Terrestrial swimming in multilegged robots

Locomotion is typically studied in continuous media where bodies and legs experience forces generated by the flowing medium, or on solid substrates dominated by friction. In the former, slipping through the medium is unavoidable and necessary for propulsion. In the latter, slip is often assumed undesirable and/or minimal. We discover in laboratory experiments that terrestrial locomotion of a multi-legged robophysical model (L = 20 to 160 cm, 2 to 8 segments) proceeds via slipping and can thus be viewed as terrestrial "swimming". Robophysical experiments varying leg and body wave amplitudes reveal that inertial effects are minimal. Numerical simulations and theoretical analysis demonstrate that periodic leg-substrate contact allows for analysis via an effective Resistive Force Theory akin to that in flowable media where the system experiences an effective viscous drag and acquired drag anisotropy. Analysis of body and leg dominated propulsion reveals body undulation can buffer the robot against vertical heterogeneities (randomly distributed leg scale vertical obstacles), which extends the terrestrial swimming capabilities of the robot beyond flat frictional terrain.