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16242
01/01/2018

Joint position and velocity bounds in discrete-time acceleration/torque control of robot manipulators

A.DEL PRETE

GEPETTO

Revue Scientifique : IEEE Robotics and Automation Letters, Vol.3, N°1, pp.281-288, Janvier 2018, DOI: 10.1109/LRA.2017.2738321 , N° 16242

Lien : https://hal.archives-ouvertes.fr/hal-01356989

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This letter deals with the problem of controlling a robotic system whose joints have bounded position, velocity, and acceleration/torque. Assuming a discrete-time acceleration control, we compute tight bounds on the current joint accelerations that ensure the existence of a feasible trajectory in the future. Despite the clear practical importance of this issue, no complete and exact solution has been proposed yet, and all existing control architectures rely on hand-tuned heuristics. We also extend this methodology to torque-controlled robots, for which joint accelerations are only indirectly bounded by the torque limits. Numerical simulations are presented to validate the proposed method, which is computationally efficient and hence suitable for high-frequency control.

140657
17381
23/10/2017

2PAC: Two point attractors for center of mass trajectories in multi contact scenarios

S.TONNEAU, A.DEL PRETE, J.PETTRE, N.MANSARD

GEPETTO, IRISA

Rapport LAAS N°17381, https://hal.archives-ouvertes.fr/hal-01609055, Octobre 2017, 10p.

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Synthesizing motions for legged characters in arbitrary environments is a long-standing problem that has recently received a lot of attention from the computer graphics community. We tackle this problem with a procedural approach that is generic, fully automatic and independent from motion capture data. The main contribution of this paper is a point-mass-model-based method to synthesize Center Of Mass trajectories. These trajectories are then used to generate the whole-body motion of the character. The use of a point mass model often results in physically inconsistent motions and joint limit violations. We mitigate these issues through the use of a novel formulation of the kinematic constraints which allows us to generate a quasi-static Center Of Mass trajectory, in a way that is both user-friendly and computationally efficient. We also show that the quasi-static constraint can be relaxed to generate motions usable for applications of computer graphics (on average 83% of a given trajectory remain physically consistent). Our method was integrated in our open-source contact planner and tested with different scenarios-some never addressed before-featuring legged characters performing non-gaited motions in cluttered environments. The computational efficiency of our trajectory generation algorithm (under ten ms to compute one second of motion) enables us to synthesize motions in a few seconds, one order of magnitude faster than state-of-the-art methods.

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17406
17/10/2017

Estimation et stabilisation de l'état d'un robot humanoïde compliant

A.MIFSUD

GEPETTO

Doctorat : INP de Toulouse, 17 Octobre 2017, 113p., Président: C.CHEVALLEREAU, Rapporteurs: T.HAMEL, Examinateurs: J.SOLA, P.B.WIEBER, Directeurs de thèse: F.LAMIRAUX, M.BENALLEGUE , N° 17406

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Cette thèse traite de l’estimation et de la stabilisation de l’état des compliances passives présentes dans les chevilles du robot humanoïde HRP-2. Ces compliances peuvent être vues comme un degré de liberté unique et observable, sous quelques hypothèses qui sont explicitées. L’estimateur utilise des mesures provenant de la centrale inertielle située dans le torse du robot et éventuellement des capteurs de forces situés dans ses pieds. Un filtre de Kalman étendu est utilisé pour l’estimation d’état. Ce filtre utilise un modèle complet de la dynamique du robot, pour lequel la dynamique interne du robot, considérée comme parfaitement connue et contrôlée, a été découplée de la dynamique de la compliance passive du robot. L’observabilité locale de l’état a été montrée en considérant ce modèle et les mesures provenant de la centrale inertielle seule. Il a de plus été montré que l’ajout des mesures des capteurs de forces dans les pieds du robot permet de compléter l’état avec des mesures d’erreurs dans le modèle dynamique du robot. L’estimateur a été validé expérimentalement sur le robot humanoïde HRP-2. Sur cet estimateur a été construit un stabilisateur de l’état de la compliance d’HRP-2. L’état commandé est la position et vitesse du centre de masse (contrôle indirecte de la quantité de mouvement) du robot, l’orientation et la vitesse angulaire de son tronc (contrôle indirecte du moment cinétique), ainsi que l’orientation et la vitesse angulaire de la compliance. Les grandeurs de commande sont l’accélération du centre de masse du robot et l’accélération angulaire de son tronc. Un régulateur quadratique linéaire (LQR) a été utilisé pour calculer les gains du retour d’état, basé sur un modèle appelé "pendule inverse flexible à roue d’inertie" qui consiste en un pendule inverse dont la base est flexible et où une répartition de masse en rotation autour du centre de masse du robot représente le tronc du robot. Des tests ont été effectués sur le robot HRP-2 en double support, utilisant l’estimateur décrit précédemment avec ou sans les capteurs de forces.

