A robotic platform is described that will be used to study the hydrodynamic performance—forces and flowfields—of the swimming California sea lion. The robot is a model of the animal's foreflipper that is actuated by motors to replicate the motion of its propulsive stroke (the 'clap').
The California sea lion (Zalophus californianus), is an agile and powerful swimmer. Unlike many successful swimmers (dolphins, tuna), they generate most of their thrust with their large foreflippers. This protocol describes a robotic platform designed to study the hydrodynamic performance of the swimming California sea lion (Zalophus californianus). The robot is a model of the animal's foreflipper that is actuated by motors to replicate the motion of its propulsive stroke (the 'clap'). The kinematics of the sea lion's propulsive stroke are extracted from video data of unmarked, non-research sea lions at the Smithsonian Zoological Park (SNZ). Those data form the basis of the actuation motion of the robotic flipper presented here. The geometry of the robotic flipper is based a on high-resolution laser scan of a foreflipper of an adult female sea lion, scaled to about 60% of the full-scale flipper. The articulated model has three joints, mimicking the elbow, wrist and knuckle joint of the sea lion foreflipper. The robotic platform matches dynamics properties—Reynolds number and tip speed—of the animal when accelerating from rest. The robotic flipper can be used to determine the performance (forces and torques) and resulting flowfields.
While scientists have investigated the basic characteristics of sea lion swimming (energetics, cost of transport, drag coefficient, linear speed and acceleration1-3, we lack information about the fluid dynamics of the system. Without this knowledge, we limit potential high-speed, high-maneuverability engineering applications to body-caudal fin (BCF) locomotion models4. By characterizing a different swimming paradigm, we hope to expand our catalog of design tools, specifically those with the potential to enable quieter, stealthier forms of swimming. Thus, we study the fundamental mechanism of sea lion swimming through direct observation of the California sea lion and laboratory investigations using a robotic sea lion foreflipper5,6 .
To do this, we will employ a commonly used technique for exploring complex biological systems: a robotic platform7. Several locomotion studies—both of walking8,9 and swimming10—have been based on either complex11 or highly simplified12 mechanical models of animals. Typically, the robotic platforms retain the essence of the model system, while allowing researchers to explore large parameter spaces13-15. While not always characterizing the entire system, much is learned through these platforms that isolate a single component of a locomotive system. For example, the fundamental functioning of unsteady propulsors, like the back-and-forth sweeping of a caudal fin during carangiform swimming, has been intensely explored through experimental investigations of pitching and/or heaving panels12,16,17,18. In this case, we can isolate certain modes of this complex motion in ways that animal based studies cannot. Those fundamental aspects of propulsion can then be used in the design of vehicles which do not need the biological complexity evolution provides.
In this paper, we present a novel platform for exploring the 'clap' phase of the sea lion thrust-producing stroke. Only a single foreflipper—the 'roboflipper'—is included in the platform. Its geometry is derived exactly from biological scans of a California sea lion (Zalophus californianus) specimen. The roboflipper is actuated to replicate the motion of the animals' derived from previous studies1. This robotic flipper will be used to investigate the hydrodynamic performance of the swimming sea lion and to explore a wider parameter space than animal studies, particularly those of large aquatic mammals, can yield.
1. Digitize a Specimen of a Sea Lion Foreflipper
2. Design the Bone Structure
3. Creating a Flipper
4. Mounting
The process described above yields a robotic model of a California sea lion foreflipper. The model can be used in two different ways. One is by actuating the flipper only at the root (Figure 6a). In this case, the driving motor sets the rotational rate of the first joint, but the resulting motion of the flipper is determined by the fluid-structure interaction between the flexible flipper and the surrounding water. Additionally, we can create robotic flippers that are actuated at the two lower joints in addition to the root (Figure 6b). This is done through the tower structures printed onto the skeleton pieces. Wires connected to the towers are connected to separate motors and can actively control the camber of the flipper during the clapping motion.
