This work presents a new control approach to multi-contact balancing for torque-controlled humanoid robots. The controller includes a non-strict task hierarchy, which allows the robot to use a subset of its end effectors for balancing while the remaining ones can be used for interacting with the environment. The controller creates a passive and compliant behavior for regulating the center of mass (CoM) location, hip orientation and the poses of each end effector assigned to the interaction task. This is achieved by applying a suitable wrench (force and torque) at each one of the end effectors used for interaction. The contact wrenches at the balancing end effectors are chosen such that the sum of the balancing and interaction wrenches produce the desired wrench at the CoM. The algorithm requires the solution of an optimization problem, which distributes the CoM wrench to the end effectors taking into account constraints for unilaterality, friction and position of the center of pressure. Furthermore, the feedback controller is combined with a feedforward control in order to improve performance while tracking a predefined trajectory, leading to a control structure similar to a PD+ control. The controller is evaluated in several experiments with the humanoid robot TORO.

Fig. 1: The torque-controlled humanoid robot TORO and its development stages from 2010 (DLR Biped [1]) to 2014.Abstract-This paper gives an overview on the torquecontrolled humanoid robot TORO, which has evolved from the former DLR Biped. In particular, we describe its mechanical design and dimensioning, its sensors, electronics and computer hardware. Additionally, we give a short introduction to the walking and multi-contact balancing strategies used for TORO.

We report results from a collaborative project that investigated the deployment of humanoid robotic solutions in aircraft manufacturing for some assembly operations where access is not possible for wheeled or rail-ported robotic platforms. Recent developments in multi-contact planning and control, bipedal walking, embedded SLAM, whole-body multi-sensory task space optimization control, and contact detection and safety, suggest that humanoids could be a plausible solution for automation given the specific requirements in such large-scale manufacturing sites. The main challenge is to integrate these scientific and technological advances into two existing humanoid platforms: the position controlled HRP-4 and the torque controlled TORO. This integration effort was demonstrated in a bracket assembly operation inside a 1:1 scale A350 mock-up of the front part of the fuselage at the Airbus Saint-Nazaire site. We present and discuss the main results that have been achieved in this project and provide recommendations for future work.

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