This paper explores the optimal consensus problem involving fully actuated Euler-Lagrange systems with parametric uncertainties. We propose a distributed adaptive control algorithm that incorporates a novel dynamical auxiliary system to generate reference positions. Nonlinear transformation functions for the position tracking error, along with smooth compensating terms, are introduced and integrated into the reference velocity design. The proposed control framework is distinctive in that it only requires relative position measurements from neighboring agents and does not rely on any global information, thus enabling a fully distributed implementation. A theoretical analysis demonstrates that the proposed control protocol ensures the asymptotic convergence of all agents to the optimal solution of the total cost function and the boundedness of all closed-loop signals. Simulation results for two-link revolute joint manipulators are presented to validate the effectiveness of the proposed approach.

To address complex mission tasks, multirotors benefit from in-flight reconfiguration that enhances their morphological adaptability. This paper presents the Center-Driven Scissor Extendable Airframe (CDSEA), a novel one-degree-of-freedom (DOF) morphing airframe designed to replace traditional fixed-size airframes. The CDSEA allows a quadrotor to achieve significant morphological changes during flight, with rotors deploying radially from a central point. This capability facilitates substantial variations in footprint radius and ensures smooth transitions. The paper details the mechanical design, as well as kinematic and dynamic analyses, and discusses the actuator selection strategy for the CDSEA. Experimental results with a prototype demonstrate that the CDSEA achieves a footprint-radius deformation ratio of 2.5 and a morphing time of 0.3 seconds, surpassing existing solutions. Additionally, the design improves obstacle avoidance and wind resistance. These results underscore the CDSEA's potential as an advanced solution for enhancing UAV adaptive navigation performance in complex environments.

Improving motion speed and efficiency remains a critical challenge in snake robots gait control. This paper introduces the Loop gait, a novel locomotion gait designed to enhance both speed and energy efficiency of snake robots without passive wheels. Compared to Crawler gait and S-pedal gait, which are more widely used, the Loop gait has a better motion speed (1.8 times of the Crawler gait in the same parameter) and a better motion efficiency (1.6 times of the Crawler gait in the same parameter) due to its more loop body morphology. A static stability model is developed to guide parameter optimization, addressing potential instability caused by elevated center of mass of snake robots. Experiments confirm the Loop gait’s exceptional energy efficiency and propulsion, validating the static stability model’s utility in selecting parameters.

This paper addresses the consensus issue of multiagent systems with both input amplitude and rate constraints. We propose simple yet effective distributed control algorithms that integrate a velocity damping term with nonlinear saturated functions for both undirected and directed graphs. Leveraging the interplay between Barbalat’s lemma and graph theory, we show that all agents can achieve consensus without violating predefined input amplitude and rate constraints through the presented control algorithms. Moreover, we employ the developed framework to solve the rendezvous control problem of quadrotor unmanned aerial vehicles (UAVs) with motion limits. To illustrate and validate our proposed approach, we conduct extensive simulations and comparative experiments. Note to Practitioners—Most existing control methods for multiagent systems achieve consensus but neglect the constraints on the amplitude and rate of the control signal. However, in practice, the control signals are invariably subject to limitations in their amplitude and rate due to factors such as actuator saturation, considerations for ride comfort, and actuator wear. This neglect leads to a degradation in system performance and in severe cases results in the loss of closed-loop stability. This work primarily focuses on developing new control methods that can achieve consensus without violating the predefined input amplitude and rate limitations. The experiments on rendezvous control of quadrotor UAVs show the practical applicability of the presented algorithms, which yield satisfactory control performance as verified by theoretical analysis. This research contributes to the advancement of distributed control for multiagent systems, particularly in scenarios where input constraints are a critical consideration.

This paper investigates the problem of finite-time stabilization and trajectory tracking control of a nonholonomic wheeled mobile robot (NWMR) under input constraints. Based on the hyperbolic tangent functiontanh(⋅), a bounded finite-time stabilization controller and a bounded finite-time tracking controller are proposed. Specifically, for the stabilization controller, a switching strategy is used and an explicit upper-bound estimate for the closed-loop settling time is provided. For the tracking controller, a finite-time control scheme consisting of a bounded angular velocity controller and a bounded linear velocity controller is developed. For both the stabilization controller and the tracking controller, the saturation level of the control input can be predefined based on the actuator’s capacity. Comprehensive finite-time stability analyses are provided by selecting appropriate Lyapunov functions. Finally, simulation and experimental results addressing the stabilization and trajectory tracking problem for NWMR demonstrate the effectiveness of the proposed controllers.

