Legged Robots

International audienc

Angular and Current-Target Correlations in Deep Inelastic Scattering at HERA

Correlations between charged particles in deep inelastic ep scattering have been studied in the Breit frame with the ZEUS detector at HERA using an integrated luminosity of 6.4 pb-1. Short-range correlations are analysed in terms of the angular separation between current-region particles within a cone centred around the virtual photon axis. Long-range correlations between the current and target regions have also been measured. The data support predictions for the scaling behaviour of the angular correlations at high Q2 and for anti-correlations between the current and target regions over a large range in Q2 and in the Bjorken scaling variable x. Analytic QCD calculations and Monte Carlo models correctly describe the trends of the data at high Q2, but show quantitative discrepancies. The data show differences between the correlations in deep inelastic scattering and e+e- annihilation.Comment: 26 pages including 10 figures (submitted to Eur. J. Phys. C

Plastisol Foaming Process. Decomposition of the Foaming Agent, Polymer Behavior in the Corresponding Temperature Range and Resulting Foam Properties

The decomposition of azodicarbonamide, used as foaming agent in PVC - plasticizer (1/1) plastisols was studied by DSC. Nineteen different plasticizers, all belonging to the ester family, two being polymeric (polyadipates), were compared. The temperature of maximum decomposition rate (in anisothermal regime at 5 K min-1 scanning rate), ranges between 434 and 452 K. The heat of decomposition ranges between 8.7 and 12.5 J g -1. Some trends of variation of these parameters appear significant and are discussed in terms of solvent (matrix) and viscosity effects on the decomposition reactions. The shear modulus at 1 Hz frequency was determined at the temperature of maximum rate of foaming agent decomposition, and differs significantly from a sample to another. The foam density was determined at ambient temperature and the volume fraction of bubbles was used as criterion to judge the efficiency of the foaming process. The results reveal the existence of an optimal shear modulus of the order of 2 kPa that corresponds roughly to plasticizer molar masses of the order of 450 ± 50 g mol-1. Heavier plasticizers, especially polymeric ones are too difficult to deform. Lighter plasticizers such as diethyl phthalate (DEP) deform too easily and presumably facilitate bubble collapse

Legged Robots

International audienc

Angular and Current-target Correlations in Deep Inelastic Scattering at HERA

Correlations between charged particles in deep inelastic e+ p scattering have been studied in the Breit frame with the ZEUS detector at HERA using an integrated luminosity of 6.4pb-1. Short-range correlations are analysed in terms of the angular separation between current-region particles within a cone centred around the virtual photon axis. Long-range correlations between the current and target regions have also been measured. The data support predictions for the scaling behaviour of the angular correlations at high Q2 and for anti-correlations between the current and target regions over a large range in Q2 and in the Bjorken scaling variable x. Analytic QCD calculations and Monte Carlo models correctly describe the trends of the data at high Q2, but show quantitative discrepancies. The data show differences between the correlations in deep inelastic scattering and e+e- annihilation

Mean-field transport theory for the two-flavour NJL model

By making decomposition of the Wigner function simultaneously in both the spinor and the isospin spaces we derive a set of kinetic equations for the quark distribution functions and the spin densities. A detailed analysis of the consequences imposed by the chiral invariance on the form of the transport equations is presented.Comment: Revtex, 25 pages, no figure

Inverse dynamics‐based formulation of finite horizon optimal control problems for rigid‐body systems

We propose a formulation of the finite horizon optimal control problem (FHOCP) based on inverse dynamics for general open-chain rigid-body systems, which reduces the computational cost from the conventional formulation based on forward dynamics. We regard the generalized acceleration as a decision variable and inverse dynamics as an equality constraint. To treat under-actuated systems with inverse dynamics that are well defined only to fully actuated systems, that is, to consider passive joints in this FHOCP, we add an equality constraint to zero the corresponding generalized torques. We include the contact forces in the decision variables of this FHOCP and treat the contact constraints using Baumgarte's stabilization method for numerical stability. We derive the optimality conditions and formulate the two-point boundary-value problem that can be efficiently solved using the recursive Newton–Euler algorithm (RNEA) and the partial derivatives of RNEA. We conducted three numerical experiments on model predictive control based on the proposed formulation to demonstrate its effectiveness. The first experiment involved simulating a swing-up control of a four-link arm with a passive joint and showed that the proposed formulation is effective for under-actuated systems. The second one involved comparing the proposed formulation with the conventional forward-dynamics-based formulation with various numbers of joints and showed that the proposed formulation reduces computational cost regardless of the number of joints. The third experiment involved simulating a whole-body control of a quadruped robot, a floating-base system having four contacts with the ground, and showed that the proposed formulation is applicable even for floating-base systems with contacts

Balance and Posture Control for Biped Robots

This work presents an overview of a new approach for balance and posture control by regulating simultaneously the center of mass position and trunk orientation of a biped robot. After an unknown external perturbation deviates the robot from a desired posture, the controller computes a wrench (force and torque) required to recover the desired position and orientation, according to a compliance control law. This wrench is distributed to predefined supporting contact points at the feet. The forces at these points are computed via a constrained optimization problem, adopted from the grasping literature, which minimizes the contact forces while including friction restrictions and torque limits at each joint

Vision-guided motion primitives for humanoid reactive walking: decoupled vs. coupled approaches

International audienceThis paper proposes a novel visual servoing ap-proach to control the dynamic walk of a humanoid robot. Online visual information is given by an on-board camera. It is used to drive the robot towards a specific goal. Our work is built upon a recent reactive pattern generator that make use of Model Predictive Control (MPC) to modify footsteps, center of mass and center of pressure trajectories to track a reference velocity. The contribution of the paper is to formulate the MPC problem considering visual feedback. We compare our approach with a scheme decoupling visual servoing and walking gait generation. Such a decoupled scheme consists in first, computing a reference velocity from visual servoing; then, the reference velocity is the input of the pattern generator. Our MPC based approach allows to avoid a number of limitations that appears in decoupled methods. In particular visual constraints can be introduced directly inside the locomotion controller, while camera motions do not have to be accounted for separately. Both approaches are compared numerically and validated in simulation. Our MPC method shows a faster convergence