Ribbon Fin Research Articles

Underwater collision avoidance using undulating elongated fin propulsion

Underwater navigation to survey littoral regions or complex environments such as offshore structures, coral reefs or sensitive coastal zones requires a highly maneuverable propulsion mechanism. Conventional propulsors have limited maneuverability, often incapable of providing adequate torques and forces to maneuver in such environments. Elongated ribbon fin propulsion, a bio-inspired propulsion technique, can overcome these shortcomings. In the present work, we used a robotic vessel with elongated fin propulsion to test and implement three collision avoidance strategies based on potential field methods. We found that having a circular sensing envelope in the robotic vessel helps to smoothly transition to the mission path after collision is avoided. However, under this strategy, the distance to return to the desired path tends to be longer. The selection of adequate collision avoidance strategies will depend on multiple factors, including others sensing capabilities, desired reaction time, rate of turning and objects to avoid. Although this work focused on relatively simple scenarios for obstacle avoidance, we envision that utilizing more complex maneuver techniques, including rapid turns and hovering, collision avoidance strategies for underwater vessels with undulating fin propulsion can take advantage of the full fin kinematics manipulation to maneuvers as elegant as seen on the fish counterpart.

Inspired by the Black Ghost Knifefish: Bionic Design of Undulatory Fin with 2-DOF Rays and Its Propulsion Performance

The demand for high-performance underwater thrusters in marine engineering is increasing. The concealed, mobile, and efficient underwater ability of fish provides many directions for research. The black ghost knifefish uses only wavy ventral fins to swim and can hover and roll in the water. Based on the physiological and morphological characteristics of the black ghost knifefish, we explored the structure and movement mode of the ventral fin, so as to establish a two-degree of freedom (2-DOF) structural model and kinematic model. We reveal the motion mechanism of the undulating fin propulsion through the constructed model and computational fluid dynamics. It is found that when the fin surface fluctuates, a pair of vortices with opposite directions will be formed on the concave side of the fin surface. These vortices will produce a central jet on the fin surface, provide a reverse impulse for the ventral fin, and make the fin obtain power. In addition, we found that the propulsive force of the ribbon fin along the body direction is positively correlated with the swing amplitude and frequency of the fin movement, and the propulsive torque of the ribbon fin to realize the maneuvering movement increases first and then decreases with the increase of the torsion angle. The research on the structure and motion mechanism of the ribbon fin of the black ghost knifefish provides a basis for the development of a bionic prototype of multi-DOF motion and the control strategy of high-mobility motion.

Knifefish turning control and hydrodynamics during forward swimming.

Rapid turning and swimming contribute to ecologically important behaviors in fishes such as predator avoidance, prey capture, mating and the navigation of complex environments. For riverine species, such as knifefishes, turning behaviors may also be important for navigating locomotive perturbations caused by turbulent flows. Most research on fish maneuvering focuses on fish with traditional fin and body morphologies, which primarily use body bending and the pectoral fins during turning. However, it is uncertain how fishes with uncommon morphologies are able to achieve sudden and controllable turns. Here, we studied the turning performance and the turning hydrodynamics of the black ghost knifefish (Apteronotus albifrons, N=6) which has an atypical elongated ribbon fin. Fish were filmed while swimming forward at ∼2 body lengthss-1 and feeding from a fixed feeder (control) and an oscillating feeder (75 Hz) at two different amplitudes. 3D kinematic analysis of the body revealed the highest pitch angles and lowest body bending coefficients during steady swimming. Low pitch angle, high maximum yaw angles and large body bending coefficients were characteristic of small and large turns. Asynchrony in pectoral fin use was low during turning; however, ribbon fin wavelength, frequency and wave speed were greatest during large turns. Digital particle image velocimetry (DPIV) showed larger counter-rotating vortex pairs produced during turning by the ribbon fin in comparison to vortices rotating in the same direction during steady swimming. Our results highlight the ribbon fin's role in controlled rapid turning through modulation of wavelength, frequency and wave speed.