The work in this paper delves into the challenge of whole elongated body's obstacle avoidance during path following for a class of bionic snake robots. Currently, most studies focus solely on preventing the robot's head from colliding with obstacles through designed controllers. However, due to the unique elongated structure and biomimetic locomotion modes of snake robots, it is unavoidable that the rest of the robot's body could still collide with obstacles. To resolve this problem, we propose a novel real-time optimization obstacle avoidance strategy for a class of terrestrial snake robots with multi-link elongated body using model predictive control (MPC). Moreover, by leveraging the elongated body characteristics of the robot, an improved path guidance strategy is also developed. The effectiveness of the proposed strategies is verified and validated through extensive simulations and experiments on a custom-built nine-link elongated snake robot. The results demonstrate that all links of the robot can well avoid obstacles while continuing to track the given path.

This article studies the tracking control of a quadrotor unmanned aerial vehicle (UAV) under time-varying external disturbances. An event-triggered sliding mode control (SMC) strategy is proposed by introducing a new triggering condition form of desired trajectory, quadrotor position, and velocity. In the sense of Lyapunov theory, the stability of the entire closed-loop control system is analyzed, and it is proved that the tracking error is adjusted to an adjustable set around zero. We show that the Zeno phenomenon can be avoided; that is, a positive minimum inter-event time is assured. One of the salient features of the proposed strategy is that it can reduce the update frequency of the control efforts, thereby ensuring desirable tracking performance under limited communication bandwidth. Comparative simulation and experimental results are provided to show the efficacy of our framework.

This paper presents our comparative study on how the flight performances of an in-flight morphing quad-rotor are affected by the morph induced inertia variation. A custom-built in-flight morphing quad-rotor was employed in numerical and experimental tests for the study and analysis. In these tests, the quad-rotor is controlled to follow a predefined path and/or to hover in an environment with the constant wind disturbance. Our numerical results indicate that the morphing-size-down quad-rotor exhibits more agile in flight attributed to the compact volume/size, while the big-size one shows more flight stability in a disturbed and windy environment. Compared to regular scaled aerial vehicles whose volume/size changes follow a weight change proportionally, the numerical results reveal that our morphing quad-rotor that changes its volume with identical mass can acquire more merits towards high flight performances. Experimental validations further prove that through adaptatively transform its size in a complex and constrained environment, the in-flight morphing quad-rotor is not only capable of well performing path following tasks when encountering obstacles on the path or nearby the path, but also enhances the flight performance of withstanding external torques by extending its size so as to increase its moment-of-inertia. In summary, our in-flight morphing quad-rotor can acquire higher flight performances by adaptatively morphing when flying in complex environments.

Currently, most table tennis robots concentrate on the canonical position control problem while ignoring the actual velocity control requirements. In this paper, we consider these requirements and propose a new table tennis robot framework. First, a tailor-made mechanical structure is designed such that the robot can reach large workspaces. Thereafter, in the table tennis trajectory prediction process, a clustering algorithm is introduced to screen the heterogeneous predicted hitting points and filter the invalid ones, thereby significantly improving the prediction accuracy. By using quintic polynomial trajectory planning, smooth and stable high-speed control of the robot hitting motion can be obtained. Finally, a position-based strategy and a velocity-based strategy are devised for returning the table tennis. Extensive experiments demonstrate that the accuracy of the ball's trajectory prediction algorithm is more than 92%. The success rate of returning the ball exceeds 95% at the ball velocity of 3-7 m/s, and the velocity-based strategy performs better compared with the position-based approach at the ball velocity of 7-9 m/s.

