For field tasks, it is quite challenged to operate in a complex environment for the underwater robots, especially for those with multiple control surfaces due to different response and gain characteristics. To this end, this paper develops a highly integrated robotic dolphin followed by a robust motion control system. For better maneuverability and fault-tolerant capabilities, a newly-designed robotic dolphin is presented, owning a wide array of sensors and multiple control surfaces, in which passive flukes are particularly applied. On this basis, a robust motion control system is proposed, including a depth controller based on velocity-related allocation strategies and a heading controller based on clearance compensation. In detail, considering the degradation of motion performance caused by passive flukes, a sliding mode controller for gain uncertainty and an allocation-related parameter tuning strategy for inputs response characteristics are designed. Extensive simulations and aquatic experiments are conducted, and the obtained results demonstrate the satisfied maneuverability of the designed prototype and the effectiveness of the proposed methods. This study can lay a foundation for further development of robotic dolphins with a robust motion system to execute complex tasks in the field.Note to Practitioners—This paper is inspired by the issue of robust motion control system for a newly-designed practical robotic dolphin that possesses a passive tail and redundant control surfaces. The traditional methods are usually susceptible to uncertainties in the passive tail gain, exhibiting degraded control performance. Moreover, control oscillations and slow convergence speed often occur caused by neglecting the characteristics of different control surfaces, including response patterns and clearance. This paper suggests a robust depth controller based on velocity-related allocation strategies and a robust heading controller based on clearance compensation. Specifically, an allocation-related parameter tuning strategy is given by considering inputs response characteristics, including response speed, saturations, and hydrodynamic force variation patterns. To guarantee fine regulations of heading control, a nonlinear disturbance observer (NDOB)-based clearance compensation is proposed. Extensive aquatic experiments on the newly-designed robotic dolphin verified the effectiveness of the proposed methods. It is envisioned that all these presented results can provide valuable engineering practice insights for industry practitioners.

Artificial lateral line (ALL) sensors hold the potential to enhance the perception abilities of robotic fish by capturing surface pressure gradients and identifying near-field object, such as dipole source. However, the robotic fish’s free-swimming motion introduces periodic low-frequency noise into the ALL data, while dipole sources with time-varying positions generate pressure signals with complex time-frequency characteristics. This paper proposes a complete solution to these challenges that would enable freely swimming robotic fish to locate dipole source. Firstly, an ALL system consisting of pressure sensors is integrated into the robotic fish, further constructing a real-time data acquisition and processing system. Secondly, to effectively estimate and remove the swimming-induced noise from the ALL data, a noise estimation model is developed based on the bionic motion mode and unsteady Bernoulli equation. Subsequently, short-time Fourier transform is applied to the high-quality data after noise elimination, followed by developing a convolution regression neural network for feature extraction and dipole source localization. Finally, extensive simulations and experiments are conducted to validate the effectiveness of the proposed methods and perform the positive impact of the noise estimation model. Remarkably, within the range of perception, the average accuracy of dipole source location can reach 13.6 mm, providing a promising reference for improving the perception abilities of underwater robots.Note to Practitioners—This paper is motivated by the problem of blind zones in near-field perception of underwater robots. The existing perception methods as visual sensing are limited by the dark and cloudy underwater environment, and are powerless in near-field localization. In addition, the artificial lateral line, as a potential near-field sensor, is challenging to be applied in self-propelled robots due to the swimming noise. This paper proposes an integrated near-field sensory system that includes an ALL sensor, a swimming noise elimination method and a dipole source localization method. Specifically, a fish-inspired ALL sensor is designed by high-accuracy pressure sensors and integrated into the robotic fish. To enhance localization performance, a swimming noise elimination model is constructed based on unsteady Bernoulli equation. Furthermore, a convolution regression network is developed for accurate localization of near-field objects. A series of simulations and experiments demonstrate the effectiveness and superiority of the proposed near-field sensory system. Hopefully, our proposed methods can provide valuable guidance and support for near-field object localization to improve the intelligent operation ability of underwater bionic robots, such as cooperative control, underwater navigation, environment exploration, and so forth.

A heuristic fast marching (FM*)-based comprehensive path planning system involving task allocation, initial planning, and replanning is presented for the robotic floating garbage cleaning mission. There are three primary contributions in this paper. First, to tackle the invalidation of the Euclidean distance metric in the obstacle environment, the task allocation is modeled as a travelling salesman problem (TSP) employing the FM*-based distance metric in order to obtain an optimal travel sequence. Second, to meet the maneuverability constraint from the surface robot and avoid the collision, a Gaussian filter is employed to adjust the curvature radius of the generated path. Third, for an efficient replanning, a neural network-based replanning point generator with the input of garbage movement vector is provided to strike a compromise for the distance cost and the computational burden. Moreover, a case study and a virtual obstacle experiment in the laboratory water tank demonstrate the feasibility of the proposed comprehensive path planning system. This work lays a firm foundation for the development of intelligent equipment for aquatic environment protection.

In this article, an extended state observer (ESO)-based controller with a model predictive governor is designed for 3-D trajectory tracking of underactuated underwater vehicles. The proposed control scheme takes three primary challenges including underactuated property, velocity constraint, and lumped disturbance into consideration. With respect to the model predictive governor, an underactuated kinematic tracking error model is utilized to produce reference velocities. Meanwhile, a heading angle compensation mechanism is utilized to avoid the steady tracking errors resulting from dynamics coupling of the vehicle. Besides, an ESO is designed to estimate the lumped disturbances and unmeasured velocity states. Based on the ESO, a kinetic controller is offered to accomplish the precise velocity tracking only in virtue of the position and orientation information. Note that this article details both the design process of the control scheme and rigorous theoretical analysis. Eventually, simulation and experimental results demonstrate the feasibility and superiority of the proposed method. Notably, this work lays the foundation for the underactuated trajectory tracking control in complicated and turbulent underwater environments.

In this article, a robot system for intelligent water surface cleaner named IWSCR is developed to collect floating plastic garbage. It is able to accomplish three major tasks autonomously, i.e., cruise and detection, tracking and steering, and grasping and collection. The challenges behind these tasks involve how to realize the accurate and real-time garbage detection, how to resist the disturbances while IWSCR conducts vision-based steering, and how to grasp the floating garbage reliably despite the turbulent conditions on the surface of the water. To overcome these difficulties, three key techniques are proposed for IWSCR. First, the YOLOv3 network, which is widely applied in the high speed and accuracy object detection field, is trained on the proposed floating garbage dataset to realize accurate and real-time garbage detection. Next, to improve the ability of resisting disturbances, a control law based on the sliding-mode controller is proposed for vision-based steering. Furthermore, inspired by the stability of floating bottles in fluid, a feasible grasping strategy is utilized for IWSCR. Finally, the experimental results demonstrate that IWSCR is competent to carry out the task of water surface cleaning.