Performance Enhancement of a Morphing Limb for an Amphibious Robotic Turtle

Terrestrial and aquatic animals exhibit appendages adapted to the propulsion physics of their primary habitats. Terrestrial appendages typically assume stiff and load-bearing form factors, while aquatic appendages tend to adopt flexible and streamlined profiles. Bio-inspired robots with synthetic appendages often mirror this dichotomy of specialization: they are designed with fixed legs or flippers for locomotion on land or in water, respectively. Appendages that adjust their shape and stiffness can serve to specialize a robot's propulsion physics on demand, enabling transitions between multiple environments. Herein, we report a morphing limb combining layer jamming and pouch-based pneumatic actuation that rapidly and efficiently switches between a flexible flipper for swimming and a rigid leg for walking/crawling. The internal pouch actuator contributes to pressure that jams the external layers of the limb, which we refer to as positive pressure-reinforced jamming. We quantify the extent of shape-morphing conferred by the pouch actuator, the maximum load-bearing capability of the limb in leg mode, and the hydrodynamic characteristics of the limb in flipper mode. We find that the new limb boasts better performance than previous designs with respect to morphing shape, speed, efficiency, and hydrodynamics. Crucially, we also find that positive pressure-reinforced jamming increases the leg's compressive strength by 30% relative to just jamming the layers via negative pressure. With its own lightweight and compact electronics system, the morphing limb is a plug-and-play component for building an untethered multi-environment robot.