A passive, asymmetrically-compliant knee joint improves obstacle traversal in an insect-scale legged robot

Insects can locomote readily in challenging envi-ronments, such as over steep inclines and across obstacle-laden terrains, which still frustrate robots of similar size. In this work, inspired by the passive compliant properties of insect limbs, we use the insect-scale Harvard Ambulatory Microrobot and multilayer microfabrication techniques as platform to study the ability of passive mechanisms to improve open-loop running in rough terrains. We tested the performance of different limb designs in vivo, exploring how the magnitude, directionality, and distribution of compliance incorporated into the leg impacted robot performance. Limbs were evaluated on both a featureless substrate and an increasingly-adversarial 3D-printed terrain designed to mimic natural environments. We tested the limbs using a trotting gait in the quasi-static (2 Hz) and body dynamics (25 Hz) stride frequency regimes. Performance was reported as bodylengths traveled per gait cycle on the featureless substrate, and as the largest feature height the robot was able to overcome in the terrain. The work presented here provides design principles for a passive limb that expands the terrain accessible to small robot; we find a limb with a single asymmetrical joint is able to improve quasi-static terrain traversal by 203 % relative to a rigid limb.