A Penetration-free Nonsmooth Dynamics Method for Frictionless Contact/Impact Problems

International audienceThis paper studies numerical algorithms for the simulation of mechanical systems including rigid and flexible bodies, kinematic joints and frictionless contact conditions. The condition of impenetrability of the bodies in contact is expressed as a unilateral constraint, with the consequence that impacts and/or instantaneous changes in the velocities may arise in the dynamic response. When dynamic contacts are analyzed between elastic solids and structures, the gap velocity is necessarily discontinuous otherwise a non-physical penetration of the bodies would occur. When contacts between rigid bodies are studied, e.g., in multibody systems, impulsive reaction forces can also occur leading to an instantaneous change in the linear and angular momenta of each body. Standard schemes, such as the Newmark, HHT or generalized-α methods, are not consistent in these cases since the numerical response may artificially generate energy when a contact occurs. In order to analyze impact phenomena, nonsmooth time integration methods have been proposed in the literature and can be classified into two main groups, namely, event-driven schemes and time-stepping schemes. Event-driven schemes are accurate for the free flight smooth motions and are especially suitable for small multi-body systems with a limited number of events, but they become inefficient if frequent transitions occur in a short time. Time-stepping methods, such as Moreau–Jean scheme, have been proven to be convergent and robust even for a large number of events, and are extensively applied as the solution to nonsmooth system models

The nonsmooth generalized-α scheme with a simultaneous enforcement of constraints at position and velocity levels.

International audienceIn this work, we present a formalism for the numerical time integration of nonsmooth dynamical systems composed of rigid and flexible bodies, kinematic joints and frictionless contact conditions. The proposed algorithm guarantees the exact satisfaction of the bilateral and unilateral constraints both at position and velocity levels, extending a previous work on the development of the generalized-α scheme for computational contact mechanics. Following the idea of Gear, Gupta and Leimkuhler, the equation of motion is reformulated so that the bilateral and unilateral constraints appear both at position and velocity levels which amounts to solving two complementarity conditions at both position and velocity levels at each iteration using a monolithic semi-smooth Newton procedure

Implicit representation priors meet Riemannian geometry for Bayesian robotic grasping

Robotic grasping in highly noisy environments presents complex challenges, especially with limited prior knowledge about the scene. In particular, identifying good grasping poses with Bayesian inference becomes difficult due to two reasons: i) generating data from uninformative priors proves to be inefficient, and ii) the posterior often entails a complex distribution defined on a Riemannian manifold. In this study, we explore the use of implicit representations to construct scene-dependent priors, thereby enabling the application of efficient simulation-based Bayesian inference algorithms for determining successful grasp poses in unstructured environments. Results from both simulation and physical benchmarks showcase the high success rate and promising potential of this approach.Comment: 4 pages, 5 figures, submitted to the workshop Geometric Representations at ICRA 202

An inertial human upper limb motion tracking method for robot programming by demonstration

peer reviewedThis paper proposes an inertial human motion tracking for robot programming by demonstration (PbD). An original element called heading reset is proposed to catch the drift around gravity direction. It is based on a hypothesis made on the human arm motion during a task demonstration. It is used to overcome the non-use of the magnetometer due to magnetic disturbances from robotic environment. This element is implemented in an orientation estimation algorithm and compared with three other IMU algorithms and a commercial MARG algorithm. The human arm trajectory is estimated through three IMUs sensors directly set on the arm to estimate the segment orientation (hand, forearm and arm). A specific inertial-2-segment procedure is presented as well as a procedure to estimate the transformation from human reference frame to task frame, necessary for a PbD process. Experimental tests, using a robot as a reference, have been conducted to validate the different part of the method. The heading reset and the orientation algorithm show good results. The inertial-2-segment procedure is shown to be robust. Finally, experimental tests on a human arm and physical robot validate the complete method

EFFECT OF A STRETCHING PROGRAM ON OVERHEAD ATHLETES WITH A STIFF AND PAINFUL SHOULDER

Overhead athletes are more at risk of suffering shoulder injuries or/and developing shoulder pain. Stretching programs have been shown to successfully reduce pain for this population. However no study has provided a global overview of the effects of stretching. The present study evaluated the evolution of the pain, the strength of the shoulder muscles, potential instabilities or lesion of the shoulder as well as the 3D scapular kinematics of 10 overhead athletes having a painful and stiff dominant shoulder. Our results show a significant decrease of the pain and of the positive impingement syndrome tests. The kinematics of the scapula reveals a more upward rotation of the scapula after the stretching, which may reduce the risk of sub-acromial conflicts. The strength of the internal and external rotators of the shoulder is not modified by the program

