This paper investigates the cooperative movingtarget enclosing control problem of networked unicycle-type nonholonomic vehicles with constant linear velocities. The information of the target is only known to some of the vehicles, and the topology of the vehicle network is described by a directed graph. A dynamic control law is proposed to steer the vehicles, such that they can get close to orbiting around the target while the target is moving with a time-vary velocity. Besides, the constraint of bounded angular velocity for the vehicles can always be satisfied. The proposed control law is distributed in the sense that each vehicle only uses its own information and the information of its neighbors in the network. Finally, simulation results of an example validate the effectiveness of the proposed control law.

Superior dexterity and extreme flexibility are typical advantages for concentric cable-driven manipulators working in confined spaces. However, its inverse kinematics and configuration planning are very complicated. In this article, we propose a spatial biarc method for the above problem. The distinguishing feature of this method is that input parameters are two positions and two direction vectors in three-dimensional (3-D) space, and the output is a reasonable spatial biarc for controlling a concentric cable-driven manipulator in 3-D space. This method has the following three advantages. First, the positions and direction vectors of the base and inner distal tip are considered simultaneously. In addition, the length and ratio of the overlapped section and separated section can be adjusted by changing the length of the direction vectors. Furthermore, by judging the angular value of the direction vectors, one can predetermine whether the spatial configuration of the entire arm is C- or S-shaped. The proposed method realizes the parameterization of a concentric cable-driven manipulator, which makes it convenient to intuitively control the manipulator to achieve interference-free motion trajectory planning in confined spaces. Finally, trajectory tracking inspections are simulated and experimentally executed. It can be seen from results that the proposed spatial biarc method can provide reasonable solutions for concentric cable-driven manipulators. The method is especially favorable in terms of 3-D-pose-determination problem and trajectory-planning problem. It can also be applied to other manipulators with similar configurations. Without loss of generality, when the given points and direction vectors are coplanar, the proposed spatial biarc method can be transformed to a planar biarc method.

Sailing robots can contribute significantly to maritime surface exploration, due to its potential for long-range and long-duration motions in the environment with abundant wind. However, energy, the critical factor for their long-term missions, shall be carefully investigated, so as to achieve sustainability in distance and time. In this survey, we have conducted a comprehensive investigation on numerous sailing robots, developed in academia and industry. Some of them have achieved long-term operation, and some are motivated by, but still on the way to this ambitious goal. Prototypes are grouped in each team, so as to view the development path. We further investigate the existing design and control strategies for energy sufficiency from three perspectives: actuation, harvesting, and energy management. In propulsion and steering, i.e., two major actuations, researchers have accumulated effective sail and rudder designs. The motorized propeller and wave-glider–inspired mechanism also contribute as compliments for propulsion. Electricity harvesting based on solar or wind energies is also discussed to gather more power from nature. Pros and cons in strategies of energy management, which are valuable tools to enhance power utilization efficiency, are elaborated. This article is hoped to provide researchers in long-term robotic sailing with a comprehensive reference from the perspectives of energy.

The concentric cable-driven manipulator is becoming increasingly important in minimally invasive surgery. Obviously, one of the basic requirements is to track trajectory and avoid collision at the same time. However, the unavoidable difficulty is to choose an inverse kinematics solution that fits inside the confined environment from infinitely many solutions due to its redundancy. This article proposes a combined planning method based on biarc curve and Bézier curve for the concentric cable-driven manipulator tracking desired trajectory in confined 2-D and 3-D. First, the new proposed structure and the nomenclature are introduced. Then, the kinematic modeling of the concentric cable-driven manipulator in actuator space, configuration space, and task space is detailed. Thereafter, using the proposed biarc curve, the relationship between the end direction and bending angles is comprehensively analyzed. The Bézier curve is then adopted to plan the desired path for the concentric cable-driven manipulator moving in confined 2-D and 3-D environments. Finally, the proposed combined planning method based on biarc curve and Bézier curve is experimentally verified in a trajectory tracking task simulating a large intestine examination in both the typical “C” configuration (2-D environment) and the “S” configuration (3-D environment). The results prove that the combined planning method based on biarc curve and Bézier curve can be applied to generate a reasonable solution for concentric cable-driven manipulators that are confined both 2-D and 3-D environments.

Ensuring safety of unmanned surface vehicles (USVs) during nearshore field operations is challenging as they face unknown risks of collision with surface and underwater obstacles. To address this problem, we propose a bio-inspired collision risk-assessment method for ensuring safe USVs operation in nearshore clean waters. Collision risks are typically unknown due to the imprecision of underwater measurements for the water-air boundary light refraction, and disturbances caused by wave-induced USV motion. To tackle these two causes of imprecision, our collision risk-assessment method employs one single stereo camera and an IMU. We use visual information from the stereo camera to model the light rays at the water-air interface, rectify the underwater measurements, and monitor for surface obstacles, and IMU data to model wave disturbances via time-frequency analysis. We verify the performance of this new method in both the indoor and outdoor experiments, which demonstrates that it achieves a 65.44% improvement in the depth accuracy at the water-air interface compared with raw depth measurements, and provides a range of safety levels for guiding USV operations. This new collision risk-assessment method significantly expands the USVs working areas while ensuring safety during field exploration. Additionally, the simplicity of the sensor setup and thus minimal cost of this method underscore its utility for supporting USV-based various explorations in clear waters.

A solar tracker significantly increases the amount of energy harvested by a floating photovoltaic system by adjusting the pose of its photovoltaic (PV) panels to optimize their exposure to the solar rays. Conventional solar trackers perform such an adjustment using actuators, which is energy-consuming and involves complex structures. In this paper, a novel dual-axis wave-driven solar tracker is proposed where the photovoltaic (PV) panel is adjusted by the inertia force and gravity. Actuators are replaced by brakes to fix the pose of the PV panel. The kinematics and dynamics of the system are investigated, which are further used for the state feedback and the formulation of the control strategy. A sliding-mode observer is used to estimate the effect of winds on the system, which enables the robustness of the system to be enhanced. A motion planning strategy is also developed to track the solar position and minimize the movements of two joints. Indoor experiments are conducted to test the accuracy of the state feedback and dynamic model. The performance of the uncertainty observer is also tested by experiments. At last, field experiments on the real water surface are implemented to verify the feasibility of the proposed system. The results show that the solar tracker is able to adjust the PV panels at a velocity of 8.89 deg/s under the effect of limited base motions with amplitude less than 4 $^{\circ}$ . Note to Practitioners—Traditional solar trackers can be driven passively by chemical energy or actively using actuators. The former cannot function stably in the field environment, and the latter is complex in structure and energy-consuming. For the FPV system under the effect of waves, the solar tracker can utilize the motion of the base to align its PV panel to the solar position. Thus, we present a wave-driven solar tracker without using actuators, and brakes are applied to lock the position of joints. The proposed solar tracker is energy-efficient, has a simpler structure and hardware than the active one, and is more robust than the passive one. We model the system and propose a control scheme to drive the PV panel using inertia and gravity. We also observe the effect of winds to enhance the system robustness. The motion planning strategy is investigated to minimize the movements of the joints. The impact of base motion and control period on the energy-harvesting efficiency is analyzed by simulations. Both indoor and field experiments are implemented to verify the feasibility of the system. The experimental results show that the system performs well even when the base shacking is less than 4 $^\circ$ . Note that the control algorithm is a model-based algorithm, which means the model parameters (inertia, gravity, etc.) need to be reidentified for different PV panels and system properties. That is a challenge for large-scale deployment with different configurations. Future research will address the system identification problem and enable the model parameters to be updated automatically.