Force-based Control of Bipedal Balancing on Dynamic Terrain with the "Tallahassee Cassie" Robotic Platform

Out in the field, bipedal robots need to travel on terrain that is uneven, non-rigid, and sometimes moving beneath their feet. We present a force-based double support balancing controller for such dynamic terrain scenarios for bipedal robots, and test it on the robotic bipedal platform "Tallahassee Cassie." The presented controller relies on minimal information about the robot model, requiring its kinematics and overall weight, but not inertias of individual links or components. The controller is pelvis-centric, commanding pelvis positions in Cartesian space, which a model-free PD controller converts to motor torques in joint space. By commanding forces, torques, and a frontal center of pressure in this fashion, Tallahassee Cassie is capable of balancing on a variety of scenarios, from a lifting/sliding platform, to soft foam, to a sudden drop. These results show the potential for bipedal control to balance successfully despite minimal model information, the presence of large dynamic impacts-e.g., falling through trap door, and soft series-spring deflections. These results motivate future work for walking and running controllers on dynamic terrain with relatively low reliance on modeling information.