Anisotropic collapsible leg spines for increased millirobot traction

Collapsible leg spines found on insects and spiders provide a passive mechanism for increased traction while running over complex terrain. In this paper we use this architectural advantage as biological inspiration to increase the useful work in pulling a load with a VelociRoACH, a high speed terrestrial robot. These spines exhibit anisotropic properties in the fore-aft and lateral directions, with a 2:1 holding-to-release force ratio on corkboard (0.2 N to 0.1 N). This increase in effective friction coefficient at the foot-to-surface contact points has decreased the Cost of Pulling for the VelociRoACH by ten times, allowing it to transport loads using less energy. The VelociRoACH with spines is able to engage the surface and pull up to 0.36 N whereas without spines it slips while pulling 0.2 N, demonstrating that the robot´s performance with spines is now torque limited rather than friction limited. The spines also allow the robot to remain dynamically stable and resist torque disturbances.