2019 2nd IEEE International Conference on Soft Robotics (RoboSoft)
COEX, Seoul, Korea, April 14-18, 2019

Toward a bio-inspired variable-stiffness morphing limb for amphibious
robot locomotion
Robert L. Baines1 , Joran W. Booth1 , Frank E. Fish2 , and Rebecca Kramer-Bottiglio1
Abstract— Most robots operate either exclusively on land
or in water. Toward building an amphibious legged robot,
we present a morphing limb that can adapt its structureand stiffness for amphibious operation. We draw inspiration
for the limb’s design from the morphologies of sea turtle
flippers and land-faring tortoise legs. Turtles and tortoises
have rigid hulls that can be emulated in amphibious robots toprovide a convenient, protected volume for motors, electronics,
power supply, and payloads. Each of these animals’ limbs
are tailored for locomotion in their respective environments.
A sea turtle flipper has a streamlined profile to reduce drag,making it apt for swimming. A land tortoise leg boasts a strong,
expanded cross-section conducive to load-bearing. We capture
the morphological advantages of both animals’ limbs in ourmorphing limb via a variable stiffness composite coupled to apneumatic actuator system that enables on-demand transitions
between leg and flipper configurations. We control the degreeof stiffness of the limb by varying electrical input to flexible
heaters bound to the thermally responsive variable stiffness
composite. The proposed morphing amphibious limb design is
promising for enabling the next generation of hybrid soft-rigid
robots to adapt to unstructured environments.

Fig. 1. We developed a morphing robotic limb intended for amphibious
robots. Starting in a flipper state, the variable stiffness composite in the limb
is subjected to heat via an embedded Joule-heated flexible heater, whichsoftens it. Then, a pneumatic actuator pair is inflated to morph the limb to
a transition state where it is held until the composite cools. At that point,pressure is no longer required to hold the leg configuration. Heating up theleg configuration again returns it to the flipper state.

I. INTRODUCTION
Amphibious robots represent a growing area of interest for
roboticists. The ability to move both on land and in waterwith the same autonomous system has significant ramifications for security, industry, defense, and transportation. A
looming challenge with amphibious robots is that in orderto move through varying media, a robot must fundamentally
adjust its means of locomotion. A classic solution, which
has been explored extensively, is to rely on multiple distinct
parts, such as wheels, rotors, and jets, to move on land or inwater [1], [2], [3].
It is an attractive prospect to integrate multiple locomotion functionalities into one actuation space to prevent
overburdening a robot’s physical architecture. For example,
serpentine robots can both swim and commute on land byvarying their oscillation frequency and amplitude [4], [5].
In addition, fish [6] and salamander [7] locomotor strategies
have inspired robots composed of connected modules whichvary undulations to move both on land and in aquatic
environments. Unfortunately, the many degrees of freedomof these robots creates a challenging control problem, diminishing their application scope. Moreover, oscillating serpentinspired robot designs are not conducive to carrying larger
payloads.

A second approach to building amphibious robots strives
to mechanically integrate swimming and walking mechanisms of legged robots, which have relatively simplified
controller dynamics and have the capacity to transport payloads. One turtle-mimetic robot was designed with rigid legs
equipped with passive hinged flaps that enabled both swimming and walking [8]. On the upstroke during swimming, thehinged flaps allowed the leg to move through water without
much resistance. On the power stroke, the flap snapped into
place against the leg, providing a large surface area for
propulsive force. Another robot was shown to transform its
hybrid wheel-leg system to accommodate different terrains[9]. Yet another robot morphed its legs from a walking to aswimming configuration using clutch-activated interlocking
rigid segments [10].
The aforementioned approaches to integrate walking andswimming functionality pose elegant solutions, but they
exhibit a lack of environmental versatility because they rely
on rigid components. A wealth of useful environmental
adaptations are untenable since their mechanisms have afixed stiffness associated with each limb state. For instance, if
a robot is swimming in water with obstacles, it is preferable
to decrease its limb stiffness so that the system can sustain
impacts without breaking. In nature, such structural compliance protects the appendages of a number of biological

1 Department of Mechanical Engineering & Materials Science, Yale
University, 10 Hillhouse Avenue, New Haven, CT 06520, USA. (email:rebecca.kramer at yale.edu).
2 Department of Biology, West Chester University, 730 S High St West
Chester, PA 19383, USA.

