Postural Balance Strategies in Response to Disturbances in the Frontal Plane and Their Implementation With a Humanoid Robot

We examine postural reaction and balance recovery patterns occurring when a standing upright human is subjected to a sudden disturbance within the frontal plane, with the aim of developing balance control strategies for humanoid robots. Five patterns are identified and related to the magnitude of the disturbance. Three of the patterns are modeled and implemented with a small humanoid robot HOAP-2. The models are based on inverted-pendulum and double-pendulum equations, in combination with variable stiffness/damping elements for ensuring appropriate reactions and balance recovery patterns. Supporting foot reaction is minimized within the reaction null space formulation. Special attention is paid to the transitions between the reaction patterns. The experimental data show that the models and the respective controllers can ensure smooth reaction control under both impact-force and continuous-force disturbances.