Linear Bellman combination for control of character animation

Controllers are necessary for physically-based synthesis of character animation. However, creating controllers requires either manual tuning or expensive computer optimization. We introduce linear Bellman combination as a method for reusing existing controllers. Given a set of controllers for related tasks, this combination creates a controller that performs a new task. It naturally weights the contribution of each component controller by its relevance to the current state and goal of the system. We demonstrate that linear Bellman combination outperforms naive combination often succeeding where naive combination fails. Furthermore, this combination is provably optimal for a new task if the component controllers are also optimal for related tasks. We demonstrate the applicability of linear Bellman combination to interactive character control of stepping motions and acrobatic maneuvers.Singapore-MIT GAMBIT Game LabNational Science Foundation (U.S.) (Grant 2007043041)National Science Foundation (U.S.) (Grant CCF-0810888)Adobe SystemsPixar (Firm

Intelligent model-based control of complex three-link mechanisms

The aim of this study is to understand the complexity and control challenges of the locomotion of a three-link mechanism of a robot system. In order to do this a three-link robot gymnast (Robogymnast) has been built in Cardiff University. The Robogymnast is composed of three links (one arm, one torso, one leg) and is powered by two geared DC motors. Currently the robot has three potentiometers to measure the relative angles between adjacent links and only one tachometer to measure the relative angular position of the first link. A mathematical model for the robot is derived using Lagrange equations. Since the model is inherently nonlinear and multivariate, it presents more challenges when modelling the Robogymnast and dealing with control motion problems. The proposed approach for dealing with the design of the control system is based on a discrete-time linear model around the upright position of the Robogymnast. To study the swinging motion of the Robogymnast, a new technique is proposed to manipulate the frequency and the amplitude of the sinusoidal signals as a means of controlling the motors. Due to the many combinations of the frequency and amplitude, an optimisation method is required to find the optimal set. The Bees Algorithm (BA), a novel swarm-based optimisation technique, is used to enhance the performance of the swinging motion through optimisation of the manipulated parameters of the control actions. The time taken to reach the upright position at its best is 128 seconds. Two different control methods are adopted to study the balancing/stablising of the Robogymnast in both the downward and upright configurations. The first is the optimal control algorithm using the Linear Quadratic Regulator (LQR) technique with integrators to help achieve and maintain the set of reference trajectories. The second is a combination of Local Control (LC) and LQR. Each controller is implemented via reduced order state observer to estimate the unmeasured states in terms of their relative angular velocities. From the identified data in the relative angular positions of the upright balancing control, it is reported that the maximum amplitude of the deviation in the relative angles on average are approximately 7.5° for the first link and 18° for the second link. It is noted that the third link deviated approximately by 2.5° using only the LQR controller, and no significant deviation when using the LQR with LC. To explore the combination between swinging and balancing motions, a switching mechanism between swinging and balancing algorithm is proposed. This is achieved by dividing the controller into three stages. The first stage is the swinging control, the next stage is the transition control which is accomplished using the Independent Joint Control (IJC) technique and finally balancing control is achieved by the LQR. The duration time of the transition controller to track the reference trajectory of the Robogymnast at its best is found to be within 0.4 seconds. An external disturbance is applied to each link of the Robogymnast separately in order to study the controller's ability to overcome the disturbance and to study the controller response. The simulation of the Robogymnast and experimental realization of the controllers are implemented using MATLAB® software and the C++ program environment respectively

Pre-computation for controlling character behavior in interactive physical simulations

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 129-136).The development of advanced computer animation tools has allowed talented artists to create digital actors, or characters, in films and commercials that move in a plausible and compelling way. In interactive applications, however, the artist does not have total control over the scenarios the character will experience. Unexpected changes in the environment of the character or unexpected interactions with dynamic elements of the virtual world can lead to implausible motions. This work investigates the use of physical simulation to automatically synthesize plausible character motions in interactive applications. We show how to simulate a realistic motion for a humanoid character by creating a feedback controller that tracks a motion capture recording. By applying the right forces at the right time, the controller is able to recover from a range of interesting changes to the environment and unexpected disturbances. Controlling physically simulated humanoid characters is non-trivial as they are governed by non-linear, non-smooth, and high-dimensional equations of motion. We simplify the problem by using a linearized and simplified dynamics model near a reference trajectory. Tracking a reference trajectory is an effective way of getting a character to perform a single task. However, simulated characters need to perform many tasks form a variety of possible configurations. This work also describes a method for combining existing controllers by adding their output forces to perform new tasks. This allows one to reuse existing controllers. A surprising fact is that combined controllers can perform optimally under certain conditions. These methods allow us to interactively simulate many interesting humanoid character behaviors in two and three dimensions. These characters have many more degrees of freedom than typical robot systems and move much more naturally. Simulation is fast enough that the controllers could soon be used to animate characters in interactive games. It is also possible that these simulations could be used to test robotic designs and biomechanical hypotheses.by Marco Jorge Tome da Silva.Ph.D

