Body compliance helps snake robots traverse large steps

Snake robots still struggle to traverse complex 3-D terrain such as earthquake rubble and construction sites. By contrast, snakes traverse similar terrain like mountains and forests at ease. In both cases, how well the body engages the terrain is critical to successful traversal. Here, we use robophysical experiments to test the hypothesis that body compliance helps better engage and traverse complex 3-D terrain. We developed a snake robot with one-way wheels capable of traversing a large step using a partitioned gait that we recently discovered in snakes. An adjustable suspension was added to the wheels to vary compliance between the body and terrain. When traversing steps as high as 35 ± 3 % body length (BL), higher compliance allowed the suspension to compress more (from 1.0 ± 0.3 mm to 2.1 ± 0.6 mm) to better maintain contact with ground below and above the step (from 81 ± 8 % to 87 ± 5 % of the body), which increased traversal probability (from 63 ± 19 % to 90 ± 0 %) at the cost of 12 ± 6 % more power consumption (P < 0.05, ANCOVA). For larger step height (40% BL), however, the larger compression increased the chance of the rigid body edges being caught by the step corner. This is a limitation of the discrete robot body and highlights the need for a continuum compliant body.