Modeling and Control of a Flexible Space Robot to Capture a Tumbling Debris

RÉSUMÉ La conquête spatiale des 60 dernières années a généré une grande quantité d’objets à la dérive sur les orbites terrestres. Leur nombre grandissant constitue un danger omniprésent pour l’exploitation des satellites, et requiert aujourd’hui une intervention humaine pour réduire les risques de collision. En effet, l’estimation de leur croissance sur un horizon de 200 ans, connue sous le nom de “syndrôme de Kessler”, montre que l’accès à l’Espace sera grandement menacé si aucune mesure n’est prise pour endiguer cette prolifération. Le scientifique J.-C. Liou de la National Aeronautics and Space Administration (NASA) a montré que la tendance actuelle pourrait être stabilisée, voire inversée, si au moins cinq débris massifs étaient désorbités par an, tels que des satellites en fin de vie ou des étages supérieurs de lanceur. Parmi les nombreux concepts proposés pour cette mission, la robotique s’est imposée comme une des solutions les plus prometteuses grâce aux retours d’expérience des 30 dernières années. La Station Spatiale Internationale (ISS) possède déjà plusieurs bras robotiques opérationnels, et de nombreuses missions ont démontré le potentiel d’un tel système embarqué sur un satellite. Pour deux d’entre elles, des étapes fondamentales ont été validées pour le service en orbite,et s’avèrent être similaires aux problématiques de la désorbitation des débris. Cette thèse se concentre sur l’étape de capture d’un débris en rotation par un bras robotique ayant des segments flexibles. Cette phase comprend la planification de trajectoire et le contrôle du robot spatial, afin de saisir le point cible du débris de la façon la plus délicate possible. La validation des technologies nécessaires à un tel projet est quasiment impossible sur Terre, et requiert des moyens démesurés pour effectuer des essais en orbite. Par conséquent, la modélisation et la simulation de systèmes multi-corps flexibles est traitée en détails, et constitue une forte contribution de la thèse. À l’aide de ces modèles, une validation mixte est proposée par des essais expérimentaux, en reproduisant la cinématique en orbite par des manipulateurs industriels contrôlés par une simulation en temps réel. En résumé, cette thèse est construite autour des trois domaines suivants : la modélisation des robots spatiaux, le design de lois de contrôle, et leur validation sur un cas test. Dans un premier temps, la modélisation de robots spatiaux en condition d’apesanteur est développée pour une forme “en étoile”.----------ABSTRACT After 60 years of intensive satellite launches, the number of drifting objects in Earth orbits is reaching a shifting point, where human intervention is becoming necessary to reduce the threat of collision. Indeed, a 200 year forecast, known as the “Kessler syndrome”, states that space access will be greatly compromised if nothing is done to address the proliferation of these debris. Scientist J.-C. Liou from the National Aeronautics and Space Administration (NASA) has shown that the current trend could be reversed if at least five massive objects, such as dead satellites or rocket upper stages, were de-orbited each year. Among the various technical concepts considered for debris removal, robotics has emerged, over the last 30 years, as one of the most promising solutions. The International Space Station (ISS) already possesses fully operational robotic arms, and other missions have explored the potential of a manipulator embedded onto a satellite. During two of the latter, key capabilities have been demonstrated for on-orbit servicing, and prove to be equally useful for the purpose of debris removal. This thesis focuses on the close range capture of a tumbling debris by a robotic arm with light-weight flexible segments. This phase includes the motion planning and the control of a space robot, in order to smoothly catch a target point on the debris. The validation of such technologies is almost impossible on Earth and leads to prohibitive costs when performed on orbit. Therefore, the modeling and simulation of flexible multi-body systems has been investigated thoroughly, and is likewise a strong contribution of the thesis. Based on these models, an experimental validation is proposed by reproducing the on-orbit kinematics on a test bench made up of two industrial manipulators and driven by a real-time dynamic simulation. In a nutshell, the thesis is built around three main parts: the modeling of a space robot, the design of control laws, and their validation on a test case. The first part is dedicated to the flexible modeling of a space robot in conditions of weightlessness. A “star-shaped” multi-body system is considered, meaning that the rigid base carries various flexible appendages and robotic arms, assumed to be open mechanical chains only. The classic Newton-Euler and Lagrangian algorithms are brought together to account for the flexibility and to compute the dynamics in a numerically efficient way. The modeling step starts with the rigid fixed-base manipulators in order to introduce the notations, then, détails the flexible ones, and ends with the moving-base system to represent the space robots

