Morphable Legs with Directional Compliance for Multi-Terrain Multilegged Robot Swimming  

Two- and four-legged robots depend heavily on sensing and control to adapt to complex terrain. Multi-legged robots (>6 limbs) emergently display robust mobility across complex terrain due to propulsive limb redundancy [Chong et al., 2023]; because limbs continuously slip during contact, these robots are effectively "terrestrial swimmers". This swimming is dominated by dissipation, thus inertial effects are negligible during steps. To discover principles by which such systems can swim in aquatic environments where inertia can aid locomotor performance, we developed a waterproof 5-link, 10 leg robophysical model (body length = 1.38 m). Each body segment is coupled to its neighbor via a rotational actuator, and each limb is driven by its own actuator. Each of the morphable legs consists of eight torsionally coupled segments. The geometry of each segment generates directionally compliant limbs which can only be curved in one direction. A motor driven cable threaded through each limb allows it to reconfigure from rigid C-shaped to straight. Compliance in each of these morphological states can be actively tuned via adjusting cable tension. Preliminary tests reveal that the robot displays comparable locomotor performance on flat ground to models with rigid C-shaped limbs. We hypothesize that inertial aquatic swimming will be enabled by loosening the limb cable such that the limb is stiff during retraction and loose during protraction, thus enabling large thrust and minimal limb drag.