Zero spin angular momentum control: definition and applicability

In this paper, we seek control strategies for legged robots that produce resulting kinetics and kinematics that are both stable and biologically realistic. Recent biomechanical investigations have found that spin angular momentum is highly regulated in human standing, walking and running. Motivated by these biomechanical findings, we argue that biomimetic control schemes should explicitly control spin angular momentum, minimizing spin and CM torque contributions not only local in time but throughout movement tasks. Assuming a constant and zero spin angular momentum, we define the zero spin center of pressure (ZSCP) point. For human standing control, we show experimentally and by way of numerical simulation that as the ZSCP point moves across the edge of the foot support polygon, spin angular momentum control changes from regulation to non-regulation. However, even when the ZSCP moves beyond the foot support polygon, stability can be achieved through the generation of restoring CM forces that reestablish the CM position over the foot support polygon. These results are interesting because they suggest that different control strategies are utilized depending on the location of the ZSCP point relative to the foot support polygon.