Rapid legged locomotion in an isotropic frictional environment

The Purcell three-link swimmer is a fundamental model for studying locomotion in homogeneous environments such as viscous fluids and granular media. Without drag anisotropy (directional differences in resistive forces at body segments), such systems cannot translate. To locomote on isotropic environments (e.g., dry Coulomb friction), prior work demonstrated that periodic lifting/lowering of added appendages can create drag anisotropy and enable “frictional swimming” [Chong et al, PNAS, 2023]. Here, we observe an alternative mechanism using non-actuated appendages. Specifically, static limbs (0.20 BL long) added to the first and last links allow the 3-link swimmer to effectively translate forward (0.34±0.03 BL/cycle). We hypothesize that locomotion is induced by lateral oscillations between the robot's center of geometry (CoG) and center of mass (CoM). To test this, we manipulated the CoM by adding mass (1 kg) to different links of the 3-link robophysical device (0.5 kg). Effective translation occurs only when the weight is at the middle joint, where the CoG-CoM oscillation is the largest. Further, our quasi-static model predicts that non-actuated limb-induced CoG-CoM oscillations can be generalized to N-link swimmers, leading to effective translation. Our experiments demonstrate the efficacy of an alternative mechanism to create drag anisotropy with non-actuated appendages, which could be practical when leg actuation is difficult (e.g., microrobots, load carrying).