Reactive velocity control reduces the energetic cost of transport for a direct-drive robot on granular media

The Minitaur robot has direct-drive legs which can be used as force sensors to detect ground properties of interest to geoscientists studying erosion in deserts. However, Minitaur overheats quickly when running on sand. We used a combination of simulations, physical emulations with a ground emulator robot, and experiments on granular media to show that the energetic cost of transport for direct-drive robots on highly dissipative substrates like sand can be consistently reduced by at least 20% with no loss to jump height by adding a virtual damping force to slow the intrusion of the robot’s foot. The simulations used a bulk-behavior model of the forces exerted by granular media in response to intrusion. In the emulations, a single leg jumped on a highly transparent low gear-ratio ground robot leg programmed to exert a simplified version of the bulk-behavior granular media force model using PD control. The depth-dependent force determined the proportional gain and the velocity-dependent force determined the derivative gain through the leg kinematics. An actuated ratchet prevented restoring forces during stance. Physical granular media experiments were performed with glass beads in a bed prepared between jumps.