How do people, and how should legged robots, avoid falling down?

Andy Ruina


What actuators does a person or legged robot have available to help prevent falls? Only ones that can more the relative horizontal position of the support point and the center of mass. What are these? Ankle torques, distortions of the upper body (bending at hips, swinging arms), stepping and pushing off. Of these, by far the biggest control authority is in stepping and pushing off. And these can be well understood, and well approximated, by a point mass model. Why? Because the same things that can’t help much, namely ankle torques and upper body distortions, can’t hurt much either. Thus, we believe we can design a robust balance controller using foot placement and pushoff and nothing else. And, reverse engineering, we think this explains most of what people do also, at least when recovering from large disturbances. A balanced broomstick, a Segway, a bicycle, a walking robot and a walking person all use the same basic idea.