Beyond the Kármán gait: knifefish swimming in periodic and irregular vortex streets.

Neotropical freshwater fishes such as knifefishes are commonly faced with navigating intense and highly unsteady streams. However, our knowledge on locomotion in apteronotids comes from laminar flows, where the ribbon fin dominates over the pectoral fins or body bending. Here, we studied the 3D kinematics and swimming control of seven black ghost knifefish (Apteronotus albifrons) moving in laminar flows (flow speed U∞≈1-5 BL s-1) and in periodic vortex streets (U∞≈2-4 BL s-1). Two different cylinders (∼2 and ∼3 cm diameter) were used to generate the latter. Additionally, fish were exposed to an irregular wake produced by a free oscillating cylinder (∼2 cm diameter; U∞≈2 BL s-1). In laminar flows, knifefish mainly used their ribbon fin, with wave frequency, speed and acceleration increasing with U∞. In contrast, knifefish swimming behind a fixed cylinder increased the use of pectoral fins, which resulted in changes in body orientation that mimicked steady backward swimming. Meanwhile, individuals behind the oscillating cylinder presented a combination of body bending and ribbon and pectoral fin movements that counteract the out-of-phase yaw oscillations induced by the irregular shedding of vortices. We corroborated passive out-of-phase oscillations by placing a printed knifefish model just downstream of the moving cylinder, but when placed one cylinder diameter downstream, the model oscillated in phase. Thus, the wake left behind an oscillating body is more challenging than a periodic vortex shedding for an animal located downstream, which may have consequences on inter- and intra-specific interactions.

Dynamic modeling of biomimetic undulatory ribbon fin underwater propulsor actuated by IPMC

Abstract In this paper, electro-mechanical dynamic modeling of a biomimetic propulsor based on undulating ribbon fin is proposed. Kinematics of the system is considered as the sinusoidal undulating motion of the ribbon fin which is guided by the actuating ionic polymer metal composite material. This is a smart material that actuates when an input voltage is applied and composed of a simple RC circuit. Elongated body theory is used to produce system dynamics considering the kinematics and also the hydrodynamic parameters like drag is undertaken as the application aspect is underwater environment. The proposed model has been used to evaluate the speed and thrust of the biomimetic system. Also, the thrust efficiency can be obtained for the different input frequencies. The proposed model may contribute to the understanding of an undulating ribbon fin inspired by IPMC for the underwater locomotion.

Computational research on a combined undulating-motion pattern considering undulations of both the ribbon fin and fish body

In this work, the combined undulating-motion pattern (CUMP) of the undulatory ribbon-fin propulsion is numerically investigated, inspired by the swimming of the large-mouthed catfish (Silurus soldatovi meridionalis Chen). The swimming of a juvenile large-mouthed catfish is observed and recorded to study the undulation of its elongated anal fin, commonly known as the “ribbon fin”. Compared with the knifefish, which undulates the ribbon fin to swim while maintaining a relatively rigid fish body, the large-mouthed catfish performs a CUMP propulsion, which comprises the undulation of the fish body and the active undulation of the ribbon fin. Accordingly, a fin–body fish model is used to study the effect of the CUMP propulsion on the swimming performance. Simulation results show that the phase-angle difference between the undulations of the fish body and the ribbon fin has a significant impact on the thrust and heave forces. More specifically, an in-phase CUMP propulsion, which is commonly found in the swimming of actual large-mouthed catfish, can dramatically improve the thrust. In addition, the effects of the angular amplitude of ribbon fin and the swimming frequency are considered as well. Finally, the reasons for the thrust augmentation in the in-phase CUMP propulsion are detailed and analyzed.