The purpose of the robotic flipper is to explore the hydrodynamics of the propulsive stroke of the California sea lion as described in Friedman, 20141. One way to do this, qualitatively, is through dye-based flow visualization. The robotic flipper is mounted to a recirculating water flume (Figure 7), using the assembly described above. The motor and flow speed, are set to explore a given parameter space—such as the Reynolds number based on the flipper chord (Re = cU/ν where ν is the dynamic viscosity of water) or angular velocity, ω, or acceleration, α.
The dye visualization shown in Figure 9 uses fluorescent dye injected just upstream of the leading edge of the flipper. The dye is entrained into the shear layer at the surface of the flipper and allows us to visualize the vortex structure of the wake. Figure 9a shows the stream of dye being injected upstream (to the right), of the flipper. The disturbances seen on the left side of the image are the result of the previous cycle. As the flipper moved through the injection location (Figure 9b), low pressure on the upper surface of the flipper causes the dye to be pulled around the flipper. Finally, (Figure 9c), a vortex forms as the flipper moves fully out of the plane. This structure convects downstream with the mean flow. These results demonstrate how this technique can be used to qualitatively determine the flowfield surrounding a sea lion during the propulsive stroke.
In addition to the qualitative measurements of the flipper wake, we can use particle image velocimetry (PIV) to measure the velocity field surrounding the flipper. Thus, we can obtain qualitative data about the hydrodynamics of sea lion swimming for a variety of reproducible situations.
Figure 1: Flipper Bottom Comparison. A left foreflipper from a specimen of a female California sea lion is used to determine the robotic flipper's geometric parameters. The top panel (a) is a high resolution, two-dimensional image of the flipper. The lower panel (b) is a three-dimensional, computer-aided design rendering of the flipper from the laser scan. Please click here to view a larger version of this figure.
Figure 2: Wire. The digital image of the scanned flipper retains the geometric features of the animal's foreflipper. This image shows a wire-frame view of the digital flipper. Nine evenly spaced cross sections are shown in grey (every centimeter from the base to the tip of the foreflipper). The two isometric views (cross section 1 and 7) show that the flipper has an airfoil-like shape, with a thicker, rounded leading edge. The flipper is cambered, with its upper surface more convex and its inner surface concave. Please click here to view a larger version of this figure.
Figure 3: Mold. The mold used to create the flexible portion of the robotic flipper is created from the scanned flipper specimen. The mold has two parts: an upper (purple) and a lower section (green) that are aligned with male and female posts, respectively. The robot skeleton (Figure 4) is aligned inside the mold before the silicon mixture is poured into the mold. Please click here to view a larger version of this figure.
Figure 4: Skeleton. The flexible robotic flipper is supported by a skeleton printed in three pieces: the base (a), the middle (b) and the tip (c). The base and middle, and the middle and tip, are connected by dowels through knuckles at their joints. This allows for flexibility about those locations of the completed flipper. Please click here to view a larger version of this figure.
Figure 5: Skeleton Assembly. After printing, the skeleton parts, the knuckles are reinforced with carbon threads (a), they are connected at the knuckles with axels (b), guide-tubes are affixed to the base and middle pieces (c) and Kevlar threads are connected to the towers (d). Please click here to view a larger version of this figure.
Figure 6: Robotic Flipper. The robotic flipper is made of flexible silicone (white) with an imbedded plastic supporting structure (blue). The shaft at the base rotates, emulating the rotation at the elbow and shoulder of the animal. The robotic flipper can be passive (a), where it is only actuated at the root and the resulting motion is based of fluid-structure interactions, or active (b) where Kevlar wires connect to the knuckles provide the necessary changes in camber. Please click here to view a larger version of this figure.
Figure 7: Flume. Flow experiments are conducted in the recirculating water flume at the George Washington University. The flume has a working section of 0.60 (width) by 0.40 (depth) meters, is 10 meters long, and can run at flow velocities of up to 1 m/s. Flow is from right to left, in the figure. The robotic flipper is mounted using the assembly shown in Figure 8 to the rails at the top of the test section. Please click here to view a larger version of this figure.