978-1-5386-9260-8/19/$31.00 ©2019 IEEE704

Fig. 2. Manufacturing the morphing limb can be separated into four distinct steps. First the actuators were cast from elastomer in an acrylic mold. Thenthey were attached to a fabric sleeve. Next, unidirectional fabric was bonded to the top of the actuators. Lastly, the variable stiffness composite-foil heatersystem was inserted into designated pockets in the fabric sleeve.

flippers with profiles apt for efficient lift-based swimming[13]. Thin and cambered, sea turtle flippers can cut throughwater with minimal drag [14]. On the other hand, tortoises
have highly stable cylindrical cross-section legs that allow
them to bear and walk under the often substantial weightof their carapaces. Herein, we create a limb which cantransform from the shape of a sea turtle flipper to a tortoiseleg (Figure 1). We accomplish this through use of a variable
stiffness material-pneumatic actuator system. In particular,
the limb can morph from a flipper to a leg phase whenthe variable stiffness materials inside are softened via Jouleheated flexible copper heaters, and the antagonistic pneumatic actuator pair on the exterior is inflated, deforming
the materials. The limb can sturdily retain a leg shapeafter the variable stiffness material cools below its glasstransition temperature (Tg ) of 30-40◦ C and hardens. Holding
the air pressure inside the actuators is thus no longer needed.
Morphing back to the flipper phase is achieved by re-heating
the variable stiffness elements past Tg , which relaxes themback to the initial, flat geometry in which they were cast.
This described sequence of operation for the morphing limb
prototype is pictured in Figure 1.
With this work, we unlock an unprecedented spectrum oflimb stiffnesses and topologies which we can use to optimize
amphibious robot locomotion within a particular environment: water, land, or littoral zones. An additional benefitof our approach is robustness, specifically when compared
to previously reported amphibious robotic limbs composedof rigid metal components. The inherently corrosion-resistantand vibration-immune materials in our system make it highly
viable for unstructured, dynamically changing amphibious
environments.

Fig. 3. The morphing limb is approximately 170 mm long, 85 mm wideat the bottom, tapered to 75 mm wide at the top, 30 mm thick in the flipper
stage, and expands to a ∼70 mm diameter in the leg stage.

organisms, notably winged insects, from shattering when
they are subjected to large impacts [11]. Similarly, if therobot is transitioning from water to land, an intermediatelimb stiffness has been shown to be optimal for moving
efficiently [12]. It is therefore useful to be able to tune thestiffness of a locomotion mechanism on an amphibious robot.
Recent work has demonstrated how variable stiffness limbs
can allow an amphibious robot to move effectively throughvarious environments [12]. The author’s presented approach
utilizes a mechanism with five stiffness presets. Changes in
stiffness are accomplished by adjusting the height of thelimbs relative to their connection on the robot. This coupling
between geometry and stiffness constrains a robot’s adaptive
abilities. For instance, the robot cannot use the length ofits fully extended leg at a high stiffness. It is desirable tohave independent control over geometry and stiffness in an
amphibious robotic limb. Further, the only way to change
between stiffness presets on the robot is to do so manually,prior to deploying the robot. As a result, the stiffness of its
leg cannot change in-situ. To fully realize the advantage ofautonomous legged amphibious robots, limb stiffness changemust be able to occur without manual intervention.
We approach the challenge of developing amphibious
robots by observing the limb shapes of two morphologicallysimilar animals: sea turtles and tortoises. Sea turtles have

