IEEE Robotics & Automation Magazine - September 2016 - 45

compliant legs. RHex has undergone multiple series of design iterations, some of which embed
mor phing appendages to increase
adaptability. For example, Sprawl-Hex
[37] and a sprawl tuned autonomous
robot (STAR) [38] [Figure 2(b)] are
six-legged robots with a variable
sprawl angle that combines the
advantages of both vertical and inplane locomotion. At low sprawl
angles, the robots' stability and velocity are comparable to a similar
wheeled robot, but, at higher sprawl
angles, they have better grip and can
overcome higher obstacles. Also, at
small sprawl angles, the flattened
body of the robot can move through
narrow gaps. Galloway et al. [39]
implemented a RHex robot with legs
that adapt their stiffness when
morphed to run effectively over a
broad range of terrains. Inspired by
caterpillars [23], GoQBot [40] is a
robot that morphs before rolling.
Combining a silicone soft body with
shape-memory alloy (SMA) actuators, the long and slender robot can
be morphed into a wheel and propelled using a ballistic rolling
behavior [Figure 2(c)].

Wheel
Transformer
Transformable Wheel
(a)

11 cm

(b)

(c)

Figure 2. Examples of terrestrial robots with morphing structures. (a) A robot
equipped with circular wheels that can morph in whegs to negotiate obstacles [33].
(b) A robot with controllable sprawl angle to combine the benefit of vertical and in-plane
locomotion [38]. (c) A GoQBot during rolling locomotion [40].

Adaptive Morphology in Multimodal Locomotion
Several organisms operate with high efficiency across different substrates. Due to variations in physical parameters, different environments often put conflicting selection pressure
on the morphology of the locomotion appendages. Therefore, it is not surprising that multimodal species evolved
morphing appendages to facilitate the transition between
environments. For multimodal animals, adaptive morphology compensates for environmental variability to reduce tradeoffs that would derive from a highly optimized but fixed
morphology. A good example of the benefits of adaptive
morphology is given by terrestrial or aquatic species with
aerial competences. Terrestrial and aquatic movements
require a streamlined body to enhance traveling through
cluttered terrains [41] and reduce drag during swimming
[42], [43]. In contrast, flight requires large lift-generating
surfaces to support the animal's weight [17]. Therefore, several multimodal aerial species resort to morphing structures,
which are folded during terrestrial or aquatic locomotion to
occupy small spaces and can be deployed during flight to
maximize support.
For instance, arboreal animals often utilize a deployable
membrane, the patagium, to enable aerial locomotion.
Frogs belonging to the genus Rhecophorus have enlarged

hands and feet with strong
webbing that is deployed
For multimodal animals,
when the frog leaps
through the air. Experiadaptive morphology
ments with living animals
and models show that
compensates for
deployed webbed feet
improve traveling disenvironmental variability
tance and maneuverability
during gliding [44]. Mamto reduce tradeoffs that
malian gliders utilize a
larger and more effective
would derive from a
p at a g i u m s p a n n i n g
between the forelimbs and
highly optimized but
hind limbs [45]. For example, in the flying lizards of
fixed morphology.
the genus Draco, the
patagium is supported by
elongated ribs controlled by a complex muscular system
[46], [47]. Decoupling the large patagium from the limbs
seems to be a clever solution to prevent hindering of
terrestrial locomotion [48]. Among arboreal animals
equipped with patagia, bats achieve remarkable flight competences [49] while also displaying various degrees of
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Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2016