In this paper, we investigate the consensus problem of multiple unmanned aerial vehicles (UAVs) in the presence of environmental constraints under a general communication topology containing a directed spanning tree. First, based on a position transformation function, we propose a novel dynamic reference position and yaw angle for each UAV to cope with both the asymmetric topology and the constraints. Then, the backstepping-like design methodology is presented to derive a local tracking controller for each UAV such that its position and yaw angle can converge to the reference ones. The proposed protocol is distributed in the sense that, the input update of each UAV dynamically relies only on local state information from its neighborhood set and the constraints, and it does not require any additional centralized information. It is demonstrated that under the proposed protocol, all UAVs reach consensus without violation of the environmental constraints. Finally, simulation and experimental results are provided to demonstrate the performance of the protocol.

In this paper, we investigate the perception-aware path finding, planning and following for a class of snake robots autonomously serpentining in an unmodeled and unknown environment. In the work, the onboard LiDAR sensor mounted on the head of the snake robot is utilized to reconstruct the local environment, by which and the modified rapidly-exploring random tree method, a feasible path from the current position of the robot to a local selected target position can be obtained. Next, the parametric cubic spline interpolation path-planning method and potential functions are applied to make the path more smooth so as to prevent the multi-link and elongated robot body from hitting obstacles. To steer, a time-varying line-of-sight control law is designed to ensure that the robot moves to the local target position along the generated path by the perception-aware method. The robot will repeatedly perform the above search-find-move strategy until it reaches the final predefined target point. Simulation and experimental results demonstrate a good performance of the proposed perception-aware approach, that is, the elongated and underactuated snake robot is capable of autonomously navigating in an unknown environment.

In this paper, the distributed control problem of nonlinear strict-feedback multiagent systems is addressed under directed and time-invariant communication graphs. With the utilization of the prescribed performance control methodology, a control algorithm is proposed to ensure predefined bounds of overshoot, convergence rates, and steady-state values of the neighborhood synchronization errors in the presence of unknown dead-zone inputs. The algorithm is fully distributed in the sense that the control input for each agent is independent of any global information of the communication graph and is solely based on local relative output information from its neighborhood set. The approximating structures, e.g., neural networks or fuzzy systems, and the command filters that are typically incorporated to avoid the need for analytical derivatives in the backstepping design are not employed here, resulting in a low-complexity design. Simulation results are included to verify the algorithm.

This paper investigates the curve path following problem for a class of planar underactuated bio-inspired snake robots. The time-varying line-of-sight (LOS) guidance law and the cubic spline interpolation (CSI) path-planning method are employed. Existing studies focus on straight line path following which only gives a solution for snake robot motion control in relatively simple environments. Considering the snake robot's many degrees of freedom and excellent mobility in terrains, we propose a more applicable solution of curve path following for snake robots on the ground. The improved LOS helps the snake robot to steer aggressively at a sharp turning point. Furthermore, to avoid the sideslip of the snake robot caused by the ground friction change, an integral controller is introduced in the design of the heading reference. Simulations and experiments on an 8-link custom-built snake robot are conducted and the results demonstrate and validate the effectiveness of the proposed curve path following algorithm.

In this paper, a simple tracking control scheme is proposed for quadrotor systems with uncertain dynamics. It precludes the necessity for prohibitive analytic computation of the derivatives of the desired (virtual) attitude that is typically employed in controlling quadrotor systems. Moreover, this control scheme is approximation-free in the sense that it does not incorporate any adaptive laws, observers, or command filters to compensate for unknown parameters in the dynamics and the absence of the analytic differentiation, thus exhibiting remarkably low complexity levels and making its implementation straightforward. The thrust saturation is approached in the position control design which also enables the singularity in desired attitude extraction to be avoided entirely. It is demonstrated that based on the proposed scheme, the tracking errors can be made arbitrarily small by appropriately selecting design parameters. Extensive simulations and experiments are performed to verify the effectiveness of our scheme.

This paper investigates the straight path following problem for a class of underactuated bio-inspired snake robots on ground with unknown and varied friction coefficients. Existing works usually design control input requiring the exact values of these friction coefficients, which however rely on the specific operating terrain and may not always be known a priori. By virtue of backstepping technique, we present a novel adaptive controller that can compensate for unknown and varied friction coefficients in real-time. Moreover, it is proved via LaSalle-Yoshizawa theorem that the path following errors converge to zero asymptotically and all the parameter estimates are bounded. Simulations and experiments on an 8-link snake robot are carried out to illustrate the effectiveness of the proposed controller.