II. FABRICATION OF COMPONENTS OF THE MORPHING
LIMB

The morphing limb comprises three major subsystems.
The first subsystem is composed of two actuators that705

induce the bending deformation of the limb, allowing it totransition from a flipper to a leg state. The second subsystemconstitutes the variable stiffness layers and paired heatersthat are used to soften the variable stiffness material on
demand. The third subsystem is the fabric sleeve that holds
the actuators and variable stiffness layers together. Figure 2
details how all of these subsystems are integrated together
in the manufacturing process of the morphing limb, andFigure 3 shows the final prototype in its two configurations.
In subsequent sections, we elaborate on each component usedin the manufacturing process.
1) Actuator: There are two identical actuators on the morphing limb. Each actuator is an extension of the traditional
Pneunet actuator [15], but with added unidirectional fabricon its top to further direct inflation deformation along thelongitudinal axis of the limb. More recent updates to the
Pneunet actuator, such as fiber wrapped actuators [16], could
not be applied because it is impractical to wrap tension
limiters around the length of a large, planar area.
The actuators were cast in laser-cut acrylic molds froma platinum-cure silicone rubber with Shore hardness 10(Dragon Skin 10A, Smooth-On). The mold was degassed ina vacuum chamber after which a flat lid and heavy weight
was placed on top of the mold to improve flatness anddimensional accuracy between batches. Then, the actuators
were attached to a fabric sleeve, which acted as the primarystrain limiting layer to induce bending. Next, a unidirectional
fabric was bonded to the top of the actuators such that thefibers were aligned along the length of the limb. The additionof this fabric helped direct actuator deformation. Lastly, thevariable stiffness composite and heater system was inserted,
completing the assembly.
2) Variable Stiffness Composite and Resistive Heater:
The variable stiffness composite is composed of a two-partepoxy system mixed with graphite particles, imbued in aplain-weave cotton fabric. The epoxy is a thermoset that
decreases by two orders of magnitude in bending stiffness
past a Tg of 30-40◦ C [17]. This quality allows the material
to be deformed into different geometries when it is softened.
Upon cooling, it retains the newly deformed shape. Whenheated yet again, stress relaxation in the material forces itback to its initial configuration with some hysteresis. We
utilize this mechanism to morph back and forth between the
flipper and leg states.
The variable stiffness layer was created by mixing 28%
wt. Jeffamine D400 (Hunstaman International, LLC), 69%
wt. EPON 828 (Momentive Performance Materials Inc.) and
3% wt. expanded graphite. The thermoset epoxy enables
variable stiffness; the low percentage conductive carbon filler
enhances stiffness and thermal conductivity. The carbon
filler was created by expanding sulfuric-acid intercalated
graphite (Sigma-Aldrich) at 800◦ C, adding the expandedgraphite to cyclohexane to create a 1% wt. graphite mixture,and subjecting it to sonication for 4 hr (QSonica Q700
Ultrasoniator). The resulting graphite nanoparticles haveapproximately 50 nm diameter. Lastly, the mixture was dried
and blended into a fine powder to elicit the final graphite

Fig. 4. Flexible heaters were fabricated by laser etching thin kaptonclad copper foil to produce a serpentine conductive trace pattern, as shownin the microscope top view inset. We adhered these heaters to the variable
stiffness composite for on-demand limb stiffness changes. We validated thatthe heater could effectively and uniformly heat the material through thermal
imaging. Here, the heater was subjected to 72 W for one min in freeconvection conditions. The infrared image (top-down view) was translated
to a elevation map (3D) that indicates nearly all of the composite system
is above the max end of the glass transition temperature, save extreme
peripheral regions.

