Federico L. Moro

A human-like walking for the COmpliant huMANoid COMAN based on CoM trajectory reconstruction from kinematic Motion Primitives

Research on humanoid locomotion made significant improvements over the last years. In most cases, though, the gait of state-of-the-art robots is still far from being human-like due to two main reasons. These are, the mechanical incompatibilities between the human and the engineered humanoid platforms, and the lack of clear understanding of the highly complex human walking motion itself. This work attempts to address the latter by using a novel method to construct locomotion trajectories for a humanoid robot based on kinematic Motion Primitives (kMPs) derived from humans locomotion trajectories. The work demonstrates how from a small set of invariant primitives it is possible to reconstruct all the joint trajectories to obtain different gaits, with different speed, and while accomplishing different tasks at the same time. We then used the proposed method to reconstruct a human-like CoM trajectory, and evaluate it on our COmpliant huMANoid robot, COMAN. Experimental results are presented to demonstrate the execution of a stable, fast walking, with knee straightening at toe-off to push forward, that strongly resembles a more human-like walking. Furthermore, and taking inspiration from the inherent passive compliance in the joints of the COMAN robot, the energy consumption of the robot was investigated by varying the frequency of the gait generated basing on the human kinematic primitives. The idea was to exploit the natural dynamics of the compliant humanoid platform, and minimize its energy consumption. Experimental results for different stepping frequencies in terms of energy consumption over a fixed time, and to walk for a fixed distance are presented.

An attractor-based Whole-Body Motion Control (WBMC) system for humanoid robots

This paper presents a novel whole-body torque-control concept for humanoid walking robots. The presented Whole-Body Motion Control (WBMC) system combines several unique concepts. First, a computationally efficient gravity compensation algorithm for floating-base systems is derived. Second, a novel balancing approach is proposed, which exploits a set of fundamental physical principles from rigid multi-body dynamics, such as the overall linear and angular momentum, and a minimum effort formulation. Third, a set of attractors is used to implement both balance and movement features such as to avoid joint limits or to create end-effector movements. Superposing several of these attractors allows to generate complex whole-body movements to perform different tasks simultaneously. The modular structure of the proposed control system easily allows extensions. The presented concepts have been validated both in simulations, and on the 29-dofs compliant torque-controlled humanoid robot COMAN. The WBMC has proven robust to the unavoidable model errors.

On the Kinematic Motion Primitives (kMPs) - Theory and Application

Human neuromotor capabilities guarantee a wide variety of motions. A full understanding of human motion can be beneficial for rehabilitation or performance enhancement purposes, or for its reproduction on artificial systems like robots. This work aims at describing the complexity of human motion in a reduced dimensionality, by means of kinematic Motion Primitives (kMPs). A set of five invariant kMPs are identified for periodic motions, and a set of two kMPs for discrete motions. It is shown how these two sets of kMPs can be combined to synthesize more complex motion as the simultaneous execution of the periodic and the discrete motions. The results reported are an evidence of the theory of Central Pattern Generators (CPG), showing its effects on the kinematics, and are related to what presented in the literature on the Motor Primitives extracted from EMG signals. Experimental tests with the COmpliant huMANoid (COMAN) were performed to show that the kMPs extracted from human subjects can be used to transfer the features of human locomotion to the gait of a robot.

Efficient human-like walking for the compliant huMANoid COMAN based on linematic Motion Primitives (kMPs)

Research in humanoid robotics in recent years has led to significant advances in terms of the ability to walk and even run. Yet, despite the general achievements in locomotion and control, energy efficiency is still one important area that requires further attention, especially as it is one of the major steeping stones leading to increased autonomy. This paper examines, and quantifies, the energetic benefits of introducing passive compliance into bipedal locomotion using COMAN, an intrinsically COmpliant huMANoid robot. The novelty of the method proposed consists of: i) the use of a method of gait synthesis based on kinematic Motion Primitives (kMPs) extracted from human, ii) the frequency tuning of the resultant trajectories, to excite the physical elasticity of the system, and the subsequent analysis of the energetic performance of the robot. The motivation is to assess the possible effects of using dynamic human-like, and human derived, trajectories, with significant Center of Mass (CoM) vertical displacement, regulated in frequency around the frequency band of the system resonances, on the excitation of the compliant actuators, and subsequently to measure and verify any energetic benefit. Experimental results show that if the gait frequency is close to one of the main resonant frequencies of the robot, then the total work contribution of the elastic compliant element to the overall motion of the robot is positive (15% of the work required is generated by the springs).

