Rapid two-anchor crawling from a milliscale prismatic-push-pull (3P) robot

Many crawling organisms such as caterpillars and worms use a method of movement in which two or more anchor points alternately push and pull the body forward at a constant frequency. In this paper we present a milliscale push-pull robot which is capable of operating across a wide range of actuation frequencies thus enabling us to expand our understanding of two-anchor locomotion beyond the low-speed regime. We designed and fabricated a milliscale robot which uses anisotropic friction at two oscillating contact points to propel itself forward in a push-pull fashion. In experiments we varied the oscillation frequency,f, over a wide range (10-250 Hz) and observe a non-linear relationship between robot speed over this full frequency range. At low frequency (f< 100 Hz) forward speed increased linearly with frequency. However, at an intermediate push-pull frequency (f> 100 Hz) speed was relatively constant with increasing frequency. Lastly, at higher frequency (f> 170 Hz) the linear speed-frequency relationship returned. The speed-frequency relationship at low actuation frequencies is consistent with previously described two-anchor models and experiments in biology and robotics, however the higher frequency behavior is inconsistent with two-anchor frictional behavior. To understand the locomotion behavior of our system we first develop a deterministic two-anchor model in which contact forces are determined exactly from static or dynamic friction. Our experiments deviate from the model predictions, and through 3D kinematics measurements we confirm that ground contact is intermittent in robot locomotion at higher frequencies. By including probabilistic foot slipping behavior in the two-anchor friction model we are able to describe the three-regimes of robot locomotion.