Legged Robots Change Locomotor Modes To Traverse 3-D Obstacles With Varied Stiffness

When traversing complex 3-D terrains such as grass-like beams, cockroaches often transition between multiple locomotor modes, such as push through, climb over, and roll to maneuver through slits. Here, we use robophysical experiments to study the use of diverse locomotor modes when terrain properties vary and better understand locomotor transitions in nature. We developed a cockroach-inspired legged robot and challenged it to traverse grass-like beams whose torsional stiffness is precisely controlled and systematically varied. We discovered that the robot ran over compliant beam obstacles, quickly rolled to maneuver through medium stiffness beams, but pitch up substantially before rolling to traverse stiff beams. Traversal time increased from 1.0 ± 0.0 s to 3.1 ± 1.2 s as beams became stiffer (P < 0.001, ANOVA). We developed a locomotion energy landscape model to explain these observations, and found that locomotor mode is governed by competition between potential energy barriers from body-terrain interaction and kinetic energy fluctuations from intermittent body/leg-terrain contact. Our study is a major step towards the vision of locomotion energy landscapes as a physics framework to understand and predict locomotor transitions in complex 3-D terrains.