Physics of animal and robot locomotor transitions in complex terrain

Robots are still poor at traversing complex terrain like earthquake rubble and construction sites for applications like search and rescue and structural examination. By contrast, animals move through nature by transitioning between different forms of movement. This is largely due to a lack of understanding of the physics of locomotor transitions in complex terrain. Here, we propose locomotion energy landscape as a framework for understanding how macroscopic, self-propelled, legged locomotors physically interact with terrain to probabilistically transition between locomotor modes. By integrating animal and robophysical experiments and physics modeling, we discovered that: (1) Different locomotor modes have different “terradynamic favorability”, measured by the potential energy barrier that must be overcome to traverse using each mode; and (2) Kinetic energy fluctuation from body vibration, induced by seemingly wasteful, oscillatory leg movement, helps animals and robots overcome mode-separating potential energy barriers and transition from less favorable to more favorable modes. Our study is a step in establishing terradynamics of locomotion in complex 3-D terrain. Such physics-based terrain traversal complements geometry-based obstacle avoidance and expands the reaches of robots.