Achieving fast and agile swimming still remains extremely challenging for a self-propelled robotic fish due to the constraint of actuator’s propulsion capability. In this article, we report an untethered bioinspired robotic fish, which combines a high-frequency oscillation and a compliant passive mechanism to realize fast swimming, high pitch maneuvers, and even the leaping motion. For pursuing the explosive propulsion of the robotic fish, we propose an actuation system with a powerful output and a compact structure. A dynamic model is established and indicates that the compliant joint is able to modulate the power transmitted to a caudal fin to affect its velocity in the return stroke for generating more peak thrust. The design is validated with extensive experimental results. Namely, the robotic fish can surprisingly reach up to a speed of 3.8 body lengths per second (BL/s). Compared to the case with a rigid joint, dramatic improvements, involving a speed of 1.2 BL/s and a swimming distance of 141.2 m (70.6%), have been obtained, which reveal that besides the high-frequency oscillation, the compliant passive mechanism is also of great significance to perform high-speed swimming. Additionally, the robotic fish demonstrates its high pitch maneuvers by performing an agile front flip motion with a radius of 0.4 BL and an average angular velocity of 439 $^\circ$ /s. Most importantly, with a simple control strategy, our robotic fish can remarkably leap out of water completely. Results from this study provide significant insights into the innovative designs of next-generation robotic fishes, which require high speed and maneuverability.

For bioinspired underwater robots, it is a great challenge to achieve both high swimming speed and steering maneuverability. To this end, this article presents an untethered high-performance robotic tuna through mechanism optimization and steering strategies design. First, combining the advantages of single-joint and multijoint robotic fish, a novel mechanism of redundant joints is designed to enrich the swimming patterns of the robotic tuna. Next, a three-dimensional dynamic model is established, and the model parameters are accurately identified by experimental data. With the aid of the model and experiments, the speed performance is optimized under the key parameters of tail fin, such as size and flexibility. Further, from the perspective of imitating tuna and the demand of actual operations, two steering strategies are proposed to improve the steering performance. Finally, extensive simulation and experiments verify the effectiveness of the proposed methods. The obtained results reveal that the untethered robotic tuna can achieve both high swimming speed with 2.26 m/s (equivalent to 3.13 body lengths per second) and outstanding steering maneuverability with 0.48 body lengths in turning radius, providing valuable insight into performing special missions in cluttered ocean environments.

Underwater energy supplements for autonom- ous underwater vehicles (AUVs) are significant for ocean exploitation owing to energy storage and data communication limitations. Aiming at the energy supplement for bionic robotic fish, an autonomous recharging-oriented docking approach is proposed in this article. First, an omnidirectional docking system containing a supporting robotic fish capable of visual recognition and wireless charging and a docking platform is presented. Next, five locomotive modes are designed and analyzed based on the hybrid swimming and gliding patterns for the robot fish. Utilizing the five modes, a docking procedure is planned, and an onboard-visual-based autonomous docking control strategy is presented. Aquatic experiments, including docking with the platform and refloating a small robot, are performed to verify the effectiveness of the proposed approach. The obtained results offer valuable insights into the development of autonomous docking of bionic robots, laying a solid foundation for the permanent operation of various underwater devices and robots.

The broad and powerful pectoral fins of manta rays are crucial to their efficient and maneuverable swimming. However, very little is currently known about the pectoral-fin-driven 3-D locomotion of manta-inspired robots. This study is focused on the development and 3-D path-following control of an agile robotic manta. First, a novel robotic manta with 3-D mobility is constructed, of which the distinctive pectoral fins provide the only propulsion. Specifically, the unique pitching mechanism is detailed in which the time-coupled coordination movement of the pectoral fins is applied. Second, based on a 6-axis force measuring platform, the propulsion characteristics of the flexible pectoral fins are analyzed. Then, the force-data-driven 3-D dynamic model is further established. Third, a control scheme combined with a line-of-sight (LOS) guidance system and a sliding-mode fuzzy controller is conceived, addressing the 3-D path-following task. Finally, various simulated and aquatic experiments are conducted, demonstrating the superior performance of our prototype and the effectiveness of the proposed path-following scheme. This study will hopefully generate fresh insights into the updated design and control of agile bioinspired robots performing underwater tasks in dynamic environments.

Many aquatic animals are able to leap out of water effortlessly, however, it is still exceedingly challenging for a swimming robot. Inspired by the fast-swimming mechanism of fish, in this letter, we develop an untethered high-speed swimming robot with the integration of high-frequency oscillation and compliant passive components for performance improvements in swimming speed, efficiency and even fish-like leaping motion. The stiffness optimization of compliant components is conducted via experiments. As a result, both swimming speed and efficiency have been significantly improved and the robot surprisingly reaches a swimming speed of 1.88 m/s corresponding to 7.1 body lengths per second (BL/s). That is, the compliant components’ stiffness is of great importance to the performance improvement of a swimming robot at high-frequency oscillation. Additionally, the leaping motion is successfully performed with the optimized swimming robot and the maximum height of center of mass (CM) can reach up to 0.23 m. A simplified model is developed to analyze the leaping height with different speeds and body lengths, which suggests that the robot with small size leaps out of water more easily. The obtained results in this letter will offer some significant insights into the design and optimization for a high-speed swimming robot, as well as the cross-domain motions between water and air.

Biological fish exhibit remarkable adaptability and exceptional swimming performance through their powerful and flexible bodies. Therefore, designing a continuum flexible body is significantly important for the development of a robotic fish. However, it is still challenging to replicate these functions of a biological body due to the limitations of actuation and material. In this paper, based on a tensegrity structure, we propose a bionic design scheme for a continuum robotic fish body with a property of stiffness variation. Its detailed structures and actuation principles are also presented. A mathematical model was established to analyze the bending characteristics of the tensegrity structure, which demonstrates the feasibility of mimicking the fish-like oscillation propulsion. Additionally, the stiffness variation mechanism is also exhibited experimentally to validate the effectiveness of the designed tensegrity fish body. Finally, a novel bionic robotic fish design scheme is proposed, integrating an electronic module-equipped fish head, a tensegrity body, and a flexible tail with a caudal fin. Subsequently, a prototype was developed. Extensive experiments were conducted to explore how control parameters and stiffness variation influence swimming velocity and turning performance. The obtained results reveal that the oscillation amplitude, frequency, and stiffness variation of the tensegrity robotic fish play crucial roles in swimming motions. With the stiffness variation, the developed tensegrity robotic fish achieves a maximum swimming velocity of 295 mm/s (0.84 body length per second, BL/s). Moreover, the bionic tensegrity robotic fish also performs a steering motion with a minimum turning radius of 230 mm (0.68 BL) and an angular velocity of 46.6°/s. The conducted studies will shed light on the novel design of a continuum robotic fish equipped with stiffness variation mechanisms.