A Robotic Platform to Study the Foreflipper of the California Sea Lion

Summary

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').

Abstract

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.

Introduction

While scientists have investigated the basic characteristics of sea lion swimming (energetics, cost of transport, drag coefficient, linear speed and acceleration^1-3, 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 o....

Protocol

1. Digitize a Specimen of a Sea Lion Foreflipper

1. Scan a specimen of a Sea lion foreflipper.
   1. Obtain a specimen of a sea lion flipper from a deceased individual (Figure 1a).
      NOTE: In our case, they were obtained from the Smithsonian Zoological Park in Washington, D.C.
   2. Hang the foreflipper vertically from its base (where the foreflipper attaches to the animal's body). This both allows the flipper to be straight when scanned, and exposes the entire surface for scanning.
   3. Scan flipper using a high-resolution structured light scanner, with an accuracy of approximately 0.5 mm, and error of approximately 0.1 mm (....

Results

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 flipp.......

Discussion

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.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
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 0.1 mm
Artec Spider	Artec 3D		3D light scanner with resolution of 0.5 mm
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