Proprioception and Tail Control Enable Extreme Terrain Traversal by Quadruped Robots

Legged robots leverage ground contacts and the reaction forces they provide to achieve agile locomotion. However, uncertainty coupled with contact discontinuities can lead to failure, especially in real-world environments with unexpected height variations such as rocky hills or curbs. To enable dynamic traversal of extreme terrain, this work introduces 1) a proprioception-based gait planner for estimating unknown hybrid events due to elevation changes and responding by modifying contact schedules and planned footholds online, and 2) a two-degree-of-freedom tail for improving contact-independent control and a corresponding decoupled control scheme for better versatility and efficiency. Simulation results show that the gait planner significantly improves stability under unforeseen terrain height changes compared to methods that assume fixed contact schedules and footholds. Further, tests have shown that the tail is particularly effective at maintaining stability when encountering a terrain change with an initial angular disturbance. The results show that these approaches work synergistically to stabilize locomotion with elevation changes up to 1.5 times the leg length and tilted initial states.

I. Introduction

Quadrupedal robots have great potential because of their ability to traverse challenging environments that wheeled and tracked robots cannot. This makes them ideal for tasks such as environmental monitoring and disaster relief. But performing these tasks requires the ability to safely traverse extremely uneven terrains, such as rocky hills or curbs (the left panel of Fig. 1). While legged locomotion controllers can be robust, most of them rely on predefined nominal contact schedules or heuristic-based nominal footholds [3]–[5]. These methods are good at walking in relatively flat laboratory environments, but cannot easily handle extreme terrains, where elevation changes are large enough to invalidate nominal contact schedules and footholds. In contrast, animals can easily traverse these environments using various strategies, including placing feet in repeated locations to ensure reliable contact [6], using tails to reject disturbances [7], [8], and distributed limb control to promote rapid and reactive behaviors [9]. These biological phenomena inspire us to propose new approaches for the perception and control of quadruped robots to improve robustness across extreme terrains. Fig. 1.

Left: A quadruped robot successfully traverses an unforeseen cliff using animal-inspired proprioception and tail. Right: Schematic diagram of the control modeling of the tailed robot.