This paper proposes a novel actuation mechanism based on high-frequency flow-induced flutter in a gradient-discontinuous soft pipe. The actuator of the bionic fish is inspired by the tentacles of pipe anemones. The actuator consists of a gradient-discontinuous soft pipe with a thin membrane attached to its outer surface. Fluid flow through the pipe induces high-frequency flutter, which drives the attached membrane and generates propulsive force. First, an absolute nodal coordinate formulation (ANCF)-based dynamic model is established to predict the vibration response of the gradient-discontinuous soft pipe conveying fluid. The optimal configuration of the gradient-discontinuous pipe for enhanced propulsion is determined through theoretical predictions. The effects of internal flow velocity on the pipe response are then investigated theoretically, and the model is experimentally validated at an internal flow velocity of 5 m/s. Subsequently, numerical simulations are performed to investigate the effects of membrane dimensions on the vibration response and hydrodynamic performance and to identify the optimal membrane geometry. Finally, swimming experiments are conducted using membranes of different widths. The results identify a bending angle of 60°, a membrane length of 65 mm, and a membrane width of 45 mm as the optimal design parameters at an internal flow velocity of 5 m/s. The designed bionic fish achieves the highest mean swimming velocity of 20.3 cm/s (3.38 BL/s) with the 45-mm-wide membrane. The present study provides a novel driving approach for achieving high-frequency responses in flexible actuators. Keywords: Bionic robot; flow-induced flutter; flexible actuator; gradient-discontinuous pipe; high-frequency responses.
Flying fish evade predatory threats through cross-medium gliding manoeuvres, in which the ventral fins play a pivotal regulatory role. Although previous studies have examined the influence of ventral fin configuration on gliding stability, the dynamic characteristics of escape trajectories and the regulatory effects of ventral fin posture on gliding attitude remain poorly understood. Here, we employ a biomimetic approach to elucidate the gliding patterns exhibited by flying fish during escape manoeuvres. We systematically investigate how ventral fin posture regulates gliding attitude and identify an optimal ventral fin mounting configuration. Combining full-factor experimental testing with computational fluid dynamics (CFD) simulations, we propose that flying fish adopt distinct gliding trajectories to evade predators in different media, with dynamic ventral fin adjustments acting as a key regulatory mechanism. Furthermore, by extending the parameter space beyond that accessible through biological observation, this study provides new theoretical guidance for the aerodynamic design and optimisation of multi-fin biomimetic robots.
Abstract Reliable aerial perching on variable-diameter curved surfaces requires both sufficient grasping preload and conformal interfacial contact, whereas conventional underactuated bird-claw-inspired mechanisms primarily accommodate in-plane geometric variations and therefore have limited adaptability to surfaces with spatially varying curvature. This study presents a bending–torsion coupled bird-claw-inspired attachment mechanism for variable-diameter curved surfaces. The key design combines direction-selective compliant joints with a bilateral force-balanced tendon transmission, allowing asymmetric contact reactions to passively reorient individual claw segments. This passive reorientation improves interfacial conformity during grasping and provides more favorable contact and sealing conditions for dry adhesion and vacuum suction. A grasping-stability model was employed to optimize the key structural parameters, followed by mechanical characterization of the compliant joints and attachment tests on smooth acrylic cylinders with diameters of 60–120 mm and cones with apex angles of 20°–60°. The equivalent bending stiffness of the compliant joint was tunable from 0.2 to 1.0 N·mm/°, while high torsional compliance was maintained within the small-angle working range. The maximum circumferential attachment moment reached 1.65 N·m on a 120 mm diameter cylinder and 0.60 N·m on a 60° apex-angle cone. A manually controlled flight experiment further demonstrated successful perching on a 20° conical surface. These results demonstrate that bending–torsion coupled compliance can broaden the geometric adaptation range of underactuated claw-based attachment mechanisms and provide a feasible structural approach for aerial perching on variable-diameter curved surfaces.
Abstract Biological systems achieve great efficiency with tactile sensing by multiplexing receptors along shared afferent pathways, whereas soft robots, on the other hand, still rely on bulky, one-to-one sensor routing. To introduce this capability to soft-engineered systems, we present a soft optical network that utilizes input-agnostic junctions to multiplex time-of-flight signals. By connecting elastomeric waveguides into a network, we consolidate multiple sensory zones into a single shared afferent channel. We developed a curve-fitting optimization approach to deconvolve overlapping time-of-flight signals. The amplitude and arrival time of those signals convey information about the deformation state of each of the sensor paths in the robot. Testing with a prototype incorporating hard optical junctions demonstrates that the system disambiguates press and stretch deformations across four sensing zones. The prototype network produced 3 distinct signal peaks associated with the different zones. While each zone behaved differently, the system could generally detect changes at 20 g increments of pressing, and signal peaks 1 and 3 showed sensitivity to stretching. Finally, we discuss an open-face molding approach for fabricating fully soft junctions, and describe its limitations, providing insights to guide the future development of fully soft, monolithic, proprioceptive, and exteroceptive networks.
This study presents a bio-inspired open knee joint for biofidelic robots with an anatomically correct range of motion. Bio-inspired joints contribute to robot performance and understanding living organisms by providing dynamic motion data as the real-world physical twin. The proposed joint structure features the integration of the open joint and ligamentous constraint. In particular, the spherical envelope of the condyles contributes to the rotation other than pitch rotation. By applying a bio-inspired ligament arrangement, the developed knee joint performed pitch and yaw motion. The relationship between the ligament length and joint orientation was derived by forward kinematics and the ligament wrapping. The arrangement of the bio-inspired collateral ligaments was determined with the iterative calculation method considering the object wrapping, which predicted the yaw range of motion. Verification of the proposed approach includes the evaluation of the range of motion on the robotic knee joint equipped with the motor-driven tendons. The robot validation tests include the rotational motions around the pitch and yaw axes. The tests showed that the proposed joint structure can be a relevant approach for replicating the range of motion of a human knee joint. Future work includes the motion control of the joint system and integration into a real robot.