particulates. The epoxy and graphite particulates were mixed
vigorously and imbued into a muslin fabric sheet by rod
coating. The fabric serves as reinforcement to enhance thestrength of the composite material, reduce its brittleness, andprovide a host structure for achieving a thin composite. The
uncured composite was then placed in an incubator for 12 hr
at 70◦ C, with a heavy flat plate on top to disperse the resin
homogeneously through the fabric during cure.
A resistive heater was coupled to the variable stiffness
composite to change its stiffness on-demand. The resistive
heater is thin and flexible. As a result, it is easily integrated
with the variable stiffness layer. The resistive heater was created by surface etching 0.127 mm thick copper-clad kapton(DigiKey) in a ProtoLaser U4 (LPKF laser and electronics).
A serpentine patterned trace 0.70 mm wide, covering an
area of 4110.05 mm2 was found to produce uniformly heatdissipating specimens with a resistance of 12 Ω. A thinlayer of thermal grease (Chemplex 1381 DE, FUCHS) was
dispersed between the variable stiffness composite and theheater to facilitate heat transfer. We validated that the heater
was able to heat above Tg and homogeneously heat thematerial it was attached to. Figure 4 showcases the thermal
map of the heater system in free-convection, after having
been subjected to 72 W for one min. This composite-resistive
heater stack-up was then placed inside a pocket in the fabricsleeve detailed in the next section.
7063) Fabric Sleeve: The fabric sleeve was designed tohold the variable stiffness layers and actuators together andprovide a hinge along each edge of the actuators. The fabricsleeve consists of two layers of fabric with seams that create
two pockets, as well as a cavity that allows the limb toexpand without resistance when it is transforming to theleg phase. Variable stiffness layers reside in the larger two
pockets. The actuators were bonded to the exterior of thesleeve.
III. T EST P ROCEDURES AND R ESULTS
1) Compression analysis of morphing limb: To characterize the load-deformation relationship of the flipper andleg phases of the morphing limb, we conducted quasi-static
compression tests with an Instron 3345 fitted with a 5 kN
load cell. Additionally, we varied the temperature input tothe material system in the flipper configuration to highlightthe effects of geometric stiffness versus material stiffness.
Custom-fabricated parallel plates served to constrain the
flipper from sliding as it was being compressed. The plates
contained a notch where the top and bottom of the flipper
fit snugly. Five trials at three distinct temperatures, 21◦ C(room temperature), 30◦ C, and 40◦ C, were conducted for theflipper, for a total of 15 independent tests. We specificallychose a swath of temperatures so as to include the top
and bottom temperatures in the glass transition range ofthe variable stiffness composite (30◦ C to 40◦ C). We precycled the flipper with embedded variable stiffness composite
heated to 40◦ C 10 times before running tests to remove early
cycle hysteresis effects. A test of five trials of the flipper
without any variable stiffness material inserts was conducted
to serve as a baseline comparison. With the limb placed
between the custom-made parallel plates, we compressed it
at 40 mm/min, stopping when the force dropped beyond a
2% threshold or more (i.e. when the flipper buckled), or when
it reached 50 mm displacement.
In addition to the flipper phase, five compress-to-failure
trials at room temperature were performed on the leg configuration with different variable stiffness materials each time.
We did not subject the leg phase to different temperatures
because the leg geometry is intended for load-bearing on
solid land. Additionally, the mechanism by which the leg
phase morphs back to the flipper phase is stress relaxationinduced by heating (i.e. a warm leg configuration cannot besustained, since no input pressure would hold it in place).
Note that in tests for both the leg and flipper conditions, we
did not inflate the pneumatic actuators; presented is only thestiffness of a final transformed geometry.
Figure 5A shows the results of the compression tests
conducted on the flipper phase of the morphing limb. The
darker colored trend line indicates the mean, and the lighter
color clouds indicate one standard deviation from the mean.
Evidently, the flipper without any material insert (black)exhibits lowest stiffness and does not buckle before 50 mm.
The flipper with variable stiffness material inserts heateduniformly to the top end of Tg (red) is slightly stiffer anddoes not buckle either. The flipper achieves an intermediate

Fig. 5. A) Compression test results for the flipper state of the morphinglimb at various temperatures, as well as the system without the variable
stiffness material as a comparison baseline. The higher the induced temperature, the softer the system becomes. In the absence of a variable stiffness
material, the limb is the softest. B) Compression test results for the leg stateof the morphing limb at a single temperature (21◦ C). Note the leg is much
stiffer than the flipper. This result is a direct consequence of the geometricstiffness provided by the cylindrical geometry of the leg.