Robotic device shows lack of momentum enhancement for gymnotiform swimmers

Many fish generate thrust by undulating one or multiple elongated fins while keeping their body straight. This propulsion mechanism has stimulated interest in both biology and bio-inspired marine propulsion because its maneuverability and efficiency at low speed. Analytical studies have found that a fin attached to a rigid flat body can produce substantially higher thrust compared to a fin without a body, three- to four-fold for natural swimmers. However, this momentum enhancement has not been confirmed experimentally. In this work, a robotic ribbon fin model with an adjustable-height body was used to test the momentum enhancement for gymontiform swimmers where the undulating fin runs along the ventral side of the body. In a series of experiments, the force generated by the robotic device was measured as the body height of the robot, the undulating fin frequency and the flow speed were changed. It was found that the thrust generated by the ribbon fin is not affected by the presence of a body, thereby resulting in no momentum enhancement due to the fin-body interaction. These results suggest that if there is a benefit at a specific fin-body height ratio of the fishes, the momentum enhancement is not the reason. This result has broader implications in understanding the evolutionary adaption of undulatory fin propulsion and underwater vehicles designs.

Propulsive performance of an under-actuated robotic ribbon fin

Many aquatic animals propelled by elongated undulatory fins can perform complex maneuvers and swim with high efficiency at low speeds. In this propulsion, one or multiple waves travel along an elastic fin composed of flexible rays. In this study, we explore the potential benefits or disadvantages of passive fin motion based on the coupling of fluid-structure interactions and elasto-mechanical responses of the undulatory fin. The motivation is to understand how an under-actuated undulating fin can modify its active and passive fin motion to effectively control the hydrodynamic force and propulsive efficiency. We study the kinematics and propulsive performance of an under-actuated ribbon fin using a robotic device. During two experimental sets for fully-actuated fin and under-actuated fin respectively, we measured fin kinematics, surge forces and power consumption. Our results show that under-actuated fin can generate smaller thrust but consume less power comparing to a fully-actuated counterpart. The thrust generated by an under-actuated fin scales similarly to a fully-actuated fin—linear with the enclosed area and quadratic with the relative velocity. Power consumption scales with cube of lateral tangential velocity. Furthermore, we find that the under-actuated fin can keep the same propulsive efficiency as the fully-actuated fin at low relative velocities. This finding has profound implications to both natural swimmers and underwater vehicles using undulating fin-based propulsion, as it suggests that they can potentially exploit passive fin motion without decrementing propulsive efficiency. For underwater vehicles with undulatory fins, an under-actuated design can greatly simplify the mechanical design and control complexity of a versatile propulsion system.

Fin Ray Stiffness and Fin Morphology Control Ribbon-Fin-Based Propulsion.

Ribbon-fin-based propulsion has rich locomotor capabilities that can enhance the mobility and performance of underwater vehicles navigating in complex environments. Bony fishes using this type of propulsion send one or multiple traveling waves along an elongated fin with the actuation of highly flexible rays that are interconnected by an elastic membrane. In this work, we study how the use of flexible rays and different morphology can affect the performance of ribbon-fin propulsion. We developed a physical model composed of 15 rays that are interconnected with an elastic membrane. We tested four different ray flexural stiffness and four aspect ratios. The robotic model was tested in a low-turbulence flume under two flow conditions ([Formula: see text] wavelength/s). In two experimental sets, we measured fin kinematics, net surge forces, and power consumption. Using these data, we perform a thrust and power analysis of the undulating fin. We present the thrust coefficient, power coefficient, and propulsive efficiency. We find that the thrust generation was linear with the enclosed area swept by the fin, and square of the relative velocity between the incoming flow and traveling wave. The thrust coefficient levels off around 0.5. In addition, for our parameter range, we find that the power consumption scales by the cube of the effective tangential velocity of the rays [Formula: see text] (A is the amplitude of the ray oscillating motion, and [Formula: see text] is the angular velocity). We show that a decay in stiffness decreases both thrust production and power consumption. However, for rays with high flexural stiffness, the difference in thrust compared with rigid rays is minimal. Moreover, our results show that flexible rays can improve the propulsive efficiency compared with a rigid counterpart. Finally, we find that the morphology of ribbon fin affects its propulsive efficiency. For the aspect ratio considered in our experiments, [Formula: see text] was the most efficient compared with [Formula: see text]. Our results suggest that there could be an optimal morphology for a given ribbon fin kinematics. Therefore, both natural swimmers and underwater vehicles using ribbon-fin-based propulsion can take advantage of flexible rays and optimal aspect ratio to improve propulsive performance.

