Passivity-based symmetric posture control and the effects of mass distribution and internal impedance on balance

This paper proposes a balance control method that consists of an imaginary full-actuated controller and an input transformation method. The controller is designed for maintaining the balance of bipeds under the assumption that bipeds have a direct control input to the torso. This controller has a symmetric structure that is independent of the stance phase and includes internal impedance of the chest and hip to increase the controllers robustness with respect to external disturbances. The transformation method converts the full-actuated controller to a realizable under-actuated controller and guarantees that the power provided by the under-actuated controller equals that by the full-actuated controller in the case that bipeds contact the environment at multiple points. Simulations were executed to validate the proposed input transformation and the controller to clarify the effects of the mass distribution models and internal impedance on balance.