In this paper, a novel compliant joint with two identical torsion springs is proposed for a biomimetic multi-joint robotic fish, which enables imitatation of the swimming behavior of live fish. More importantly, a dynamic model based on the Lagrangian dynamic method is developed to explore the compliant passive mechanism. In the dynamic modeling, a simplified Morrison equation is utilized to analyze the hydrodynamic forces. Further, the parameter identification technique is employed to estimate numerous hydrodynamic parameters. The extensive experimental data with different situations match well with the simulation results, which verifies the effectiveness of the obtained dynamic model. Finally, motivated by the requirement for performance optimization, we firstly take advantage of a dynamic model to investigate the effect of joint stiffness and control parameters on the swimming speed and energy efficiency of a biomimetic multi-joint robotic fish. The results reveal that phase difference plays a primary role in improving efficiency and the compliant joint presents a more significant role in performance improvement when a smaller phase difference is given. Namely, at the largest actuation frequency, the maximum improvement of energy efficiency is obtained and surprisingly approximates 89%. Additionally, the maximum improvement in maximum swimming speed is about 0.19 body lengths per second. These findings demonstrate the potential of compliance in optimizing joint design and locomotion control for better performance.

In this paper, we present the design and implementation of an innovative bio-inspired gliding robotic fish and a gliding angle controller. Through mimicking a whale shark, the gliding robotic fish is designed with diversified control surfaces and a streamline shape instead of the conventional wings with large wingspan aiming at both high maneuverability and distinguished gliding performance. The mechatronic design of the whale shark-inspired gliding robotic fish is first provided. A dynamic model for gliding motion is established and the hydrodynamic analysis is performed via computational fluid dynamics simulation. The effects of both the deflection of the pectoral fins and the displacement of the movable mass on the gliding motion are analyzed. Then, based on the model and the analysis, a gliding angle control strategy is constructed via the backstepping methodology and the sliding mode methodology, aiming at the robustness of the control system to unknown perturbation. Afterwards, a control allocation law is designed and solved by Newton method. Simulations are conducted to evaluate the effectiveness of the proposed controller. Experiments are also executed to verify the dynamic model and illustrate the remarkable motion capability of the newly developed robot prototype. The results of the study supply clues to the improvement of the locomotive ability for the gliding robotic fish in both shape design and motion control.

This article proposes the mechanical design and dynamic model of an innovative manta-inspired robot system for both efficient fast swimming and high spatial maneuverability. Inspired by some biological studies, a pair of unique pectoral fins with six separate degrees of freedoms (DOFs) are developed. The novel design is characterized by an improved crank-rocker mechanism and a distinctive horizontal DOF. The former not only endows the robot with high swimming speed, but also guarantees efficient flapping patterns which are close to manta rays. The latter is employed to coordinate with the flapping movement, allowing remarkable pitch adjustment. Further, the basic motion strategy is presented by detailed analyses to the pectoral fins. Besides, based on the Morrison equation and infinitesimal method, a complete dynamic model for robotic manta with flexible pectoral fins is established, whose parameters are determined through experimental data. Moreover, the linear swimming and pitching experiments are conducted, demonstrating the prominent movement performance of the presented design and the effectiveness of the dynamic model. The obtained results shed light on updated design and control of next-generation agile underwater vehicles and robots capable of multimodal motions in dynamic and complex aquatic environments.

The intrinsically reciprocating motion in fishlike propulsion causes severe attitude instability of a robotic fish, which poses enormous challenges for environmental perception and autonomous operation. To address this issue, in this article, we propose a reaction-wheel-based control framework for guaranteeing the roll stability of the robotic fish. The mechatronic design and dynamic model of the designed robotic fish with an internal rotor are presented. By means of the simplified model and frequency domain analysis, the effect factors about roll stability are concretely analyzed. More importantly, a hybrid controller that combines a sliding mode controller with a neural network feedforward compensator is developed to reject the severe disturbance on roll angle. Then, the Lyapunov stability theory is utilized to analyze the stability and convergence property of the closed-loop system. Finally, the experimental results show that the proposed methods possess more significant performances than the passive stabilization method, which provides a valuable reference for attitude stabilization control and robust environmental perception of underwater robots.

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.

Remora, a bony fish, is well known for the remarkable hitchhiking behavior, by which it can transport itself over large distances without too much effort. In this article, a novel robotic remora with a stable adhesion system and its motion control methods are proposed for transferring the hitchhiking behavior to the engineered system. In terms of the mechatronic design, a robotic prototype with the uncoupled planar and vertical motion mechanisms is created to realize the precise and agile movement. Furthermore, a reliable adhesion system possessing low preload demand, considerable adhesive force ( $\sim \text{250}\text{ N}$ ), and reversible adhesion ability is developed, and then, the design principle of the adhesive disc is investigated. In the aspect of motion control, a mode-switch-based planar pose (position and orientation) control method and an antidisturbance depth control method are designed. The combination of these two controllers endows the robotic remora with the ability to precisely arrive at any position in a 3-D space, which paves the way for the hitchhiking task. Extensive experimental results are presented to support the performance benefits of the proposed robotic remora and control methods. Finally, the hitchhiking behavior is implemented successfully by robotic remora. To the best of our knowledge, this is the first engineered implementation of hitchhiking for a fin-actuated underwater robot. The results obtained will promote the design and control of the future underwater adhesion robot and offer valuable insights into the long-endurance robotic fish and mother–son multirobot system.