James L. Tangorra

Fish biorobotics: kinematics and hydrodynamics of self-propulsion

SUMMARY As a result of years of research on the comparative biomechanics and physiology of moving through water, biologists and engineers have made considerable progress in understanding how animals moving underwater use their muscles to power movement, in describing body and appendage motion during propulsion, and in conducting experimental and computational analyses of fluid movement and attendant forces. But it is clear that substantial future progress in understanding aquatic propulsion will require new lines of attack. Recent years have seen the advent of one such new avenue that promises to greatly broaden the scope of intellectual opportunity available to researchers: the use of biorobotic models. In this paper we discuss, using aquatic propulsion in fishes as our focal example, how using robotic models can lead to new insights in the study of aquatic propulsion. We use two examples: (1) pectoral fin function, and (2) hydrodynamic interactions between dorsal and caudal fins. Pectoral fin function is characterized by considerable deformation of individual fin rays, as well as spanwise (along the length) and chordwise (across the fin) deformation and area change. The pectoral fin can generate thrust on both the outstroke and instroke. A robotic model of the pectoral fin replicates this result, and demonstrates the effect of altering stroke kinematics on the pattern of force production. The soft dorsal fin of fishes sheds a distinct vortex wake that dramatically alters incoming flow to the tail: the dorsal fin and caudal fin act as dual flapping foils in series. This design can be replicated with a dual-foil flapping robotic device that demonstrates this phenomenon and allows examination of regions of the flapping performance space not available to fishes. We show how the robotic flapping foil device can also be used to better understand the significance of flexible propulsive surfaces for locomotor performance. Finally we emphasize the utility of self-propelled robotic devices as a means of understanding how locomotor forces are generated, and review different conceptual designs for robotic models of aquatic propulsion.

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

SUMMARY We 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 500× stiffness fin rays and lift was maximized by the 1000× stiffness fin rays.

The effect of fin ray flexural rigidity on the propulsive forces generated by a biorobotic fish pectoral fin.

SUMMARY A biorobotic pectoral fin was developed and used to study how the flexural rigidities of fin rays within a highly deformable fish fin affect the fins propulsive forces. The design of the biorobotic fin was based on a detailed analysis of the pectoral fin of the bluegill sunfish (Lepomis macrochirus). The biorobotic fin was made to execute the kinematics used by the biological fin during steady swimming, and to have structural properties that modeled those of the biological fin. This resulted in an engineered fin that had a similar interaction with the water as the biological fin and that created close approximations of the three-dimensional motions, flows, and forces produced by the sunfish during low speed, steady swimming. Experimental trials were conducted during which biorobotic fins of seven different stiffness configurations were flapped at frequencies from 0.5 to 2.0 Hz in flows with velocities that ranged from 0 to 270 mm s–1. During these trials, thrust and lift forces were measured, kinematics were recorded in three dimensions, and digital particle image velocimetry was used to evaluate flow hydrodynamics. The results of the trials revealed that slight changes to the fins mechanical properties or to the operating conditions can have significant impact on the direction, magnitude and time course of the propulsive forces. In general, the magnitude of the 2-D (thrust and lift) propulsive force scaled with fin ray stiffness, and increased as the fins flapping speed increased or as the velocity of the flow decreased.

Robotic Models for Studying Undulatory Locomotion in Fishes

Many fish swim using body undulations to generate thrust and maneuver in threedimensions.Thepatternofbodybendingduringsteadyrectilinearlocomotion has similar general characteristics in many fishes and involves a wave of increasing amplitude passing from the head region toward the tail. While great progress has been made in understanding the mechanics of undulatory propulsion in fishes, the inability to control and precisely alter individual parameters such as oscillation frequency, body shape, and body stiffness, and the difficulty of measuring forces on freely swimming fishes have greatly hampered our ability to understand the fundamental mechanics of the undulatory mode of locomotion in aquatic systems. In this paper, we present the use of a robotic flapping foil apparatus that allows these parameters to be individually altered and forces measured on self-propelling flapping flexible foils that produce a wave-like motion very similar to that of freely swimming fishes. We use this robotic device to explore the effects of changing swimming speed, foil length, and foil-trailing edge shape on locomotor hydrodynamics, the cost of transport, and the shape of the undulating foil during locomotion. We also examine the passive swimming capabilities of a freshly dead fish body. Finally, wemodel fin-fininteractionsin fishesusingdual-flappingfoilsandshowthatthrust can be enhanced under correct conditions of foil phasing and spacing as a result of the downstream foil making use of vortical energy released by the upstream foil.

A biorobotic model of the sunfish pectoral fin for investigations of fin sensorimotor control

A comprehensive understanding of the control of flexible fins is fundamental to engineering underwater vehicles that perform like fish, since it is the fins that produce forces which control the fishs motion. However, little is known about the fins sensory system or about how fish use sensory information to modulate the fin and to control propulsive forces. As part of a research program that involves neuromechanical and behavioral studies of the sunfish pectoral fin, a biorobotic model of the pectoral fin and of the fins sensorimotor system was developed and used to investigate relationships between sensory information, fin ray motions and propulsive forces. This robotic fin is able to generate the motions and forces of the biological fin during steady swimming and turn maneuvers, and is instrumented with a relatively small set of sensors that represent the biological lateral line and receptors hypothesized to exist intrinsic to the pectoral fin. Results support the idea that fin ray curvature, and the pressure in the flow along the wall that represents the fish body, capture time-varying characteristics of the magnitude and direction of the force created throughout a fin beat. However, none of the sensor modalities alone are sufficient to predict the propulsive force. Knowledge of the time-varying force vector with sufficient detail for the closed-loop control of fin ray motion will result from the integration of characteristics of many sensor modalities.