Control of a launcher in atmospheric ascent with guardian maps

This paper describes the synthesis of a SISO scheduled controller for a launcher vehicle. The problem consists in designing a control law which will be valid on the atmospheric ascent trajectory from time 25 s to time 60 s, while ensuring robustness and performance requirements. Moreover a flexible model with two bending modes is considered, making the problem more challenging. An algorithm based upon guardian maps has been retained in order to find an a priori fixed architecture controller. The algorithm yields a sequence of controllers that ensures that pole confinement constraints are fulfilled for any time between 25 s and 60 s. The user can then interpolate those controllers to find a scheduled controller with respect to time

Contrôle d'attitude d'un lanceur en phase atmosphérique approche par applications gardiennes

RÉSUMÉ: Dans un premier temps, la modélisation d’un lanceur spatial met en avant l’importance des variations des paramètres au cours de l’ascension. Ils sont en effet fortement dépendants de la variation de la masse avec la consommation des ergols. Le modèle utilisé prend ainsi en compte cette évolution en linéarisant les équations autour de 6 points de vol principaux, pour obtenir des modèles linéaires invariants dans le temps. Chacun de ces modèles devra être stabilisé par une unique loi de commande, tout en respectant les performances désirées. La considération des modes de flexion du lanceur rend alors la synthèse plus complexe. Une autre conséquence de l’évolution temporelle est le séquencement des gains des contrˆo- leurs obtenus. Il est en effet montré dans le mémoire qu’aucun réglage constant du correcteur adopté ne permet de respecter le cahier des charges sur toute la trajectoire. Ensuite, la complexité de modélisation du lanceur complet amène à considérer les erreurs et les incertitudes de modèle. Elles sont là encore un enjeu majeur du projet puisqu’il faut pouvoir assurer les performances nominales précédentes de façon robuste. Les applications gardiennes, de par leur propriétés, se sont révélées les plus adaptées pour traiter un tel pro- blème de séquencement avec respect de performances robustes. Dans cette optique, un aspect essentiel de la Maîtrise porte sur le développement de méthodes de synthèse basées sur les applications gardiennes. Il est en effet apparu que les quelques travaux portant sur ce sujet développaient essentiellement la théorie, et que la mise en œuvre de ces méthodes pouvait être améliorée. Deux approches sont ainsi développées à travers le projet. La première s’appuie sur une vi- sualisation graphique des lieux d’annulation des applications gardiennes. Un programme basé sur l’analyse d’image permet de vérifier automatiquement les réglages des gains qui satisfont les contraintes. La seconde méthode s’appuie sur une optimisation portant sur les applications gardiennes dans des zones d’intérêt. Par itérations successives, il est alors possible de partir du système en boucle ouverte pour aboutir à un réglage des gains acceptable en boucle fermée. Les méthodes élaborées ont été testées et éprouvées sur le cas du lanceur, avec le cahier des charges fourni par ASTRIUM-ST. Cette application pratique a été motivée par la complexité du système, les contraintes variées à considérer et l’importance de la robustesse pour ce type de commande. Autant de contraintes qui ont permis de valider l’intérêt et l’efficacité des applications gardiennes sur ce type de problème.---------- ABSTRACT: In a first phase, the modelling process underlines the presence of highly time varying parameters during the ascent, due to a fast mass variation along with propellant consumption. Linearizing the dynamical equations at six main flight instants yields linear time invariant models to be considered during control design. Each of them is to be stabilized by one control law, while respecting given specifications. The synthesis becomes even more complex when the bending modes are taken into account. Moreover, scheduling appears necessary to deal with the time variations. Indeed it is shown that no single gain setting is able to respect all the specifications along the trajectory. Furthermore, increasing complexity when modelling a whole launch vehicle pushes one to consider the model’s errors and uncertainties. They represent a major issue in this study since it is asked to ensure the nominal performances in a robust fashion. Owing to their properties, guardian maps appear to be the most suitable tool to deal with such a problem of scheduling with robust performances. In light of this, the development of synthesis methods based on guardian maps is the main contribution of the project. It appears that actual state of the art in this field is focused on theoretical issues, whereas practical ones could be improved. Two approches are presented in the memoire. The first one is based on a graphical ap- proach consisting in drawing the vanishing locus of guardian maps. A program using image analysis techniques is devised to check automatically which gain settings satisfy the con- straints. The second one is based on an optimisation procedure involving guardian maps. Starting with the open loop system, the iterative process proposed ends up with a satisfactory gain setting for the closed-loop. These methods are tried and tested for the launch vehicle, with specifications from ASTRIUM-ST. Their practical application is motivated by the system complexity, the differ- ent kinds of constraints and the essential need for robustness. Many restrictions that finally bring about the interest and the efficiency of guardian maps for such a problem

