* In work package 1 we developed a generic control framework for elastic actuators. This framework was shown to be applicable to a wide range of elastic actuators, including position controlled elastic actuators. Moreover, we developed a novel analysis of impactful contact transitions, which lead to the concept of the non-slippage impact direction (NSID). We believe that this concept has applications beyond bipedal locomotion and can be applied also to manipulation tasks such as hammering or sheet metal forming. The detailed treatment of impact-aware motion planning originally was not foreseen in the work plan, but evolved gradually out of the scientific analysis of the locomotion problem.
* In work package 2 we developed algorithms for agile locomotion including motions with flight phases such as hopping and running. We compared both model-based and biologically-inspired algorithms for running and evaluated them on rigid as well as elastic robot models. These detailed simulations were utilized as a basis for the proposed robot design in work package 4. In addition to the development of running algorithms, we extended our DCM-based locomotion framework to include reactive step adaptation in order to react to external disturbances at the body and the feet (stumble reaction). Moreover, we developed an algorithm for the generation and control of angular momentum during balancing and locomotion. Finally, we investigated the combination of model-based locomotion with reinforcement learning in order to further improve the locomotion behavior. One of the challenges of this turned out to be the incompatibility of modern RL algorithms relying on massive parallelization via implementation on GPUs with the computations required for the model based whole body controller. Our solution to this problem was to reduce the number of learned parameters such that an economic reinforcement learning on the CPU was sufficient. This approach allowed to increase the step adaptation to advanced motions without requiring convexity of the step location.
* Work package 3 was devoted to whole-body control and multi-contact interaction. The whole-body control represents the interface between the high-level locomotion strategy (WP2) and the underlying elastic actuator dynamics (WP1). We developed three different whole-body control approaches, including an optimization-based (classic) inverse dynamics WBC and a passivity-based WBC. These algorithms were extended for the treatment of parallel kinematic chains within the robot structure. Such parallel mechanisms are becoming more and more popular in the design of many new humanoid robots, including commercial designs. The performance of WBC for robots driven by actuation with closed kinematic linkages was exemplified via a model of the Kangaroo robot developed by PAL robotics.
* The evaluation of the mentioned concepts was done based on extensive simulations for walking, running, and jumping motions. Based on these results, in work package 4 a detailed design of an elastic humanoid robot was developed. The mechanical design was based on a detailed analysis of the thermal behavior of the actuator units. The analysis lead to a proposed design with an elastic mechanical coupling between the knee and the ankle. For the analyzed running motions it tuned out that a constant elasticity would give a good compromise in terms of speed and torque characteristics of the actuators. The elasticity becomes in particular effective for fast running motions with toe contact, while walking could be implemented on a flat foot contact with minimal disturbance by the elasticity. We believe that this design is a good compromise between controllability and efficiency. In addition to the analysis of the actuator requirements, we also performed a detailed study of the kinematics, based on a comparison with human biomechanics. As a result, the proposed design includes a tilted knee axis having particular benefits for fast running motions with small lateral foot distance.
Exploitation and dissemination of the project results:
The increasing interest of industry in humanoid robots presents a strong potential to further exploit the project results. While dynamic running is not necessarily an important use-case for many practical applications, the developed locomotion concepts extend the general motion skills and robustness of humanoid robots and thus can be interesting for future commercialization. During the project, the results were disseminated in scientific publications as well as keynotes in scientific events.
