IEEE Robotics & Automation Magazine - September 2016 - 49

In addition to the aforementioned approaches, origami
manufacturing is emerging as a viable alternative [78]. Twodimensional laser micromachining is used to engrave the
desired crease pattern on a multilayer material. This manufacturing method allows for implementing complex 3-D
folding patterns with a simple, scalable, and affordable process. The authors developed a pocket-sized foldable quadcopter [Figure 4(d)] with origami arms [79]. The arms can
be wrapped around a central main frame and, as soon as the
propellers start to spin, they autonomously deploy in 0.3 s.
The crease pattern is tailored so that the arm locks in a stiff
configuration when deployed to ensure good controllability
of the drone.
Adaptive Morphology for Protection
Adaptive morphology can also add protection against
external threats or collisions. For instance, the pillbug
(Armadillidium vulgare) rolls into a ball when disturbed.
This conglobation behavior is triggered by strong vibrations or pressure and creates an external spherical shell
for the protection of the sensitive ventral surface of the
animal. Conglobation offers protection against predators
and lethal conditions in the external environment and
acts as a water conservation mechanism [9]. Inspired by
such capabilities, the latest version of Pillbot [80] can
assume different morphologies for protection and locomotion [Figure 5(a)]. For protection, the robot rolls up
in a spherical, shock-absorbing shell that ensures survival when the robot is hand launched. Once it has safely
landed, Pillbot opens and exposes its whegs for locomotion. Similarly, the quadruped walking robot with a
spherical shell (QRoSS) [Figure 5(b)] is a morphing
robot capable of storing its legs inside a protective spherical cage [81]. The cage protects the robot when thrown
to the floor, but after landing, the legs are deployed from
the cage and the robot can start its mission. This
approach is also exploited by Kovač et al. [82] in a steerable jumping robot. The robot is equipped with a cage
that stores and protects the leg of the robot, which is
subsequently deployed for jumping.
Shim et al. [83] developed the Buckliball, an elastic continuum conceived for drug encapsulation and protection
during delivery. As proof of concept, they developed a
shell patterned with a regular array of voids and ligaments.
The latter buckle below a certain internal pressure, leading
to a folding behavior (volume reduction up to 54%) of the
structure that can be used for drug encapsulation and
delivery. Mountcastle et al. [84] reported a biomechanical
strategy exploited by wasps to mitigate the effect of collisions on their wings. A flexible joint along the leading edge
coupled with a flexion line allows the wing tip to bend in
case of collision, reducing wear. In robotics, Stowers and
Lentink [29] showed that passively morphing wings can
absorb frontal impacts with no damage to the drone. Similarly, in the DALER multimodal drone [60], the wing morphing mechanism relies on tensioning springs that can be

Pole

Hub
Slip Ring

Actuators

Center Frame
(a)

SMA Wires

(b)

Figure 5. (a) The Pillbot and (b) QRoSS are two robots that can
morph into a protective configuration to withstand collisions
when dropped [80], [81].

used to absorb energy in case of collision, for example,
during landing.
Adaptive Morphology for Variable Transmissions
Transmissions with a variable gear ratio are the typical
engineering solution to minimize the tradeoff between
force and velocity for actuators with limited power,
enabling efficient operation under different dynamic
conditions. Usually variable transmissions are cumbersome and complex because they require auxiliary gears,
clutches, and actuators. However, animals evolved an
elegant and simple variable transmission that is directly integrated in pennate muscles and exploits adaptive morphology.
Pennate muscles [Figure 6(a)] are biological actuators
composed of muscle fibers that are obliquely oriented
with respect to the line of action of the muscle (pennation
angle α0). The contraction of pennate muscles is the result
of fibers both shortening and rotating, the latter being
associated with increasing values of the pennation angle
[α1 > α0 and α2 > α0 in
Figure 6(b) and (c),
respectively]. High valOrigami manufacturing
ues of fiber rotation
increase shortening and
is an emerging technology
velocity during muscle
contraction. On the
for the development of
other hand, during rotation, f ibers become
morphing structures.
more oblique, and their
load-bearing capability
along the line of action of the muscle is decreased. Overall,
the amount of rotation defines the tradeoff between fast
and strong contractions.
Azizi et al. [85] showed that fiber rotation passively
adapts to the loading condition on the pennate muscle,
favoring either fast contractions or high forces similarly to
a variable transmission. The value of fiber rotation selftunes to different loading conditions through a passive
adaptation of muscle morphology. During light lifting
[Figure 6(b)], the muscle shortens while it increases its
September 2016

Ieee rObOtICS & AUtOmAtION mAGAZINe

Table of Contents for the Digital Edition of IEEE Robotics & Automation Magazine - September 2016