Increased robustness of humanoid standing balance in the sagittal plane through adaptive joint torque reduction

This paper introduces and compares two control approaches that increase the robustness of humanoid standing balance in the sagittal plane with respect to impulsive perturbations by adapting joint torques. To address the question of how the range of admissible perturbations for an n-link inverted pendulum model can be enlarged, we propose two different strategies: adapting the ankle torque only and adapting all joint torques uniformly. For each, explicit-form solutions exist for nonlinear models with an arbitrary number of links. A Center of Pressure-based criterion for switching between the default feedback controller and the torque reduction strategies is introduced. In a three-link model case study, a wide range of robot poses, which are optimized either for steady state effort minimization or robustness, are considered. Simulation results show that our models are robust to impulse perturbations of between 10% and 149% greater magnitude than for an LQR default control law. However, there is a trade-off between robustness gains and steady-state balancing effort. In a second example, a joint-locked model, which uses an adaptive joint torque reduction strategy, outperforms a joint-unlocked model that only uses the default controller both in terms of robustness and control effort.