A Penetration-free Nonsmooth Dynamics Method for Frictionless Contact/Impact Problems

International audienceThis paper studies numerical algorithms for the simulation of mechanical systems including rigid and flexible bodies, kinematic joints and frictionless contact conditions. The condition of impenetrability of the bodies in contact is expressed as a unilateral constraint, with the consequence that impacts and/or instantaneous changes in the velocities may arise in the dynamic response. When dynamic contacts are analyzed between elastic solids and structures, the gap velocity is necessarily discontinuous otherwise a non-physical penetration of the bodies would occur. When contacts between rigid bodies are studied, e.g., in multibody systems, impulsive reaction forces can also occur leading to an instantaneous change in the linear and angular momenta of each body. Standard schemes, such as the Newmark, HHT or generalized-α methods, are not consistent in these cases since the numerical response may artificially generate energy when a contact occurs. In order to analyze impact phenomena, nonsmooth time integration methods have been proposed in the literature and can be classified into two main groups, namely, event-driven schemes and time-stepping schemes. Event-driven schemes are accurate for the free flight smooth motions and are especially suitable for small multi-body systems with a limited number of events, but they become inefficient if frequent transitions occur in a short time. Time-stepping methods, such as Moreau–Jean scheme, have been proven to be convergent and robust even for a large number of events, and are extensively applied as the solution to nonsmooth system models

Simulation-based Bayesian inference for robotic grasping

peer reviewedGeneral robotic grippers are challenging to control because of their rich nonsmooth contact dynamics and the many sources of uncertainties due to the environment or sensor noise. In this work, we demonstrate how to compute 6-DoF grasp poses using simulation-based Bayesian inference through the full stochastic forward simulation of the robot in its environment while robustly accounting for many of the uncertainties in the system. A Riemannian manifold optimization procedure preserving the nonlinearity of the rotation space is used to compute the maximum a posteriori grasp pose. Simulation and physical benchmarks show the promising high success rate of the approach

RELEVANCE OF EARLY STRETCHING IN OVERHEAD ATHLETES

Overhead athletes often develop, over time, a tightness of the posterior structures of the shoulder, which may be associated with injuries. Ten symptomatic (with pain) and ten asymptomatic players with a tight shoulder were compared and the effect of a selfapplied stretching program was evaluated. Before and after the stretching program, pain and stiffness of the shoulder were evaluated. Our results demonstrate that risk factors for shoulder pain such as glenohumeral internal rotation deficit and total range of motion deficit may only be limited in symptomatic athletes. The mobility of the shoulder was significantly improved after the stretching program for both groups. Pain was reduced when present. Because of the limited differences between the symptomatic and asymptomatic athletes, clinicians may find it advantageous to initiate early prevention or rehabilitation programs

HOW BIOMECHANICAL ANALYSIS MAY HELP TO IDENTIFY ABDOMINAL INJURY CAUSES IN HIGH LEVEL TENNIS

This research studies the case of a professional tennis player who has suffered from a medical tear on the left rectus abdominis muscle after a tennis serve. The goal of the study is to understand whether the could be explained by an inappropriate technique. For this purpose, we analyzed the three dimensions kinematic and kinetic of the serve. We also performed isokinetic tests of the knees. We compared the player to five other professional players. We observed a possible deficit of energy transfer because of an important anterior pelvis tilt. Some compensations of the player during serve could be a possible higher abdominal contraction and a larger shoulder external rotation. These particularities could induce an abdominal overwork that would explain the first and may provoke a new injury

INFLUENCE OF THE LABORATORY CONTEXT AND THE SIZE OF THE MARKERS SET ON THE TENNIS SERVE EVALUATION

The purpose of this study was to identify the influence on the tennis serve evaluation of 1/ the test environment and 2/ the number of the markers placed of the player. Two different studies were performed. The first compared a 4 vs. 28 marker set in a laboratory the same day. The second compared a 4 markers test in a laboratory with a 4 markers test on an official tennis court one week apart. We observed similar results between the different tests of both studies