Body compliance helps oscillating snake robots reduce roll instability to traverse large steps

Snakes traverse a diversity of complex terrain at ease, an ability that snake robots still aspire to achieve. Despite progress in arboreal (or similar) and granular environments, a major knowledge gap is how to maintain stability on large, smooth obstacles like large rocks or steps that lack “push points” to grip or brace against. Our previous study (Gart et al 2019 JEB) discovered that kingsnakes traverse large steps stably by combining lateral oscillation for propulsion with cantilevering for bridging height change. Here, to understand stability principles, we developed a snake robot as a physical model and tested two hypotheses: (1) roll stability diminishes as step becomes higher; and (2) body compliance helps maintain contact and reduce roll instability. With similar gait as the animal, the rigid body robot was able to traverse steps as high as 40% body length, but traversal probability quickly diminished with increasing step height. A locomotor transition analysis revealed that this was mainly due to an increase in roll instability. Adding body compliance improved contact, reduced roll instability, and increased traversal probability by up to 40% while maintaining speeds. Our study demonstrated that maintaining contact is important for stable locomotion in complex terrain.