A Biologically Founded Design and Control of a Humanoid Biped

During the last decade many advancements in the fields of industrial and service robotics have produced robots that are well integrated inside the industry; they can operate faster and with higher precision in comparison to human beings. Nevertheless if we take a look at the kinematic structure of these systems, it is clear that the actual machines are limited in the mobility and in the number of tasks that they can perform. This is more evident if we intend to apply those robots in an unstructured environment like home. First at all the robot should be able to move around avoiding obstacles, climbing stairs, opening doors. These movements should also be performed with a certain level of compliance for the safety of the human beings that are in the environment. Secondly, the robots should be able to use tools and other machines designed for human use, and based of the human manipulation and kinematic abilities. A possible solution for mobility, that is well applied in mobile robotics, is the choice of a wheeled traction system. This usually is a simple manner to move on flat floors, and is efficient from the energetic point of view (during the movement the center of mass acts on a straight line). However it presents important limitations, for example it is not possible for such a robot to overcome obstacles bigger than the wheels dimensions. Those limitations can be overcome if the robot is equipped with legs, that normally act by increasing the robots DOF(Degrees of Freedom). Many studies were conducted on legged robot in order to improve the efficiency and stability during walking. A pioneering contribution was done (Takanishi et al, 2004) at the laboratories of Waseda University (Tokyo). Several other modern robots are designed to walk and behave like humans (Hashimoto et al, 2002)( 3) but until now the efficiency of the human gait is still far from being reached. In this sense, the work of McGeer (McGeer, 1990) can be considered exemplar. His passive dynamic walker made a stable gait without close position control, considering the walking motion as a natural oscillation of a double pendulum; and this is actually how humans seem to walk (Gottlieb et al, 1996) (Kiriazov, 1991). His results inspired many other works, such as the stability analysis (Garcia et al, 1998) and the physical implementation ( Wisse et al, 2001) (Kuo, 1999)(Collins et al, 2001) of several prototypes.

Simulation for the optimal design of a biped robot: analysis of energy consumption

Our first aim is to develop a systematic method to estimate energy consumption of bipedal locomotion. This method is then used to evaluate the performance of materials and actuators that could be used for the design of a biped robot. Moreover, with this analysis we also demonstrated the importance of having good joint trajectories in order to reduce energy consumption. Results collected are then integrated with complementary information about materials and actuators, to finally suggest the best configurations. These indications are meant to be used for future developments of LARP, the biped of Politecnico di Milano. The method adopted, however, is general enough to produce valid results for any robot, and we hope our considerations will help in evaluating design choices for future humanoid robots.

Use of gravitational stiffness in an attractor-based Whole-Body Motion Control approach

This paper presents an analysis of the relation between effort and gravitational stiffness, two physical measures that depend on the configuration of the robot. It is shown that whenever the gravitational stiffness is maximized, the effort is indirectly minimized. A minimum effort attractor that controls the gravitational stiffness is presented. This attractor together with an attractor to zero joint momentum guarantee balance maintenance. The novel implementation of the attractor-based Whole-body Motion Control (WBMC) System is experimentally tested: first with simple models, and finally with the full-body humanoid robot COMAN in a physical simulation.

Guest Editorial of the Special Issue on Whole-Body Control for Robots in the Real World

Research in whole-body control (WBC) aims to contribute to provide robots with those capabilities that are necessary to move and perform in real world scenarios. Until recent years, limitations on hardware relegated WBC to almost purely theoretical research. Recently, a growing number of experimental platforms have become available (in particular, torque-controlled humanoids). This new opportunity has triggered the deployment on real robots of the theoretical outcomes of research in the field. This is backed up by a number of new research projects and initiatives addressing issues in this domain, including the Darpa robotic challenge (DRC). The goal of this special issue is to provide a clear representation of what is the state-of-the-art in WBC, and to help identifying what steps still need to be taken to have humanoid robots moving out of research laboratories to real world applications.