Linear dynamics of flexible multibody systems : a system-based approach

A new methodology to derive the linear model of flexible multibody system dynamics is presented in this paper. This approach is based on the twoport model of each body allowing the model of the whole system to be built just connecting the inputs/outputs of each body model. Boundary conditions of each body can be taken into account through inversion of some input-output channels of its two-port model. This approach is extended here to treat the case of closed-loop kinematic mechanisms. Lagrange multipliers are commonly used in an augmented differential-algebraic equation to solve loop-closure constraints. Instead, they are considered here as a model output, which is connected to the adjoining body model through a feedback. After a summary of main results in the general case, the case of planar mechanisms with multiple uniform beams is considered and the two-port model of the Euler-Bernouilli beam is derived. The choice of the assumed modes is then discussed regarding the accuracy of the first natural frequencies for various boundary conditions. The overall modeling approach is then applied to the well-known four bar mechanism

Incorporating delayed and multi-rate measurements in navigation filter for autonomous space rendezvous

In the scope of space missions involving rendezvous between a chaser and a target, vision based navigation relies on the use of optical sensors coupled with image processing and computer vision algorithms to obtain a measurement of the target relative pose. These algorithms usually have high latency time, implying that the chaser navigation filter has to fuse delayed and multi-rate measurements. This article has two main contributions: it provides a detailed modelization of the relative dynamics within the estimation filter, and it proposes a comparison of two delay management techniques suitable for this application. The selected methods are the Filter Recalculation method -which always provides an optimal estimation at the expense of a high computational load- and the Larsen’s method -which provides a faster solution whose optimality lies on stronger requirements. The application of these techniques to the space rendezvous problem is discussed and formalized. Finally, the current article proposes a comparison of the methods based on a Monte-Carlo campaign, aimed at demonstrating whether the loss of performance of Larsen’s method due to its sub-optimality still enables target state robust tracking

Modeling & control of a space robot for active debris removal

Space access and satellites lifespan are increasingly threatened by the great amount of debris in Low Earth Orbits (LEO). Among the many solutions proposed in the literature so far, the emphasis is put here on a robotic arm mounted on a satellite to capture massive debris, such as dead satellites or launch vehicle upper stages. The modeling and control of such systems are investigated throughout the paper. Dynamic models rely on an adapted Newton-Euler algorithm, and control algorithms are based on the recent structured H infinity method. The main goal is to efficiently track a target point on the debris while using simple PD-like controllers to reduce computational burden. The structured H infinity framework proves to be a suitable tool to design a reduced-order robust controller that catches up with external disturbances and is simultaneously compatible with current space processors capacities

Vision-based navigation for autonomous space rendezvous with non-cooperative targets

This study addresses the issue of vision-based navigation for space rendezvous with non-cooperative targets. After a brief description of the scenario and its peculiarities, the theory underlying monocular edges-based tracking for pose estimation is recalled and an innovative tracking algorithm is formally developed and implemented. This algorithm is coupled with a dynamic Kalman Filter propagating the dynamics which underlies a space rendezvous. The navigation filter increases the robustness of target position and attitude estimation, and allows the estimation of target translational velocity and rotation rate using only pose measurements. Moreover, the filter implements a computationally efficient delay management technique that allows merging the delayed and infrequent measurements typical of vision-based navigation. The performance of the algorithm is tested in different scenarios with high fidelity synthetic images

