Balancing while Executing Competing Reaching Tasks: an Attractor-based Whole-Body Motion Control System Using Gravitational Stiffness

Whole-Body Control (WBC) systems represent a wide range of complex movement skills in the form of low dimensional task descriptors which are projected on to the robot's actuator space. Using these methods allows to exploit the full capabilities of the entire body of redundant, floating-base robots in compliant multi-contact interaction with the environment, to execute any single task and simultaneous multiple tasks. This paper presents an attractor-based Whole-Body Motion Control (WBMC) System, developed for torque-control of floating-base robots. The attractors are defined as atomic control modules that work in parallel to, and independently from the other attractors, generating joint torques that aim to modify the state of the robot so that the error in a target condition is minimized. Balance of the robot is guaranteed by the simultaneous activation of an attractor to the minimum effort conguration, and of an attractor to a zero joint momentum. A novel formulation of the minimum effort is proposed, based on the assumption that whenever the gravitational stiffness is maximized, the effort is consequently minimized. The effectiveness of the WBMC was experimentally demonstrated with the COMAN humanoid robot in a physical simulation, in scenarios where multiple conflicting tasks had to be accomplished simultaneously.