Separability of drag and thrust in undulatory animals and machines.

For nearly a century, researchers have tried to understand the swimming of aquatic animals in terms of a balance between the forward thrust from swimming movements and drag on the body. Prior approaches have failed to provide a separation of these two forces for undulatory swimmers such as lamprey and eels, where most parts of the body are simultaneously generating drag and thrust. We nonetheless show that this separation is possible, and delineate its fundamental basis in undulatory swimmers. Our approach unifies a vast diversity of undulatory aquatic animals (anguilliform, sub-carangiform, gymnotiform, bal-istiform, rajiform) and provides design principles for highly agile bioinspired underwater vehicles. This approach has practical utility within biology as well as engineering. It is a predictive tool for use in understanding the role of the mechanics of movement in the evolutionary emergence of morphological features relating to locomotion. For example, we demonstrate that the drag-thrust separation framework helps to predict the observed height of the ribbon fin of electric knifefish, a diverse group of neotropical fish which are an important model system in sensory neurobiology. We also show how drag-thrust separation leads to models that can predict the swimming velocity of an organism or a robotic vehicle.

Kinematics of the ribbon fin in hovering and swimming of the electric ghost knifefish

Two errors appeared in J. Exp. Biol. 216, 823-834.During data processing, the wrong scaling factor was used when converting the fin amplitude from pixels on the video to linear displacement in millimetres [term y(x) in Eqn 1]. This error caused the angular displacement to be underestimated by a factor that varied between 2.9 and 3.6 (mean ± s.d. = 3.28 ± 0.27) for the different data sets.The error is found in Figs 2, 4, 9 and 10. In the right panels of Fig. 2, both panels of Fig. 4, Fig. 9C and the color bar in Fig. 10, the range of the y-axis is approximately three times smaller than it should have been.In the Results and Discussion, we comment on the direction of the change in amplitude at different swimming speeds. The error does not affect these comments because the amplitude at all swimming speeds was scaled by approximately the same factor.A second error was introduced when calculating the ratio of the fin length to the fin base length. The measurement of the fin base length was performed only on the first frame of every data set, instead of averaging the length across all the frames as was done for the fin length. This caused the ratio of fin length to fin base length to be larger in the hovering condition and smaller when swimming at 6 cm s−1. After the correction, the ratio is more constant than before.This error can be found in Fig. 9B, as well as in the sentence ‘The amount of stretch might not be insignificant: as Fig. 9B shows, the edge of the fin is 1.3 times the length of the fin base for a hovering fish.’ This sentence should read ‘The amount of stretch might not be insignificant: as Fig. 9B shows, the edge of the fin is 1.2 times the length of the fin base for a hovering fish.’The corrected figures appear here.The authors apologise for any inconvenience, and assure readers that these errors do not affect any other part of the analysis or conclusions.

Biomimetic and bio-inspired robotics in electric fish research

Weakly electric knifefish have intrigued both biologists and engineers for decades with their unique electrosensory system and agile swimming mechanics. Study of these fish has resulted in models that illuminate the principles behind their electrosensory system and unique swimming abilities. These models have uncovered the mechanisms by which knifefish generate thrust for swimming forward and backward, hovering, and heaving dorsally using a ventral elongated median fin. Engineered active electrosensory models inspired by electric fish allow for close-range sensing in turbid waters where other sensing modalities fail. Artificial electrosense is capable of aiding navigation, detection and discrimination of objects, and mapping the environment, all tasks for which the fish use electrosense extensively. While robotic ribbon fin and artificial electrosense research has been pursued separately to reduce complications that arise when they are combined, electric fish have succeeded in their ecological niche through close coupling of their sensing and mechanical systems. Future integration of electrosense and ribbon fin technology into a knifefish robot should likewise result in a vehicle capable of navigating complex 3D geometries unreachable with current underwater vehicles, as well as provide insights into how to design mobile robots that integrate high bandwidth sensing with highly responsive multidirectional movement.

