Effects of Object Mass on Balancing for Whole-Body Lifting Tasks

Despite the importance and prevalence of loco-manipulation tasks by humanoids, existing criteria and control methods for stability are mostly developed for unloaded legged gait. In this paper, the stability during lifting tasks is comprehensively analyzed to determine the role of the lifted object mass in balancing. The stability of a simple two-degree-of-freedom lifting model and a whole-body humanoid robot are evaluated by constructing their balanced state boundaries, which represent their specific capabilities in maintaining balance, through an optimization-based framework for varying combinations of object mass, joint torque limits, and base of support dimensions. Comparative analysis of the rate of change of the linear and centroidal angular momenta quantifies the nonlinear and nontrivial tradeoffs, i.e., contribution or obstruction, of the effects of the object mass on balancing. Overall, increasing the object mass enhances balance capability subject to the limiting factors of system kinematic and actuation limits, center of pressure within the base of support, friction cone, and unilateral normal contact forces between the feet and the ground.