A robotic fish caudal fin: effects of stiffness and motor program on locomotor performance

Abstract

SUMMARYWe designed a robotic fish caudal fin with six individually moveable fin rays based on the tail of the bluegill sunfish, Lepomis macrochirus. Previous fish robotic tail designs have loosely resembled the caudal fin of fishes, but have not incorporated key biomechanical components such as fin rays that can be controlled to generate complex tail conformations and motion programs similar to those seen in the locomotor repertoire of live fishes. We used this robotic caudal fin to test for the effects of fin ray stiffness, frequency and motion program on the generation of thrust and lift forces. Five different sets of fin rays were constructed to be from 150 to 2000 times the stiffness of biological fin rays, appropriately scaled for the robotic caudal fin, which had linear dimensions approximately four times larger than those of adult bluegill sunfish. Five caudal fin motion programs were identified as kinematic features of swimming behaviors in live bluegill sunfish, and were used to program the kinematic repertoire: flat movement of the entire fin, cupping of the fin, W-shaped fin motion, fin undulation and rolling movements. The robotic fin was flapped at frequencies ranging from 0.5 to 2.4Hz. All fin motions produced force in the thrust direction, and the cupping motion produced the most thrust in almost all cases. Only the undulatory motion produced lift force of similar magnitude to the thrust force. More compliant fin rays produced lower peak magnitude forces than the stiffer fin rays at the same frequency. Thrust and lift forces increased with increasing flapping frequency; thrust was maximized by the 500ϫ stiffness fin rays and lift was maximized by the 1000ϫ stiffness fin rays. Supplementary material available online at

Speciation through the lens of biomechanics: locomotion, prey capture and reproductive isolation

“…Robotic systems offer the advantage of facilitating force measurement, the ability to explore a large parameter space of possible parameters, and greater control over flow visualization measurements. We believe that there will be increasing use of robotic systems in comparative biology to allow more precise understanding of the relationship between the phenotype and performance [86,87], especially where interspecific comparisons involve such distantly related species that one cannot have confidence in comparisons of biological systems or can serve as 'surrogate organisms' in cases where animal function cannot be directly observed. The design of robotic models that capture key phenotypic features of these hard-to-get species may be of use in testing the performance consequences of interspecific phenotypic differences that arise during the process of speciation.…”

Investigation of Fish Caudal Fin Locomotion Using a Bio-Inspired Robotic Model

“…The intrinsic muscle controlling the caudal fin has evolved into such a sophisticated mechanism that it can achieve complex 3D motion of the caudal fin membrane [24,[41][42]. Studies on real fish and robotic models have all demonstrated that even a symmetric caudal fin can realize asymmetric hydrodynamic function to aid the fish's manoeuvrability [26,13].…”

“…Earlier research focused only on the fish caudal fin itself, including the movement patterns [13], fin ray stiffness [26], tail shape [20] and even membrane thickness [43], but overlooking the critical point that the fish tail is derived from the undulatory wave of the body and is attached to the caudal peduncle, which has a maximum lateral excursion of the undulatory wave [21]. If we take the motion of the caudal peduncle and the caudal fin as a whole, we can then introduce an important motion parameter, i.e., the phase angle Φ between the fin ray and caudal peduncle motion.…”

“…For most of the rayfinned fishes, the caudal fin does not move only in two dimensions: under the control of the intrinsic musculature, the fin rays actively move as the fish swims, forming a complex three-dimensional (3D) propulsive surface [22][23][24][25]. Earlier studies on caudal fin active deformation investigated the hydrodynamic effects of different motion patterns and fin ray stiffness at a constant flow speed and found that this kind of three-dimensional motion may contribute to the fish's manoeuvrability [13,[26][27]. However, although the musculature of the caudal fin is seperate from the posterior axial body musculature, bony fishes are able to control the caudal peduncle and the fin surface simultaneously, so the three-dimensional caudal fin motion should not be considered separately.…”

Biomimetic and bio-inspired robotics in electric fish research

“…Lastly, the bony rays of the fish are flexible and actually bend (Lauder, 2006) rather than simply rotate like the rigid rays used in many of the robotic ribbon fin models. Research in flexible rays in other fins, for example caudal fins (Esposito et al, 2012), will hopefully inspire similar work in future ribbon fin models to better match biological ribbon fin kinematics. This will facilitate additional progress in zeroing in on their mechanical impact on fish swimming.…”