Projects

The Development of a Biologically Inspired Propulsive Device for Autonomous Undersea Vehicles

Objective

To develop a propulsor for autonomous undersea vehicles (AUVs) using conductive polymers that is based on the pectoral fin of the bluegill sunfish (Lepomis macrochirus). The biorobotic fin will be used to give AUVs the ability to produce and control thrust like that for highly maneuverable fish.

Figure 1: Bluegill sunfish with right pectoral fin extended and CAD model of biorobotic pectoral fin.

Introduction

Fish represent a category of biological systems that have performance characteristics which have not been achieved using traditional engineering approaches. Many species of fish are highly maneuverable and have remarkable control over their body position even in turbulent flows. These abilities are the direct result of their fins being highly conformable control surfaces that can produce and vector thrust in three dimensions. The paired pectoral fins, in particular, are important for executing high and low speed maneuvers and for enhancing high speed stability. By using only the pectoral fins, bluegill sunfish are able to able to hover, conduct yaw turns, roll about their log axis, and translate backwards and forwards at low speeds. It is believed that by studying the movements of the pectoral fins, borrowing appropriately from their design, and employing novel actuators and fabrication methods that allow the biological architecture to be exploited, that a human engineered propulsor can be developed to give AUVs superior levels of control and maneuverability. This research program is using a four part approach to the analysis and design of a propulsor based on principles derived from bluegill pectoral fin function. First, a detailed, biological study of the pectoral fin’s anatomy, its mechanical properties, and the three dimensional kinematics exhibited during locomotion was conducted. Second, the hydrodynamics of fins on freely swimming fishes is being studied experimentally and through CFD simulations to estimate the hydrodynamic forces and to characterize the flow and vortex patterns created by the fin. Third, a suite of conducting polymer materials is being developed that will be incorporated as actuators, structural components, and power delivery mechanisms. It is recognized that to achieve a level of performance equal, or superior, to that of the fish fin, the final design will require actuators and actively controlled materials with properties and performance characteristics that exceed those of traditional devices. Fourth, robotic prototypes of the fin are being developed that can reproduce many of the complex motions used by the fin for propulsion and maneuvering. These prototypes are being used to understand better how the fin produces and controls its hydrodynamic forces, and to experiment with manufacturing, control, and actuation methods.

Design of the Biorobotic Fin.

The design of the first biorobotic fin prototype is based heavily on the anatomy of the sunfish pectoral fin (Figure 1). The sunfish pectoral fin is comprised of 14 fin rays that are sandwiched between two layers of a thin, flexible membrane. The fin rays are made of segmented bony halves called hemitrichs. The base of each hemitrich is attached to a cartilage pad which sits on top of the radial bones and scapula of the pectoral fin girdle. The girdle resides just within the body of the fish. The arrangement of the fin rays and bones in the pectoral fin girdle has obvious similarities to the fingers and metacarpals of the human hand and allows the base of the fin to be twisted and reoriented and the fin rays to still be swept forward and back. Each fin ray is actuated by two pairs of abductor and adductor muscles that attach to the fin ray base. There are no tendon attachments within the webbing of the fin. The two most dorsal fin ray fins are fused and have three extra muscles, presumably to provide additional control over the movement of these rays. The fin is naturally very flexible, but its stiffness and shape can be controlled by manipulating the fin ray elements. The fin rays behave much like bilaminar strips such that the stiffness and curvature of each fin ray can be controlled by contracting the abductor/abductor muscle pairs and displacing the base of one hemitrich relative to the other.

Figure 2: Biorobotic fin in air (L) and while being tested in flow tank (R).

The basic design of the biorobotic fin uses three to five fin rays embedded in a flexible urethane webbing (Figure 2). The webbing is pleated so that it can be expanded easily. Its linear measurements are 2 to 3 times that of the biological fin. The bases of the fin rays are attached to a compliant base mechanism that serves a similar same purpose as the radial bones and cartilage pad in the sunfish – it supports the fin rays, but is flexible and allows the base of the fin to move and be reoriented. The compliant base is mounted to a rigid foundation plate which connects to an array of servomotors. The servomotors control the fin rays via nylon tendons which pass through channels in the foundation plate and tie to the base of the fin rays. This design is a simplification of the sunfish anatomy, but the components retain similar functionality to their biological counterparts. actuation of the fin rays was designed to enable the fin to execute four motions that had been identified as fundamental to the biological fin’s movement. These component motions were identified by applying proper orthogonal decomposition (POD) to the fin’s displacements during steady swimming. The fundamental motions were 1) a sweep of the fin forward and back, 2) a cupping of the fin about its spanwise axis (Figure 3D), 3) an expansion of the fin’s area in the plane of the fin (Figure 3B), and 4) a curl of the distal end of the fin towards the fin’s base (Figure 3C). Sweep was executed simply by pulling the tendon attached to the front or back of the fin ray base. For expansion, tendons were attached lateral to the midline of each fin base so that the fin was opened like a hand fan. Curl was accomplished using a single tendon that pulled the two bases of a fin ray tightly against the head of the compliant base, and then by pulling the back base up and the forward base down. The cupping motion was created by pulling the most lateral fin rays of the fin forward and towards the midline of the fin.

Figure 3: Four component motions from which complex fin movements were created. Relaxed (A), expansion (B), curl (C), and cupping (D)

Performance of the Fin

The biorobotic fin is able to creat each of the four component motions very well and can combine the component motions to create fin movements that combine gross displacements of the fin with actively controlled conformations of the fin’s surface. The fin can produce motions that appeared very similar to those used by the bluegill sunfish, and as well is able to create movements such as rowing (feathering plus paddling) that may not be associated with bluegill sunfish, but that might, under certain conditions, be more effective for propelling or maneuvering an AUV. The ability to create a rich spectrum of movements enables this biorobotic fin to be used to discover the most optimal fin movements for an AUV. The biorobotic fin’s ability to create such a range of motions was due to the many degrees of freedom and level of control that was provided by actuating individual fin rays with multiple actuators, rather than actuating the fin as a single entity with individual pitch, flapping, and rowing motors.

Figure 4: Thrust and drag from different combinations of component motions.

Representative measures of the thrust and drag forces produced when using combinations of the component motions are shown in Figure 4. These results were used to investigate how the component motions contributed to the generation of thrust and drag. They were not intended to identify the maximum levels of thrust that could be produced by the fin.

Conclusion

By taking key design cues from the anatomy of the bluegill sunfish, a biorobotic pectoral fin has been developed that can produce motions that resembled closely the complex movements of the sunfish pectoral fin. Because of its many degrees of freedom and high level of control, the biorobotic fin is capable of producing a rich spectrum of motions and is a very effective experimental tool for understanding how hydrodynamic force could be produced and modulated. Based on the results from this first prototype, the next version of the biorobotic fin will use conductive polymer actuators in the fin’s webbing to activate the cupping mechanism. The development of this biorobotic fin is an important step towards developing propulsive devices that can produce and vector thrust in three dimension and give AUVs superior levels of maneuverability.