Addressing mental health problems among persons without stable housing in the context of the COVID-19 pandemic: study protocol for a randomised trial. RESPOND – France

Background The COVID-19 pandemic has had an impact on population-wide mental health and well-being. Although people experiencing socioeconomic disadvantage may be especially vulnerable, they experience barriers in accessing mental health care. To overcome these barriers, the World Health Organization (WHO) designed two scalable psychosocial interventions, namely the web-based Doing What Matters in Times of Stress (DWM) and the face-to-face Problem Management Plus (PM+), to help people manage stressful situations. Our study aims to test the effectiveness of a stepped-care program using DWM and PM + among individuals experiencing unstable housing in France – a majority of whom are migrant or have sought asylum. Methods This is a randomised controlled trial to evaluate the effectiveness and cost effectiveness of a stepped-care program using DWM and PM + among persons with psychological distress and experiencing unstable housing, in comparison to enhanced care as usual (eCAU). Participants (N = 210) will be randomised to two parallel groups: eCAU or eCAU plus the stepped-care program. The main study outcomes are symptoms of depression and anxiety measured using the Patient Health Questionnaire Anxiety and Depression Scale (PHQ-ADS). Discussion This randomised controlled trial will contribute to a better understanding of effective community-based scalable strategies that can help address the mental health needs of persons experiencing socioeconomic disadvantage, whose needs are high yet who frequently have limited access to mental health care services

Modeling and Control of a Flexible Space Robot to Capture a Tumbling Debris

Les débris en orbite sont actuellement une source de préoccupation majeure pour les acteurs du spatial et pour le reste de la population, comme en témoignent les articles de presse et les œuvres cinématographiques sur le sujet. En effet, la présence de ces objets menace directement les astronautes en mission et les satellites en opération. Parmi les nombreuses options déjà envisagées pour les traiter, cette thèse se concentre sur l’approche robotique, en proposant des outils et des méthodes de modélisation et de contrôle pour un satellite chasseur équipé d’un bras manipulateur. Des modèles dynamiques et des schémas de simulation optimisés sont ainsi développés pour tout système multi-corps constitué d’une base mobile supportant un nombre quelconque d’appendices rigides ou flexibles. Par la suite, les trajectoires de capture sont générées en conservant la continuité en accélération avec le mouvement naturel du point cible, dans le but de saisir aussi délicatement que possible le débris en rotation. Le suivi de cette trajectoire par l’effecteur du robot chasseur est alors assuré par une loi de contrôle à deux niveaux, dont le réglage repose sur la synthèse H1 structurée. Une étude de robustesse est également mise en place pour assurer la stabilité et les performances du système en boucle fermée, malgré les changements de configuration du bras. Enfin, la validation des travaux de thèse est réalisée par voie numérique avec un simulateur haute-fidélité, et par voie pratique avec un banc d’essais robotique incluant des composants physiques en temps réel.On-orbit debris are currently causing deep concern for space agencies, related companies, and also among the population. ¿is is evidenced by the numerous scientific articles and recent movies on the matter. Indeed, these objects pose a serious threat for the astronauts on mission and for operational satellites. Among the various technical concepts already designed to address these threats, this thesis focuses on space robotics. Tools and methods are thus introduced for the modeling and control of a chaser satellite equipped with a manipulator. Dynamic models and optimized simulation schemes are developed to handle any multi-body system made up of amoving base embedding various appendages, either rigid or flexible. ¿en, a trajectory planner is designed to ensure acceleration continuity with the natural motion of the debris in order to perform a soft capture. ¿is reference trajectory is tracked by the end-effector of the chaser using a two-level control law, which is tuned by the structured H1 synthesis. A robustness analysis is also presented to assess the stability and the performances of the closed-loop system with respect to the motion of the robotic arm. Finally, the outcome of the thesis is validated by a twofold approach: by numerical means with a highfidelity simulator, and by practical ones with a robotic test bench including physical components in real time

Advanced GNC for In-Orbit Autonomous Assembly of Flexible Vehicles – IOA-GNC

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