Kinematics of the ribbon fin in hovering and swimming of the electric ghost knifefish

Weakly electric knifefish are exceptionally maneuverable swimmers. In prior work, we have shown that they are able to move their entire body omnidirectionally so that they can rapidly reach prey up to several centimeters away. Consequently, in addition to being a focus of efforts to understand the neural basis of sensory signal processing in vertebrates, knifefish are increasingly the subject of biomechanical analysis to understand the coupling of signal acquisition and biomechanics. Here, we focus on a key subset of the knifefish's omnidirectional mechanical abilities: hovering in place, and swimming forward at variable speed. Using high-speed video and a markerless motion capture system to capture fin position, we show that hovering is achieved by generating two traveling waves, one from the caudal edge of the fin and one from the rostral edge, moving toward each other. These two traveling waves overlap at a nodal point near the center of the fin, cancelling fore-aft propulsion. During forward swimming at low velocities, the caudal region of the fin continues to have counter-propagating waves, directly retarding forward movement. The gait transition from hovering to forward swimming is accompanied by a shift in the nodal point toward the caudal end of the fin. While frequency varies significantly to increase speed at low velocities, beyond approximately one body length per second, the frequency stays near 10 Hz, and amplitude modulation becomes more prominent. A coupled central pattern generator model is able to reproduce qualitative features of fin motion and suggest hypotheses regarding the fin's neural control.

Stimulus predictability mediates a switch in locomotor smooth pursuit performance for Eigenmannia virescens

The weakly electric glass knifefish, Eigenmannia virescens, will swim forward and backward, using propulsion from an anal ribbon fin, in response to motion of a computer-controlled moving refuge. Fish were recorded performing a refuge-tracking behavior for sinusoidal (predictable) and sum-of-sines (pseudo-random) refuge trajectories. For all trials, we observed high coherence between refuge and fish trajectories, suggesting linearity of the tracking dynamics. But superposition failed: we observed categorical differences in tracking between the predictable single-sine stimuli and the unpredictable sum-of-sines stimuli. This nonlinearity suggests a stimulus-mediated adaptation. At all frequencies tested, fish demonstrated reduced tracking error when tracking single-sine trajectories and this was typically accompanied by a reduction in overall movement. Most notably, fish demonstrated reduced phase lag when tracking single-sine trajectories. These data support the hypothesis that fish generate an internal dynamical model of the stimulus motion, hence improving tracking of predictable trajectories (relative to unpredictable ones) despite similar or reduced motor cost. Similar predictive mechanisms based on the dynamics of stimulus movement have been proposed recently, but almost exclusively for nonlocomotor tasks by humans, such as oculomotor target tracking and posture control. These data suggest that such mechanisms might be common across taxa and behaviors.

1A2-M05 力学的相互作用を活用したヒレ型ロボットの自律分散制御(水中ロボット・メカトロニクス)

Although physical interaction between individual components plays an important role in autonomous decentralized control systems, a systematic way of designing it is still lacking. In order to alleviate this, we focus on an undulating fin of the black ghost knifefish, and we propose a design scheme based on the physical interaction between individual components. More specifically, we theoretically analyze the undulating fin based on a continuum model and derive optimal force distribution. Based on the optimal force distribution, we propose a decentralized control scheme of a robotic ribbon fin. The robotic ribbon fin developed shows that the proposed control scheme allows the robot to locomote efficiently.