stiffness when heated at the beginning of Tg (orange), andbuckles around 18 mm. Finally, the flipper is stiffest whenthe variable stiffness inserts are at room temperature (blue).
The maximum force it sees before buckling is 98 N. Notethat the trends that buckled (orange and blue) were truncatedto extend only as far as the specimen out of five that buckled
the earliest.
Figure 5B showcases the compress-to-failure test results
for the leg phase at room temperature (21◦ C). Five individual
specimens are reported as their own lines to visualize theextent of variation between samples, which was greater thanthat witnessed for the flipper compression tests. A primaryreason for this variation likely stems from manufacturing
processes; that non-uniformities in the fiber-epoxy composite
elicit pronounced stress concentrations when the limb issubjected to loading. This hypothesis is partly verified by
the different modes of buckling and cracking observed when
extracting the variable stiffness material specimen from the707

limb after each test. Another reason for the variation is likely
nuances in limb alignment–which are exaggerated by thehigh force scales–when placed in between the plates.
The maximum compression force seen by the leg withoutbuckling was approximately an order of magnitude higher
than the flipper at room temperature: 1100 N. Further, the leg
phases buckled under less displacement on average–aroundthree to five mm compared to the flipper. These two facts
underscore the significant role geometric stiffness plays in
the limb’s load-bearing capabilities. In fact, it is known that
hollow cylinders are an optimal load-bearing geometry forcolumns under compression, which verifies this experimental
observation.
Further insight into the buckling behavior of the two limb
states under compression at room temperature can be attainedby treating them as columns under compression. Modelingthe leg as a cylindrical cross-section column and the flipper
as a rectangular cross-section column, we have the following
area moments of inertia:π(1)
Ileg = r4
4
If lipper =bh3
12Fig. 6. Transformation operation of the morphing limb from flipper to leg
configuration and back. The black dots indicate the start of Joule heating.
The two different color trend lines represent different wattages input into
the system. Overall, the graph illustrates how greater input wattages enable
fast transitions, with the caveat that the limb takes upwards of 15 min to
fully cool and stiffen into its new geometry.

(2)2) Radius as a function of time and heat input: We studied
the radius of the morphing limb over time to grasp a senseof the time-scale of the morphing operation (Figure 6). First,
we applied a fixed pressure (117 kPa) to the flipper phase andwe waited until the flipper had expanded to the maximum
radius allowable with the stiff material inside it. Then, we
activated the heaters and differing input powers of 72 W and
18 W (calculated by multiplying the voltage setting on the
power supply by the amperage drawn) and observed how
the radius subsequently changed. We recorded the extent
of radius change until it had reached a steady state. Then,the inflated limb was allowed to cool for 15 min while thepressure was maintained to fix it into the leg geometry. Afterit had cooled, we released the pressure and noted the extent
of radius change. Then we applied wattage once more andrecorded how the leg phase relaxed to the flipper phase.
A thermocouple placed in the center of the morphing limb
gave us in-situ temperature data. We tracked the radius (thedistance between the inner edges of the actuators) using ahigh-definition camera and ImageJ. We recorded strain as aproportion of initial radius, i.e. r = δR
R0 , with R0 = 10 mm.
Figure 6 shows the radius versus time results for the limb
throughout its transformation from flipper to leg, and back(top row). The figure also includes a stacked temperaturegraph as measured by the thermocouple (bottom row). The
blue line indicates behavior associated with a low wattage of
18 W; the red line a high wattage of 72 W. Non-shaded areas
indicate where the limb radius expanded/contracted without
the presence of input power. The shaded areas represent
the times at which power was applied to the system. Blackdot marks denote where Joule heating commenced. The Tg
transition region is delineated by the fine gray lines stretchinghorizontally in each of the temperature graphs.

