Robust Gait Design For A Compass Gait Biped On Slippery Surfaces

Most current bipedal robots were modeled with an assumption that there is no slip between the stance foot and ground. This paper relaxes that assumption and undertakes a comprehensive study of the compass gait biped on slippery ground. It presents in detail the control of a biped that allows for foot slipping, and shows that feasible gaits fail on slippery ground for two causes: falling backward or requiring negative contact force which cannot be provided by the ground. To characterize a robust gait on slippery ground, three safety factors are proposed to measure the robustness: slip friction, falling friction and tolerance ability of slipping without falling. This study thus uses these factors to investigate independent influence of gait speed and step length on the robustness of the gait, and shows that gaits with small step length and moderate speed are robust and preferable on slippery surfaces. In contrast, gaits with large step length generally require large friction to maintain stable walking on slippery surfaces. Moreover, gaits with a backward swing foot velocity relative to the ground just before touch down are generally more robust than ones with a forward velocity. It is further shown that only one parameter in gait design determines the swing-backward feature, which can help design robust gaits. Models with varying physical parameters such as mass, leg length and position of center of mass (CoM) in each leg, are also studied to validate the universality of this result. (C) 2021 Elsevier B.V. All rights reserved.