Biorobotic fins for investigations of fish locomotion

Experimental analyses of propulsion in freely-swimming fishes have led to the development of self-propelling pectoral and caudal fin robotic devices. These biorobotic models have been used in conjunction with biological and numerical studies to investigate the effects of the fins kinematic patterns and structural properties on forces and flows. Data from both biorobotic fins will be presented and discussed in terms of the utility of using robotic models for understanding fish locomotor dynamics. Through the use of the robotic fins, it was shown that subtle changes to the kinematics and/or the mechanical properties of fin rays can impact significantly the magnitude, direction, and time course of the 3d forces used in propulsion and maneuvers.

A biorobotic flapping fin for propulsion and maneuvering

A series of biorobotic fins has been developed based on the pectoral fin of the bluegill sunfish. These robotic fins model physical properties of the biological fin, and execute kinematics derived from sunfish motions that were identified to be most responsible for thrust. When the physical properties of the robotic fin are tuned appropriately to operating conditions, the robotic fin, like the sunfish, produces positive thrust throughout the entire fin beat. Due to having many degrees of freedom, these fins can be used to generate and control forces for propulsion and maneuvering.

A biologically derived pectoral fin for yaw turn manoeuvres

A bio-robotic fin has been developed that models the pectoral fin of the bluegill sunfish as the fish turned to avoid an obstacle. This work involved biological studies of the sunfish fin, the development of kinematic models of the motions of the fins rays, CFD based predictions of the 3D forces and flows created by the fin, and the implementation of simplified models of the fins kinematics and mechanical properties in a physical model. The resulting robotic fin produced the forces and flows that drove the manoeuvre and had a sufficiently high number of degrees of freedom to create a variety of non-biologically derived motions. The results indicate that for robotic fins to produce a level of performance on par with biological fins, both the kinematics and the mechanical properties of the biological fin must be modelled well.

Understanding the Hydrodynamics of Swimming: From Fish Fins to Flexible Propulsors for Autonomous Underwater Vehicles

The research effort described here is concerned with developing a maneuvering propulsor for an autonomous underwater vehicle (AUV’s) based on the mechanical design and performance of sunfish pectoral fin. Bluegill sunfish (Lepomis macrochirus) are highly maneuverable bony fishes that have been the subject of a number of experimental analyses of locomotor function [5, 6]. Although swimming generally involves the coordinated movement of many fin surfaces, the sunfish is capable of propulsion and maneuvering using almost exclusively the pectoral fins. They use pectoral fins exclusively for propulsion at speeds of less than 1.1 body length per second (BL/s). The curve in Fig. 1 depicts two peaks of body acceleration of bluegill sunfish during steady forward swimming. These abilities are the direct result of their pectoral fins being highly deformable control surfaces that can create vectored thrust. The motivation here is that by understanding these complex, highly controlled movements and by borrowing appropriately from pectoral fin design, a bio-robotic propulsor can be designed to provide vectored thrust and high levels of control to AUVs. This paper will focus on analyses of bluegill sunfish’s pectoral fin hydrodynamics which were carried out to guide the design of a flexible propulsor for AUV’s

Fish Locomotion: Biology and Robotics of Body and Fin-Based Movements

The study of fish locomotion provides a rich source of inspiration for the design of robotic devices. Fish exhibit an array of complex locomotor designs that involve both diversity of structures used to generate locomotor forces, and versatile behaviors to engage with the aquatic environment. The functional design of fish includes both a flexible body exhibiting undulatory motion as well as numerous control surfaces that enable fish to vector forces and execute rapid maneuvers in roll, pitch, and yaw directions. Patterns of body undulation have often been misunderstood, and fish with propulsive mechanics as diverse as tuna and eels can display similar patterns of body bending during swimming. Many of the often-cited classical locomotor categories are based on a misunderstanding of body and fin kinematics. Fish fins can exhibit remarkably complex conformational changes during propulsion, and do not function as flat plates but have individual mobile fin rays actuated by muscles at the fin base. Fin motion and surface bending in most fish is actively controlled. Even during steady horizontal locomotion, median fins such as the dorsal and anal fins function to balance torques and can contribute to thrust. Locomotion using body undulation is not achieved independently from fin motion, and the vast majority of fish locomotor patterns utilize both the body and fins. Robotic systems derived from fish templates can range from simple flexible plastic panels to more complex models of whole body and fin design. Experimental test platforms that represent individual fins or specific components of fish locomotor design allow for detailed testing of hydrodynamic and mechanical function. Actuating and controlling complex fish robotic systems involving both the body and multiple individual fins are a major challenge for the future.