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Robotic Mechanisms 183

stability of a sprawled posture and of an increased number of legs reduce the complexity of controls
and decrease likelihood of catastrophic fall or inversion. Full hypothesizes that six legs offer an
optimal accord between stability and complexity (1999), extra legs being expensive to build,
creating potential failure points, and requiring extra energy to operate.
On the other hand, multipurpose extra limbs could serve as alternate legs for the six in case
of emergency, and in the interim they could operate as sensor antennae, or as arms tipped with
manipulators or instruments.
Similar to Sprawl, in the Rhex robot of McGill the compliance of the six legs improves the
stability and controls of the robot; but Rhex can alternately also run on two legs, in the manner of a
toddler. Such reconfigurable design promises to enable robots to be less domain-specific, and more
capable of meeting the demands of real-world operations. Reconfigurable robots are further
discussed in Section 6.2.2.

6.2.2 Flexibility, Hybrids, and Reconfiguration

Real-world performance can require flexibility and multipurpose applicability in mechanical
architecture. To achieve this flexibility, roboticists have introduced novel hybrid systems such as
wheel–leg combinations (such as the Whegs of Case Western), or reconfigurable architectures that
can morph between walking, crawling, and slithering morphologies (such as the crystal robots of
Dartmouth and MIT).
Slithering morphologies are inherently flexible, enabling robots to crawl through tight spaces,
such as arteries, caves, or sewers, in a snakelike or centipede-like manner. The snake robot S5
(1998–1999) built by Gavin Miller, demonstrates untethered legless snakelike slithering locomo-
tion (see Figure 6.3). Miller’s S1–S5 build on earlier work by Shigeo Hirose of Japan, in which
oscillatory deflections travel down the length of the snake causing the snake to move forward, while
offset to oscillations steer the device.
MAKRO, a snake robot the Fraunhofer Institute for Autonomous Intelligent Systems in St
Augustin, Germany, boasts an articulated-wheeled, segmented design that enables maneuverability

Figure 6.3 S5 by Gavin Miller ß 1999 and MAKRO of Fraunhofer, DE.