IEEE Robotics & Automation Magazine - September 2016 - 43

to enable new behaviors or functionalities. Morphology deals
with the form of things and their composition, and it greatly
influences the behavior and performance of both living organisms and robots [6]. Because different environments and functions require specific and tailored morphologies, many
animal species have evolved morphable bodies to extend their
operational range. Some animals can adapt their morphologies to facilitate locomotion in different substrates [7], to limit
biomechanical tradeoffs [8], and to enable additional functionalities, for example, protection [9]. Adaptive morphology
allows living organisms to overcome the boundaries imposed
by fixed body shapes. Therefore, it is not surprising that a
great deal of research effort has been devoted to the development of highly reconfigurable artificial systems with adaptive
morphologies. Traditionally, adaptive morphology has been
explored using self-reconfigurable robots-modular systems
that rearrange their relative position in space and connect
together [10]. Modular robots of different shapes, controlled
by dedicated planners, provide a glimpse of the potential
offered by adaptive morphology [11]-[13]. However, due to
their intrinsic mechanical complexity, rigid components, and
limited scalability, modular robots have been mostly used as
proof of concept.
Leveraging soft technologies and new manufacturing processes, adaptive morphology can be a powerful design principle for engineers who want to increase the flexibility, efficiency,
and adaptability of robotic systems. Morphing structures have
the potential to pave the way for a new class of intelligent
machines: mobile robots that accommodate critical but conflicting locomotion requirements, multifunctional robots that
reconfigure their shape on the fly, soft wearable robots, and
more. In this article, we highlight benefits and challenges of
adaptive morphology with the help of examples in the field of
locomotion and functionalization of animals and robots.
Adaptive Morphology in Locomotion
During locomotion, animals use their muscles to generate
forces that are transferred to their environments through
appendages with highly specialized morphologies. The morphology and material properties of a locomotion system are
strongly dependent on the environment and define the way
animals move, and, therefore, their dynamics (e.g., locomotion gaits and agility), energy efficiency, and control strategy
[14]. Many animals exploit adaptive morphology to extend
their dynamic envelope, to reduce tradeoffs (e.g., velocity
versus maneuverability), and to transition between multiple
substrates. Nature is constantly inspiring the design of artificial locomotion systems [15], [16]. Therefore, fundamental
knowledge and design paradigms can be elucidated and
applied for the development of more robust and versatile
mobile robots by understanding the role of adaptive morphology in animal locomotion.
Adaptive Morphology in a Single Locomotion Mode
When an animal's operational space is restricted to a single
substrate, morphing appendages are beneficial for extending

locomotion capabilities by reducing tradeoffs. A classic example is a bird's wing, which can be quickly morphed to obtain
different dynamics, extending the flight envelope of the animal [8], [17], [18], i.e., optimal performances with limited
tradeoffs over a wide variety of radically different flight conditions. A bird's wing can be morphed for different flight modes
because it is composed of an articulated skeleton controlled by
muscles and covered with feathers that can overlap [17]. Birds
exploit fully deployed wings to maximize wingspan and surface with a beneficial effect on lift and energetic efficiency at
low speed, e.g., during soaring, or to increase maneuverability
for tight turns. Wing adaptation for low speed is further
enhanced by an increased airfoil camber induced by modifications of the feather curvature, driven by dedicated tendons
[17]. However, when high speed is required, wings can be
quickly retracted and swept back and feathers flattened to
reduce drag. Partially folded wings are used during aggressive
flights, for example, to perform escaping or hunting
A bird wing can be
maneuvers. Birds exploit
folded wings also to fly in
morphed for different
very cluttered environments and traverse gaps
flight modes because
barely larger than their
bodies [19]. Wing morphit is composed of an
ing is also exploited in
flapping flight by birds
articulated skeleton
[20] and bats [21]. During
a wingbeat cycle, the
controlled by muscles and
wing is fully extended in
downstroke and folded
covered with feathers
in up stroke to reduce the
energetic costs produced
that can overlap.
by drag and inertia.
Adaptive morphology is
also used by terrestrial
species. For example, the Mount Lyell salamander (Hydromantes platycephalus) [22] and several species of leaf-roller
caterpillar [23] morph their body into a wheel to rapidly
escape predators instead of walking.
Engineers have been inspired to develop drones with
extended flight envelopes by morphing wings [24], [25]. In
small drones, passively morphing wings have been demonstrated to increase robustness against wind gusts [26]. Such
wings are composed of a flexible carbon fiber skeleton covered with a stretchable latex membrane [Figure 1(c)]. The
wings dynamically adapt to the airflow thanks to a tuned
aeroelastic washout of their compliant structure. In response
to turbulences, the membrane extends and the frame twists
to maintain an almost constant lift. Nonflapping and actively
morphing wings have been investigated in a series of drones
developed by Grant et al. [27]. Inspired by gulls, the drones
have shoulder and elbow joints that control the wing's dihedral [Figure 1(a)] and swept [Figure 1(b)] angles. The wing
is composed of a multijoint carbon fiber frame that is covered with an inextensible membrane and actuated through
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Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2016