Scombrid fish possess superior-performance propulsion characteristics, attributed to the powerful contraction of the peduncle tendon muscle which effectively transmits energy to the caudal fin. Inspired by the bionic structure, we proposed a fish-like propeller consisting of a driving joint and a peduncle joint connected in series. The peduncle joint combines magnetorheological (MR) damping with elastic components and is actuated by a sine mechanism to replicate biological motion. The intensity of the damping mechanism enhances with frequency, which can compensate for the shortcomings of insufficient elastic intensity at high frequencies. The integration of both mechanisms aims to accomplish high propulsion across a wide frequency range. Then, an MR damper is constructed to imitate the tendon muscle of the peduncle. The shear stress of the MR fluid possesses online intensity adjustment capability by regulating the current of the inner coil, thereby affecting the damping torque output by the peduncle joint. Experimental results reveal that the controllable damping torque ranges from 49.3 to 134.4 mNm. A forward thrust dynamic model is built according to the Lagrangian method to explore the effect of the shear stress on propulsion. Simulated results indicate that at a specific frequency, an optimal shear stress exists that enables the system to achieve the best propulsion. Meanwhile, as frequency increases, the best propulsion gradually enhances, and the shear stress corresponding to the best propulsion also increases. Experimental results of four caudal fins with different aspect ratios (ARs) show that the shear stress should be further increased to accomplish best propulsion as AR decreases. The max average thrust of the fish-like propeller can reach 1.956 N at a frequency of 3 Hz.

Passive electric sense, a unique underwater perception ability in aquatic species, enables fish to detect external electrical signals and perform essential life activities in dark or turbid environments. Inspired by the passive electrosensing capability of natural shark, this letter proposes a low-cost hardware solution for the robotic shark’s passive electric sense system to detect weak underwater electric fields. Furthermore, motion-induced electrode displacement generates dynamic electric field noise that interferes with measurements. We analyze the properties of this noise which is correlated with the body waves of the robotic shark, and then propose a suppression method based on the noise analysis. Next, simulation tests are conducted to demonstrate the time and frequency properties of dynamic electric field noise. Finally, through physical experiments, the robotic shark with passive electric sense system has successfully detected $\mu$V-level underwater electric fields, and the proposed suppression method has improved the signal-to-noise ratio from 6.697dB to 9.445dB compared to the unfiltered situation.

Fish can actively modulate the mechanical properties of their muscles to adapt to complex hydrodynamic environments, and extensive research has been dedicated to replicating such adaptive locomotion capabilities in bionic robotic fish. Unlike the widely investigated variable-stiffness tail designs in existing studies, this paper proposes a decoupled tunable rotational inertia mechanism for the robotic fish caudal peduncle, which enables independent regulation of tail rotational inertia without altering the global center of mass of the robot. A symmetric cable-driven transmission system equipped with a pair of oppositely moving mass spheres is adopted to realize real-time online inertia tuning while sustaining the robot’s dynamic balance. A high-fidelity fluid-structure interaction (FSI) dynamic model is established by integrating the pseudo rigid body method (PRBM) and the boundary data immersion method (BDIM). Simulation and experimental results demonstrate that rotational inertia modulation exerts a significant regulatory effect on the swimming performance of bionic robotic fish. Specifically, increased rotational inertia enhances thrust generation and forward swimming velocity within a specific tail-beat frequency range, whereas it degrades locomotion performance when the tail-beat frequency exceeds a critical threshold. Remarkably, the performance improvement enabled by enhanced rotational inertia can only be achieved via internal mass redistribution. In contrast, directly attaching additional mass to increase rotational inertia will almost invariably impair the swimming capability of the robot. This study reveals the unique regulatory effect of rotational inertia as an independent control input for bio-inspired underwater robots, providing a unique regulatory effect for the development of unified impedance-inertia adaptive control frameworks for future bionic underwater platforms.