Where r is the radius of the leg cross section, b is the widthand h is the height of the flipper cross section. The Euler
critical or maximum load which a limb can bear withoutbuckling is defined as:
Pcr =π 2 EI(kL)2(3)

Where E is the modulus of elasticity of the material (thesame for leg and flipper configurations), I is the area momentof inertia of the limb cross section, k is the effective length
factor given as 1 for two pin boundary conditions (we assume
pinned because the limb is free to rotate about the contact
point of its plate constraints), and L is the unsupported
length of the limb (again, which is the same between eachconfiguration). The ratio of the critical load for the leg to theflipper gives insight into how much more load the leg cansustain without buckling, and given the previous conditions,simplifies to:
Pcr,leg
Pcr,f lipper=

Ileg
If lipper

π 4r= 4bh3(4)

12

With r = 32 mm, b = 80 mm, and h = 23 mm, we seethat for the case of our morphing limb:
Pcr,leg= 10.15
Pcr,f lipper(5)This is approximately the ratio of buckling force exhibitedby the leg (max around 1100 N) to the flipper (max around
100 N) in Figure 5A and B, and validates experimental
results. It is straightforward to tune the thicknesses of themorphing limb system to achieve a desired force at which
each configuration, leg and flipper, buckles.
708

Fig. 7. The morphing limb has potential to enhance locomotion performance for amphibious robots across different environments. For swimming, thelimb can assume a flipper shape. It becomes flat, exploiting geometry to reduce drag. The promising drag characteristics of the limb are shown in that it
has lower drag forces FD at higher flow rates V and angles of attack φ than the leg. Here, the black marker is the flipper and the blue marker is the leg.

wall effects, we ensured the flipper was no more than 20
cm in span. Force/Torque data were collected via a 6-axis
Gamma DAQ F/T Transducer (ATI industrial). We utilized
the measured component of force in the direction of flow fordrag, and the component of force normal to the upper surfaceof the flipper for lift, as in typical hydrofoil experiments. A
National Instruments DAQ recorded the data in a LabVIEW
interface.

Overall, the instances of radius change after the initial
expansion without heat input occur primarily in the Tg
region. This effect diminishes in tandem with decreasing heat
input. Clearly, the higher power (72 W) allowed the flipper totransition between phases much faster than the lower power(18 W). In the flipper to leg transformation, we also observethat the higher wattage input allowed the limb to reach alarger radius (4.68 strain) compared to the lower wattage(3.79 strain). This finding suggests the lower wattage did
not heat the variable stiffness material uniformly enough tofully soften. We suspect free convection and the rate of heatretained by the system reached an equilibrium, causing theradius change expansion to halt.
In the leg to flipper transition, we note that a minimal
amount of radius change occurred after releasing the pressure
after the 15 min cooling period and before heating. This
demonstrates that the cooled variable stiffness material issufficiently stiff to hold the leg geometry without the aid of
the pneumatic actuator. In addition, we note the different
amounts of hysteresis depending on the applied wattage.
The high wattage case exhibits r = 1 of hysteresis, andthe low wattage, a much more significant r = 2.3. We
believe that the ability of the morphing limb to return to its
initial flipper state from the leg state depends heavily on its
ability to uniformly surpass Tg to induce widespread material
relaxation. But, it is clear that some hysteresis remains,which is likely a result of plastic deformation that occurred
when straining the thermoset.
Lastly, we suspect that the time required to cool the limb
was so long (15 min) because the fabric sleeve and pneumatic
actuators act as insulators. Immersion in a liquid medium
would likely expedite the cooling process, but might also
impact the rate of heating.
3) Flow characteristics assessment: Sea turtles swim
using dorsoventral oscillations of wing-like foreflippers for
efficient, lift-based locomotion [18]. The geometry of theirflipper–flat and thin–helps mitigate drag forces. To determine
if our flipper configuration poses better drag characteristics
than the leg configuration, we acquired drag force data of
the limb in a flow tank. The tank has a 0.87x0.25x0.25 m
working section and a speed range over 1.6 m/s. To minimize