The mechanics of the contact phase in trampolining

During the takeoff for a trampoline skill the trampolinist should produce sufficient vertical velocity and angular momentum to permit the required skill to be completed in the aerial phase without excessive horizontal travel. The aim of this study was to investigate the optimum technique to produce forward somersault rotation. A seven-segment, subject-specific torque-driven computer simulation model of the takeoff in trampolining was developed in conjunction with a model of the reaction forces exerted on the trampolinist by the trampoline suspension system. The ankle, knee, hip, and shoulder joints were torque-driven, with the metatarsal-phalangeal and elbow joints angle-driven. Kinematic data of trampolining performances were obtained using a Vicon motion capture system. Segmental inertia parameters were calculated from anthropometric measurements. Viscoelastic parameters governing the trampoline were determined by matching an angle-driven model to the performance data. The torque-driven model was matched to the performance data by scaling joint torque parameters from the literature, and varying the activation parameters of the torque generators using a simulated annealing algorithm technique. The torque-driven model with the scaled isometric strength was evaluated by matching the performance data. The evaluation produced close agreement between the simulations and the performance, with an average difference of 4.4% across three forward rotating skills. The model was considered able to accurately represent the motion of a trampolinist in contact with a trampoline and was subsequently used to investigate optimal performance. Optimisations for maximum jump height for different somersaulting skills and maximum rotation potential produced increases in jump height of up to 14% and increases of rotation potential up to 15%. The optimised technique for rotation potential showed greater shoulder flexion during the recoil of the trampoline and for jump height showed greater plantar flexion and later and quicker knee extension before takeoff. Future applications of the model can include investigations into the sensitivity of the model to changes in initial conditions, and activation, strength, and trampoline parameters

