and a finite reaching time to the sliding manifold, and in HOSMC the estimation of high‐order derivatives of states is usually difficult and it still exhibits chattering in the presence of parasitic dynamics. In this study by employing a new sliding manifold including a time function, the chattering is attenuated as well as keeping the robustness. Finally, a second‐order nonlinear dynamical system subject to disturbance is simulated to highlight the validity and applicability of the proposed method.

This study proposes a U-control–based Composite Nonlinear Feedback (U-CNF) design procedure. This U-CNF control establishes a double feedback loop framework for generalisation and simplification in designing the CNF control systems. Two controllers, in terms of double dynamic inversion, are designed separately, (1) to stabilise and cancel the nonlinearities and dynamics (convert the plant into an identity matrix) in the inner closed-loop, and then (2) to improve the system transient response by specifying a second-order linear system with a monotonic nonlinear function to smoothly tune the damping ratio. Accordingly, the conventional CNF characteristics in a concise pathway are achieved. The properties show, under proper conditions, the U-CNF control is plant model-free control, applicable to nonaffine nonlinear dynamic systems, and robust against model uncertainty and external disturbance. For the initial bench tests of the first-time proposed U-CNF configuration, the simulated case studies are provided with a transparent procedure to demonstrate the consistency with the analytical results in the numerical computations and to present guidance for applications.

This study presents two new robust nonlinear control algorithms based on the theory of time-varying sliding mode control (TVSMC) strategy to stabilize the attitude of an unmanned aerial vehicle (UAV) for nuclear decommissioning applications. Constant radiation affects the UAV performance. For instance, its parameters are time-varying and subject to uncertainty all the time. This is especially important in designing sliding mode control as the motion of the control system in the reaching phase is highly sensitive against environmental disturbances and parameter uncertainties. In this study, two types of time-varying sliding manifolds are proposed to eliminate the reaching phase and to enhance the robust performance in the aforementioned phase. Therefore, two novel types of time-varying sliding surfaces are introduced based on the initial condition as intercept-varying sliding mode control (IVSMC) approaches. In the first proposed method, the reaching time from initial manifold to the desired one is similar to that of the conventional SMC method. While in the second proposed IVSMC scheme, one can accelerate or decelerate the motion of the time-varying sliding manifolds at any selected time. Furthermore, chattering phenomenon can be avoided using two techniques known as boundary layer and continuous SMC. Finally, to highlight the robust performance of the proposed methods, a quadrotor UAV subject to external disturbances is simulated.

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