Mots-Clés / Keywords
Estimation; Asservissement; Robotique humanoïde;

141453
17370
16/10/2017

Fundamental actuation properties of multi-rotors: force-moment decoupling and fail-safe robustness

G.MICHIELETTO, M.RYLL, A.FRANCHI

RIS

Rapport LAAS N°17370, Octobre 2017, 14p.

Lien : https://hal.laas.fr/hal-01612602

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In this paper we shed light on the fundamental actuation capabilities of multi-rotors, such as force-moment decoupling and ability to robustly fly still in place after the loss of propellers. These two actuation properties are formalized through the definition of some necessary algebraic conditions on the control force and control moment input matrices of generically tilted multi-rotors. Standard quadrotors are not able to robustly fly still at a constant spot after the loss of a propeller. The increased number of actuators of a hexarotor does not always help to overcome this limitation. To deeply understand this counterintuitive result, we apply the developed theory on the analysis of fail-safe robustness of hexarotor platforms, and clarify the role of the tilt angles and locations of the propellers in the vehicle. We show that standard star-shaped hexarotors are unable of robust static hovering after a propeller failure, while both the tilted star-shaped hexarotor and the Y-shaped hexarotor possess this important property. The analysis is validated with both simulation and experimental results testing the control of six-rotor vehicles subject to rotor loss.

141240
17347
03/10/2017

Using a memory of motion to efficiently warm-start a nonlinear predictive controller

N.MANSARD, A.DEL PRETE, M.GEISERT, S.TONNEAU, O.STASSE

GEPETTO

Rapport LAAS N°17347, Octobre 2017, 9p.

Lien : https://hal.archives-ouvertes.fr/hal-01591373

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Abstract

Predictive control is an efficient model-based methodology to control complex dynamical systems. In general, it boils down to the resolution at each control cycle of a large nonlinear optimization problem. A critical issue is then to provide a good guess to initialize the nonlinear solver so as to speed up convergence. This is particularly important when disturbances or changes in the environment prevent the use of the trajectory computed at the previous control cycle as initial guess. In this paper, we introduce an original and very efficient solution to automatically build this initial guess. We propose to rely on off-line computation to build an approximation of the optimal trajectories, that can be used on-line to initialize the predictive controller. To that end, we combined the use of sampling-based planning, policy learning with generic representations (such as neural networks), and direct optimal control. We first propose an algorithm to simultaneously build a kinodynamic probabilistic roadmap (PRM) and approximate value function and control policy. This algorithm quickly converges toward an approximation of the optimal state-control trajectories (along with an optimal PRM). Then, we propose two methods to store the optimal trajectories and use them to initialize the predictive controller. We experimentally show that directly storing the state-control trajectories leads the predictive controller to quickly converges (2 to 5 iterations) toward the (global) optimal solution. The results are validated in simulation with an unmanned aerial vehicle (UAV) and other dynamical systems.

141128
17340
03/10/2017

Geometric and Numerical Foundations of Movements

J.P.LAUMOND, N.MANSARD, J.B.LASSERRE

GEPETTO, MAC

Ouvrage (éditeur) : Geometric and Numerical Foundations of Movements, Springer, N°ISBN 978-3-319-51546-5, Octobre 2017, 425p. , N° 17340

Lien : https://hal.laas.fr/hal-01590779

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Abstract

This book aims at gathering roboticists, control theorists, neuroscientists, and mathematicians, in order to promote a multidisciplinary research on movement analysis. It follows the workshop “ Geometric and Numerical Foundations of Movements ” held at LAAS-CNRS in Toulouse in November 2015[1]. Its objective is to lay the foundations for a mutual understanding that is essential for synergetic development in motion research. In particular, the book promotes applications to robotics --and control in general-- of new optimization techniques based on recent results from real algebraic geometry.

