Madhumita Pal

This paper proposes a novel Adaptive Nonsingular Fixed-Time Path Following Control scheme for underactuated unmanned surface vehicles navigating under unknown uncertainties, environmental disturbances, and asymmetric input saturation constraints. The main contribution is the integration of a Fixed-Time Line-of-Sight guidance law with a homogeneity-based Adaptive Full-order Integral Terminal Sliding Mode control framework to achieve robust fixed-time path-following performance. Unlike conventional Line-of-Sight guidance methods with fixed or empirically selected look-ahead distances, the proposed Fixed-Time Line-of-Sight guidance law incorporates adaptive sideslip compensation and a dynamically adjusted look-ahead distance based on real-time cross-track error, enabling rapid and accurate heading correction under time-varying disturbances. Furthermore, a novel variable-exponent Adaptive Full-order Integral Terminal Sliding Mode controller is developed to guarantee nonsingular fixed-time convergence while simultaneously ensuring accurate tracking of both the desired heading angle and yaw rate. An adaptive gain mechanism is introduced to estimate and compensate unknown uncertainties online, eliminating the requirement for conservative prior bounds and mitigating control-gain overestimation. To address practical implementation issues, an asymmetric saturation function is incorporated directly into the control design to explicitly account for actuator limitations. Compared with existing fixed-time and finite-time path-following approaches, the proposed method simultaneously addresses adaptive sideslip compensation, fixed-time convergence, uncertainty rejection, singularity avoidance, second-order heading dynamics tracking, and asymmetric actuator saturation within a unified framework. A homogeneity-based variable-exponent Lyapunov analysis is developed to rigorously establish Fixed-Time Stability of the closed-loop system. Comparative simulation studies demonstrate that the proposed Adaptive Nonsingular Fixed-Time Path Following Control scheme achieves superior path-following accuracy, robustness, and disturbance-rejection capability compared with existing benchmark methods under constrained and uncertain marine environments.

This paper uses a fixed-time (FxT) dynamic event-triggered control(DETC) framework to study distributed multi leader-follower multi-group consensus for multi-agent systems(MASs) under disturbances. A normalized state-dependent variable-exponent (SDVE) design, which avoids the conservative tuning associated with constant-exponent techniques and modifies the convergence rate online in accordance with the system evolution, enforces FxT convergence. To improve resilience against matching uncertainties and external perturbations, a variable-exponent super-twisting algorithm (VSTA) is devised on nonlinear sliding manifolds. In order to facilitate simultaneous intra-group agreement and inter-group coordination while tracking different leader trajectories, agents are divided into several groups and communicate over a complex-valued communication graph. The FxT control architecture incorporates a DETC mechanism that ensures consensus without constant information transmission, hence reducing communication overhead. By establishing a positive infimum of the inter-event intervals and guaranteeing global FxT group consensus using just local information, the suggested technique eliminates Zeno behavior. Comparative numerical studies with conventional sliding mode and event-triggered approaches demonstrate significant communication savings, enhanced resilience, faster convergence, and a substantial reduction in chattering.

An event-triggered nonsingular fixed-time sliding mode control (ET-NFxT-SMC) scheme is developed for trajectory tracking of quadrotor unmanned aerial vehicles (QUAVs). The proposed controller employs a state-dependent variable exponent (SDVE) framework to ensure fixed-time stability (FxTS). Unlike constant-exponent methods, it adapts the convergence rate dynamically to the system state, offering a simplified structure with reduced computational complexity. The proposed approach addresses altitude and attitude tracking problems in the presence of external disturbances and system uncertainties. An event-triggering mechanism is integrated to significantly reduce unnecessary control updates by up to 52% without compromising stability. Thus, computational load and signal transmission bandwidth are further reduced, while maintaining robust performance. The Lyapunov-based analysis ensures FxT convergence and confirms a positive lower bound on inter-event times, thus avoiding Zeno behavior. Simulation results demonstrate that the proposed controller achieves accurate and robust trajectory tracking with fewer control updates, leading to improved resource efficiency and practical implementation feasibility.

The problem of altitude and attitude tracking problem of nonlinear quad rotor type unmanned aerial vehicles (QUAV) is addressed by dynamic event-triggered based nonsingular terminal sliding mode control (TSMC) scheme. In order to achieve fast reaching of desired trajectories in presence of unknown disturbances and uncertainties, nonsingular terminal sliding surface is defined. Moreover, the superior performance is achieved with limited resource of computation and communication channel band width by using dynamic event-triggered (DET) strategy. Fast convergence of tracking errors for the proposed control strategy is ensured via Lyapunov stability. The Zeno phenomena is proved to be excluded through precise analysis for the proposed dynamic event-triggered based TSMC scheme. The effectiveness of DET mechanism is examined over static and time triggered implementation of TSMC law for QUAV system.

A finite-time consensus protocol is presented in this paper for a network of connected heterogeneous agents with fixed and switching topology. In this study, two cases directed networks with fixed topology and directed networks with switching topology are examined. Convergence analysis for both the cases are presented. An independent tracking controller, which assures that an agent follows an ideal trajectory designed by sliding mode based consensus control, is devised. Agents stability are independent of the distributed consensus algorithm. Under the proposed control methodology, the measurement vectors of heterogeneous agents converge to the agreement value in a finite amount of time. The comparison results with the existing works show the proposed algorithm’s improved performance in terms of convergence time in presence of disturbances.

Summary: The main contribution of the paper is to propose a scheme of attitude controller for a class of unmanned aerial vehicles based on an adaptive version of the super‐twisting algorithm. This controller is based on a very recent second‐order sliding mode controller, which is robust in spite of uncertainties and perturbations, ensures finite time convergence, reduces the chattering, increases the accuracy, and does not require time derivative of the sliding variable. A very important feature of the controller is its adaptive gain, which allows to design the controller without knowing bounds of the uncertainties and perturbations. This controller is validated by experimental results. Comparisons versus PID‐based controller are made in order to evaluate the robustness of the closed‐loop system when similar perturbations are acting. Copyright © 2015 John Wiley & Sons, Ltd.

Development of computationally efficient and accurate attitude rate estimation algorithm using low-cost commercially available star sensor arrays and processing unit for micro-satellite mission is presented. Our design reduces the computational load of least square (LS)-based rate estimation method while maintaining the same accuracy compared to other rate estimation approaches. Furthermore, rate estimation accuracy is improved by using recently developed fast and accurate second-order sliding mode observer (SOSMO) scheme. It also gives robust estimation in the presence of modeling uncertainties, unknown disturbances, and measurement noise. Simulation study shows that rate estimation accuracy achieved by our LS-based method is comparable with other methods for a typical commercially available star sensor array. The robustness analysis of SOSMO with respect to measurement noise is also presented in this paper. Simulation test bench for a practical scenario of satellite rate estimation uses moment-of-inertia variation and environmental disturbances affecting a typical micro-satellite at 500 km circular orbit. Comparison studies of SOSMO with 1-SMO and pseudo-linear Kalman filter show that satisfactory estimation accuracy is achieved by SOSMO.