An insect-scale robot reveals the effects of different body dynamics regimes during open-loop running in feature-laden terrain.

Insects and robots can traverse heterogeneous terrains using open-loop dynamics. However, robots typically operate within a narrow range of running frequencies dictated by body and leg resonances; the impact of different dynamics regimes is largely unknown. We used the quadrupedal Harvard Ambulatory MicroRobot to study trot and pronk gaits at vertical and pitch/roll body resonances and above resonance. We tested open-loop running on a homogeneous substrate and two 3D printed heterogeneous terrains with foot-size features. The "pebbly" terrain features were shorter than limb amplitude while the "alpine" terrain features increased linearly to above limb amplitude. Distance traveled per gait cycle on the homogeneous and pebbly terrains were linearly related, suggesting above-resonance running which performs well in laboratory settings will also be successful on natural surfaces, provided the features are low enough to step over. In the alpine terrain, running in the body resonance regime bypassed the tallest obstacles. These results indicate that with rudimentary knowledge of terrain feature sizes, locomotors can target different body dynamics for effective open-loop running. This strategy may be useful for animals and robots with limited sensing and computing capabilities.