141117
17095
28/09/2017

Online payload identification for quadruped robots

G.TOURNOIS, M.FIOCCHI, A.DEL PRETE, R.ORSOLINO, D.G.CALDWELL, C.SEMINI

IIT, Genova, GEPETTO

Manifestation avec acte : IEEE/RSJ International Conference on Intelligent Robots and Systems ( IROS ) 2017 du 24 septembre au 28 septembre 2017, Vancouver (Canada), Septembre 2017, 8p. , N° 17095

Lien : https://hal.laas.fr/hal-01575033

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Abstract

The identification of inertial parameters is crucial to achieve high-performance model-based control of legged robots. The inertial parameters of the legs are typically not altered during expeditions and therefore are best identified offline. On the other hand, the trunk parameters depend on the modules mounted on the robot, like a motor to provide the hydraulic power, or different sets of cameras for perception. This motivates the use of recursive approaches to identify online mass and the position of the Center of Mass (CoM) of the robot trunk, when a payload change occurs. We propose two such approaches and analyze their robustness in simulation. Furthermore, experimental trials on our 80-kg quadruped robot HyQ show the applicability of our strategies during locomotion to cope with large payload changes that would otherwise severely compromise the balance of the robot.

140693
17054
24/09/2017

A Kinodynamic steering-method for legged multi-contact locomotion

P.FERNBACH, S.TONNEAU, A.DEL PRETE, M.TAIX

GEPETTO

Manifestation avec acte : IEEE/RSJ International Conference on Intelligent Robots and Systems ( IROS ) 2017 du 24 septembre au 28 septembre 2017, Vancouver (Canada), Septembre 2017, 7p. , N° 17054

Lien : https://hal.laas.fr/hal-01486933

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We present a novel method for synthesizing collision-free, dynamic locomotion behaviors for legged robots, including jumping, going down a very steep slope, or recovering from a push using the arms of the robot. The approach is automatic and generic: non-gaited motions, comprising arbitrary contact postures can be generated along any environment. At the core of our framework is a new steering method that generates trajectories connecting two states of the robot. These trajectories account for the state-dependent, centroidal dynamic constraints inherent to legged robots. The method, of low dimension, formulated as a Linear Program, is really efficient to compute, and can find an application in various problems related to legged locomotion. By incorporating this steering method into an existing sampling-based contact planner, we propose the first kinodynamic contact planner for legged robots.

140189
17313
21/09/2017

Modification of the spontaneous seat-to-stand transition in cycling with bodyweight and cadence variations

B.WATIER, A.COSTES, N.TURPIN

GEPETTO, Toulouse III, CRIR

Rapport LAAS N°17313, doi 10.1016/j.jbiomech.2017.08.003, Septembre 2017

Lien : https://hal.laas.fr/hal-01584024

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When a high power output is required in cycling, a spontaneous transition by the cyclist from a seated to a standing position generally occurs. In this study, by varying the cadence and cyclist bodyweight, we tested whether the transition is better explained by the greater power economy of a standing position or by the emergence of mechanical constraints that force cyclists to stand. Ten males participated in five experimental sessions corresponding to different bodyweights (80%, 100%, or 120%) and cadences (50 RPM, 70 RPM, or 90 RPM). In each session, we first determined the seat-to-stand transition power (SSTP) in an incremental test. The participants then cycled at 20%, 40%, 60%, 80%, 100%, or 120% of the SSTP in the seated and standing positions, for which we recorded the saddle forces and electromyogram (EMG) signals of eight lower limb muscles. We estimated the cycling cost using an EMG cost function (ECF) and the minimal saddle forces in the seated position as an indicator of the mechanical constraints. Our results show the SSTP to vary with respect to both cadence and bodyweight. The ECF was lower in the standing position above the SSTP value (i.e., at 120%) in all experimental sessions. The minimal saddle forces varied significantly with respect to both cadence and bodyweight. These results suggest that optimization of the muscular cost function, rather than mechanical constraints, explain the seat-to-stand transition in cycling.

140919
17339
13/09/2017

A model-checking approach to analyse temporal failure propagation with AltaRica

A.ALBORE, S.DAL ZILIO, G.INFANTES, C.SEGUIN, P.VIRELIZIER

VERTICS, RIS, ONERA, IRT

Manifestation avec acte : International Symposium on Model-Based Safety and Assessment ( IMBSA ) 2017 du 11 septembre au 13 septembre 2017, Trento (Italie), Septembre 2017, 15p. , N° 17339

Lien : https://hal.archives-ouvertes.fr/hal-01590416

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Abstract

The design of complex safety critical systems raises new technical challenges for the industry. As systems become more complex—and include more and more interacting functions—it becomes harder to evaluate the safety implications of local failures and their possible propagation through a whole system. That is all the more true when we add time to the problem, that is when we consider the impact of computation times and delays on the propagation of failures. We describe an approach that extends models developed for Safety Analysis with timing information and provide tools to reason on the correctness of temporal safety conditions. Our approach is based on an extension of the AltaRica language where we can associate timing constraints with events and relies on a translation into a realtime model-checking toolset. We illustrate our method with an example that is representative of safety architectures found in critical systems.

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