Caractérisation tridimensionnelle de l’amplitude articulaire de l’épaule

L’épaule est l’articulation la plus mobile et la plus instable du corps humain dû à la faible quantité de contraintes osseuses et au rôle des tissus mous qui lui confèrent au moins une dizaine de degrés de liberté. La mobilité de l’épaule est un facteur de performance dans plusieurs sports. Mais son instabilité engendre des troubles musculo-squelettiques, dont les déchirures de la coiffe des rotateurs sont fréquentes et les plus handicapantes. L’évaluation de l’amplitude articulaire est un indice commun de la fonction de l’épaule, toutefois elle est souvent limitée à quelques mesures planaires pour lesquelles les degrés de liberté varient indépendamment les uns des autres. Ces valeurs utilisées dans les modèles de simulation musculo-squelettiques peuvent amener à des solutions non physiologiques. L’objectif de cette thèse était de développer des outils pour la caractérisation de la mobilité articulaire tri-dimensionnelle de l’épaule, en passant par i) fournir une méthode et son approche expérimentale pour évaluer l’amplitude articulaire tridimensionnelle de l’épaule incluant des interactions entre les degrés de liberté ; ii) proposer une représentation permettant d’interpréter les données tri-dimensionnelles obtenues; iii) présenter des amplitudes articulaires normalisées, iv) implémenter une amplitude articulaire tridimensionnelle au sein d’un modèle de simulation numérique afin de générer des mouvements sportifs optimaux plus réalistes; v) prédire des amplitudes articulaires sécuritaires et vi) des exercices de rééducation sécuritaires pour des patients ayant subi une réparation de la coiffe des rotateurs. i) Seize sujets ont été réalisé séries de mouvements d’amplitudes maximales actifs avec des combinaisons entre les différents degrés de liberté de l’épaule. Un système d’analyse du mouvement couplé à un modèle cinématique du membre supérieur a été utilisé pour estimer les cinématiques articulaires tridimensionnelles. ii) L’ensemble des orientations définies par une séquence de trois angles a été inclus dans un polyèdre non convexe représentant l’espace de mobilité articulaire prenant en compte les interactions entre les degrés de liberté. La combinaison des séries d’élévation et de rotation est recommandée pour évaluer l’amplitude articulaire complète de l’épaule. iii) Un espace de mobilité normalisé a également été défini en englobant les positions atteintes par au moins 50% des sujets et de volume moyen. iv) Cet espace moyen, définissant la mobilité physiologiques, a été utilisé au sein d’un modèle de simulation cinématique utilisé pour optimiser la technique d’un élément acrobatique de lâcher de barres réalisée par des gymnastes. Avec l’utilisation régulière de limites articulaires planaires pour contraindre la mobilité de l’épaule, seulement 17% des solutions optimales sont physiologiques. En plus, d’assurer le réalisme des solutions, notre contrainte articulaire tridimensionnelle n’a pas affecté le coût de calculs de l’optimisation. v) et vi) Les seize participants ont également réalisé des séries d’amplitudes articulaires passives et des exercices de rééducation passifs. La contrainte dans l’ensemble des muscles de la coiffe des rotateurs au cours de ces mouvements a été estimée à l’aide d’un modèle musculo-squelettique reproduisant différents types et tailles de déchirures. Des seuils de contrainte sécuritaires ont été utilisés pour distinguer les amplitudes de mouvements risquées ou non pour l’intégrité de la réparation chirurgicale. Une taille de déchirure plus grande ainsi que les déchirures affectant plusieurs muscles ont contribué à réduire l’espace de mobilité articulaire sécuritaire. Principalement les élévations gléno-humérales inférieures à 38° et supérieures à 65°, ou réalisées avec le bras maintenu en rotation interne engendrent des contraintes excessives pour la plupart des types et des tailles de blessure lors de mouvements d’abduction, de scaption ou de flexion. Cette thèse a développé une représentation innovante de la mobilité de l’épaule, qui tient compte des interactions entre les degrés de liberté. Grâce à cette représentation, l’évaluation clinique pourra être plus exhaustive et donc élargir les possibilités de diagnostiquer les troubles de l’épaule. La simulation de mouvement peut maintenant être plus réaliste. Finalement, nous avons montré l’importance de personnaliser la rééducation des patients en termes d’amplitude articulaire, puisque des exercices passifs de rééducation précoces peuvent contribuer à une re-déchirure à cause d’une contrainte trop importante qu’ils imposent aux tendons.The shoulder is the most mobile but instable joint of the human body due to bony constraint scarcity and soft tissue function unlocking several degrees of freedom (DoF). Shoulder mobility is a factor of performance in some sports. But its instability leads to musculoskeletal impairments, the rotator cuff tear being the most debilitating disorder. Evaluation of the shoulder range of motion (RoM) is a common indicator of shoulder function but it is often limited to a few monoplanar measurements where each DoF varies independently. These values used in computer simulation models lead to non-physiological movements. The aim of this thesis was to develop tools for caracterizing tridimensional shoulder mobility. In this purpose it was mandatory to i) provide a method and its experimental approach to assess shoulder 3D (three-dimensional) RoM with DoF interactions; ii) propose a representation allowing 3D kinematical data interprestation; iii) present normalized shoulder amplitudes; iv) implement 3D RoM into computer simulation models to generate more realistic optimal sports technique; and v) predict safe 3D RoM and vi) safe rehabilitation exercises for patients after rotator cuff repair. i) Sixteen participants performed series of active arm movements with maximal amplitude with interactions between all the shoulder degrees-of-freedom. A motion analysis system combined with an upper limb kinematic model was used to estimate the 3D joint kinematics. ii) All 3D angular poses were included into a nonconvex hull representing the RoM space accounting for DOF interactions. The combination of elevation and rotation series is recommended to fully evaluate shoulder RoM. iii) A normalized 3D RoM space was defined by including 3D poses common to 50% of the participants into a hull of average volume. iv) This average hull, defining physiologic mobility, was used in a computer simulation model to optimize the technique of a release move in gymnastics. With commonly used monoplanar constraints of shoulder mobility, only 17% of the simulations led to a physiological shoulder kinematics, while our 3D RoM constraints systematically ensures realistic shoulder kinematics without extra computational cost. v) and vi) The 16 participants performed 3D shoulder range-of-motion and passive rehabilitation exercises. Stress in all rotator cuff tendons was predicted during each movement by means of a musculoskeletal model using simulations with different type and size of tears. Safety stress thresholds were used to discriminate safe from unsafe ranges-of-motion. Increased tear size and multiple tendons tear decreased safe range-of-motion. Mostly, glenohumeral elevations below 38°, above 65°, or performed with the arm held in internal rotation cause excessive stresses in most types and sizes of injury during abduction, scaption or flexion. This thesis established an innovative representation of the shoulder mobility, which accounts for DoF interactions. Clinical evaluation will be more accurate with a large potential to better diagnose shoulder disorders. Computer simulations are now more realistic. Finally, we showed the importance of personalized rehabilitation in terms of 3D RoM, since passive early rehabilitation exercises could contribute to re-tear due to excessive stress