The hydrodynamics of ribbon-fin propulsion during impulsive motion

Weakly electric fish are extraordinarily maneuverable swimmers, able to swim as easily forward as backward and rapidly switch swim direction, among other maneuvers. The primary propulsor of gymnotid electric fish is an elongated ribbon-like anal fin. To understand the mechanical basis of their maneuverability, we examine the hydrodynamics of a non-translating ribbon fin in stationary water using computational fluid dynamics and digital particle image velocimetry (DPIV) of the flow fields around a robotic ribbon fin. Computed forces are compared with drag measurements from towing a cast of the fish and with thrust estimates for measured swim-direction reversals. We idealize the movement of the fin as a traveling sinusoidal wave, and derive scaling relationships for how thrust varies with the wavelength, frequency, amplitude of the traveling wave and fin height. We compare these scaling relationships with prior theoretical work. The primary mechanism of thrust production is the generation of a streamwise central jet and the associated attached vortex rings. Under certain traveling wave regimes, the ribbon fin also generates a heave force, which pushes the body up in the body-fixed frame. In one such regime, we show that as the number of waves along the fin decreases to approximately two-thirds, the heave force surpasses the surge force. This switch from undulatory parallel thrust to oscillatory normal thrust may be important in understanding how the orientation of median fins may vary with fin length and number of waves along them. Our results will be useful for understanding the neural basis of control in the weakly electric knifefish as well as for engineering bio-inspired vehicles with undulatory thrusters.

Omnidirectional Electric Fish

Omnidirectional Electric Fish

Biomimetic Motion Planning of an Undulating Robotic Fish Fin

This paper presents a locomotion control implementation of a robotic system mimicking the undulating fins of fish. To mimic the actual flexible fin of a real fish, we created a ribbon fin type actuation device with a series of connecting linkages and attached it to the robotic fish. By virtue of a specially designed strip withaslider, eachlinkisabletoturnandslidewithrespect to the adjacent link. The driving linkages are used to form a mechanical fin consisting of several fin segments, which are able to produce undulations, similar to those produced by actual fins. By virtue of the modular and reconfigurable fin mechanisms, two robotic fish with different fin layouts have been designed and constructed: The first prototype is a robotic stingray, swimming by undulations of a pair of lateral fins-the second is a robotic knifefish, swimming by undulations of a long anal fin. The locomotion scheme and mechatronics implementation of the robotic stingray are presented and discussed with a parametric study of the slider’s workspace and joint trajectory. Some experimental observations of the robotic knifefish are alsoshownanddiscussed. Theresults demonstrate that the designed fin mechanisms are able to undulate sections of pectoral and anal fins, with different amplitudes and phases, along the length of the fin in the direction of motion. This may lead to further developments that better mimic the fin locomotion capability of real fish.

Designing Future Underwater Vehicles: Principles and Mechanisms of the Weakly Electric Fish

Future underwater vehicles will be increasingly called upon to work in cluttered environments and to interact with their surroundings. These vehicles will need sensors that work efficiently at short range and be highly maneuverable at low speed. To obtain insights into principles and mechanisms of low-speed operation in cluttered environments, we examine a fish that excels in this regime, the black ghost knifefish Apteronotus albifrons. This fish hunts in dark or turbid water using a short-range self-generated electric field to sense its surroundings. Coupled with this unique mode of sensing is an unusual ribbon fin propulsion system that confers high multidirectional maneuverability at low speeds. To better understand the relationship between body morphology and common maneuvers of this fish, we utilized an idealized ellipsoidal body model, Kirchhoff's equations, and an optimal control algorithm for generating trajectories. We present evidence that common fish trajectories are optimal, and that these trajectories complement the sensory abilities of the fish. We also discuss prototypes of the sensing and propulsion systems of the fish with a view to providing alternative approaches for underwater vehicle design where high maneuverability in geometrically complex environments is needed.