We subjected a morphing limb in each of its two configurations, flipper and leg, to three flow rates at three
angles, repeated five times for a total of 90 independent test
trials. Flow rates were randomized to mitigate experimental
biases and inconsistencies. The flow rates were: 0.1, 0.3, and
0.5 m/s. We believe this is a representative test spectrum forflow rates encountered by a turtle robot, as actual sea turtles
have reported sustained swimming speeds of up to 0.278 m/s
but bursts up to 0.53 m/s or higher [13], [19]. Note a 0 m/s
case was included for calibration purposes for both flipper
and leg configurations. We tested the following angles ofattack: φ = 0◦ , 10◦ , 20◦ . This spectrum reflects the range ofa flipper’s angle of attack observed when green sea turtles
swim [13]. At the start of each trial, we let the impeller inthe tank run for approximately one min to allow the flow toreach steady state. We then collected force/torque data forone min.
Data presented in Figure 7 showcases the results for theflipper and leg phase flow tests. Reported points are averageforce values for each of the randomized five trials in thatangle/flow rate condition. Error bars indicate one standard
deviation above and below the mean among the trials.
The results in Figure 7 imply that the flipper phase (bluetrend) is beneficial for low-drag swimming, as the flipper
generally exhibits less drag force compared to the leg acrosstested flow rates and angles. However, at 0.1 m/s flow rate
for all angles, there is little difference between the twophases. Overall, the drag force on the limb rises at a fasterrate as flow rate increases. This finding is consistent withhydrodynamics of streamlined hydrofoils versus other nonstreamlined shapes at increasing flow velocities, according
to the relation:709

R EFERENCES

1 2
ρv CD A
(6)2
where ρ is the density of the fluid media, v is the velocityof the fluid flow, CD is the coefficient of drag, and A is thefrontal area. We attribute the better drag force profile of the
flipper to its reduced A and CD terms in comparison to theleg. Since the wetted surface area does not change on themorphing limb, the frontal area A will be reduced as thelimb transitions from leg to flipper. Also, streamlined bodies
like the flipper have a lower CD compared to cylindricalbodies like the leg [20].
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IV. C ONCLUSION AND FUTURE WORK
We presented a morphing limb intended for amphibious
robots. The limb is capable of transforming between a flipper
and leg configuration. We demonstrated that the flipper
mode has a preferable force profile in closed-tank flow
tests, suggesting its utility in aquatic locomotion. We alsopresented evidence that the leg mode could sustain higher
compression loads than the flipper mode, suggesting it isbetter for locomotion on land. A major boon of the morphinglimb design presented here is decoupled control of stiffness
and geometry. In the flipper stage, a robot equipped withthe limb can adjust its stiffness without having to change its
shape. This poses advantages for swimming or transitioning
from water to land. In the leg phase, stiffness could be controlled further by inflating/deflating the pneumatic actuators,
which was not explored in the present work. All of theabove capacities are enabled by the novel variable stiffness
composite heater system and antagonistic pneumatic actuatorpair.
The most prominent shortcoming of the presented workis the long time scale (approximately 15 min) required
to cool the material and fully set it into a transformed
geometry. We intend to address this heat transfer problem andoptimize geometry to facilitate cooling in subsequent work.
Other ongoing work includes removing the copper foil heater
itself from the system and replacing it with a monolithic,
Joule-heating variable stiffness composite. The immediatechallenge is balancing the loading of conductive filler withthe structural integrity of the limb and thin geometry. Lastly,in the near future, we intend to install this morphing limb
on an autonomous, amphibious turtle-inspired robot. We
will integrate sensors to discern whether or not the robot
is on land, in water, or in a transitional littoral zone, andautomatically change the limb morphology and actuation
scheme accordingly.
ACKNOWLEDGEMENTS
The authors would like to recognize Dr. Michelle Yuen
for her assistance in fabrication of elastomeric actuators, Dr.
Sang Yup Kim for his input regarding the synthesis of epoxy
composite materials, Morgan Upchurch for her help withdrawing figures, and Dylan Shah for his support modelingthe limb. This work was supported by the Office of Naval
Research under the Young Investigator Program (N0001417-1-2604).
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