Figure 8: Assembly. The robotic flipper is mounted to a recirculating flume with a custom mounting. The mounting holds a servomotor that is connected to the main axis of the robotic flipper (located at the root of the robotic flipper) through a belt and three pulleys. Please click here to view a larger version of this figure.
Figure 9: Dye visualization. Fluorescent dye is injected through a tube upstream of the flapping flipper. Three instances of time are shown: (a) the beginning of the cycle t= 0, (b) 40% of the way through the cycle t= 0.4, and (c) after 80% of the cycle t= 0.8. In the right panel (c), we can see a vortex that has formed around the tip of the flapping robotic flipper. Please click here to view a larger version of this figure.
The robotic flipper apparatus will allow us to understand the hydrodynamics of the swimming California sea lion. This includes the basic thrust producing stroke (the 'clap'), as well as non-physical variations that animal studies cannot investigate. The robotic flipper has been designed for experimental versatility, thus, step 3—where the flipper itself is made—is critical in obtaining the desired results. While this apparatus is, clearly, just a model of the living system, in situ studies of the California sea lion are extremely difficult and the range of possible data is quite limited.
While sometimes possible, velocity field measurements on large aquatic animals are very difficult (e.g. untrained animals, non-research grade viewing glass, no control over the environment), and the errors are higher than laboratory experiments21. Furthermore, they require access to the animals that is often impossible to obtain and in such cases robotic platforms like the one we built allow for in depth investigations. In addition to replicating the living system as faithfully as possible, robotic models allow us to modify it in unrealistic ways. For example, the mold can be modified to alter the trailing edge morphology. Or, the texture of the surface can be changed to investigate the role of the microstructure on the swimming performance.
The use of a robotic platform to investigate the performance of a biological system gives only a partial view of that system—this is a limitation of this approach. Furthermore, this particular protocol isolates the foreflipper from the rest of the sea lion body. Thus, the results will not offer a complete view of the system and the body-flipper interactions. Further limitations include the homogenous properties of the flipper and point wise actuation (as opposed to the distributed actuation of musculoskelatal systems). Additionally, that material is compliant and can lead to fluid-structure-interactions that are not present in the physical system. This is minimized by using materials that closely replicate the overall biological properties, but can never be completely controlled for. Despite these limitations, much can be learned by comparing the performance of different activation modes and flow conditions.
The robotic flipper will form the basis of a rich research project that will provide insight into the fundamental physics of a unique paradigm of efficient swimming—the California sea lion. The platform is flexible, and each flipper can be made quickly with minimal cost. Thus, a large parameter space can be tested as new research questions arise.
The authors have nothing to disclose.
The authors would like to thank the George Washington University Facilitating Fund for financial support of the project. Mr. Patel is grateful the George Washington University School of Engineering and Applied Science Summer Undergraduate Program in Engineering Research and the Undergraduate Research award for financial support. Finally, we are grateful to the GWU Center for Biomemetics and Bioinspired Engineering (COBRE) for use of facilities controlled by the center.
Dragon Skin 20 | Smooth-on | ||
Dragon Skin 20 medium | Smooth-on | ||
Object24 | Stratasys | 3D printer | |
Stand Mixer | Hamilton | ||
PKS-PRO-E-10 System | Anaheim Automation | PKS-PRO-E-10-A-LP22 | Controller and Servo Motor |
Artec Eva | Artec 3D | 3D light scanner with resolution of 0.1mm | |
Artec Spider | Artec 3D | 3D light scanner with resolution of 0.5mm | |
Steel plate | Mcmaster | ||
Carbon Tow | Fibreglast | 2393-A | |
Hardened Precision 440C Stainless Steel Shaft | Mcmaster | 6253K49 | |
Tygon PVC Clear Tubing | Mcmaster | 6546T23 | |
Kevlar Thread | Mcmaster |