Control of underactuated fluid-body systems with real-time particle image velocimetry

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2012.Cataloged from PDF version of thesis.Includes bibliographical references (p. 141-153).Controlling the interaction of a robot with a fluid, particularly when the desired behavior is intimately related to the dynamics of the fluid, is a difficult and important problem. High-performance aircraft cannot ignore nonlinear stall effects, and robots hoping to fly and swim with performance matching that seen in birds and fish cannot treat fluid flows as quasi-steady. If we wish to match the level of performance seen in nature several major hurdles must be overcome, with one of the most difficult being the poor observability of the fluid state. Fluid dynamicists have long contended with this observability problem, and have used computationally intensive Particle Image Velocimetry (PIV) to gain an understanding of the fluid behavior after the fact. However, improvement in available computational power is now making it possible to perform PIV in real-time. When PIV provides real-time awareness of the fluid state it is no longer just an analysis tool, but rather a valuable sensor that can be integrated into the control loop. In this thesis I present methods for controlling fluid-body systems in which the fluid plays a vital dynamical role, for performing real-time PIV, and for interpreting the output of PIV in a manner useful to control. The utility of these methods is demonstrated on a mechanically simple but dynamically rich experimental platform: the hydrodynamic cartpole. This system is analogous to the well-known cart-pole system in the controls literature, but through its relationship with the surrounding fluid it captures many of the fundamental challenges of general fluid-body control tasks, including: nonlinearity, underactuation, an important and unknown fluid state and a dearth of accurate and tractable models. The first complete demonstration of closed-loop PIV control is performed on this system, and there is a statistically significant improvement in the system's ability to reject fluid disturbances when using real-time PIV for closed-loop control. These results suggest that these new techniques will push the boundaries of what we can expect a robot in a fluid to do.by John W. Roberts.Ph.D

Commande Robuste et Contraintes d'Optimisation

This thesis presents an overview of my research activities carried out since my PhD in 2001. In the first section, description of the projects, my different contributions to robust control applied to the spatial field and underwater robotics, are highlighted. My research project for the coming years is then presented; I propose an original and efficient methodology to compute simple control laws by combining \textit{robust control} and \textit{global optimization}. The second part of this thesis is dedicated to the scientific aspects that will help clarify the proposed research project. As a starting point, Youla parametrization is presented as a tool to \textit{render convex} the control problem, and the subsequent work is used as a foundation to establish specifications based on the constraints related to optimization. This theme has served as a driving thread in illustrating how industrial requirements could lead to a control problem. Parallel to this, the question also arose as to the practical realization of results from these methodologies, that is, how they might be implemented in an embedded system. Ariane 5 launcher control is taken as an example for research on the structured control and validation.Ce mémoire présente un panorama des activités de recherche menées depuis ma thèse de doctorat en 2001. Dans une première partie, à travers la description des projets, sont mises en avant les différentes contributions à la commande robuste appliquée au monde spatial et au monde de la robotique sous-marine. On montre alors comment s'est construit le projet de recherche proposé pour les années à venir. Il s'agit de proposer une méthodologie originale et efficace pour régler des lois de commande simple à implémenter en combinant \textit{commande robuste} et \textit{optimisation globale}. La seconde partie de ce mémoire est consacrée à quelques aspects scientifiques qui aident à comprendre le projet de recherche proposé. On y trouve comme point de départ la paramétrisation de Youla en tant qu'outil pour \textit{convexifier} le problème de commande et les travaux qui en ont découlés pour traduire un cahier des charges en terme de contrainte dans un problème d'optimisation. Cette thématique a été un fil conducteur important pour faire le lien avec la demande industrielle de savoir comment les exigences étaient traduites dans le problème de commande. En parallèle, s'est posée la question de la réalisation pratique des résultats issues de ces méthodologies, c'est-à-dire leur implémentation sur un système embarqué. On prendra comme exemple les activités de recherche sur la structuration de correcteur et leur qualification pour les lois de pilotage des lanceurs Ariane 5

Recent Advances in Robust Control

Robust control has been a topic of active research in the last three decades culminating in H_2/H_\infty and \mu design methods followed by research on parametric robustness, initially motivated by Kharitonov's theorem, the extension to non-linear time delay systems, and other more recent methods. The two volumes of Recent Advances in Robust Control give a selective overview of recent theoretical developments and present selected application examples. The volumes comprise 39 contributions covering various theoretical aspects as well as different application areas. The first volume covers selected problems in the theory of robust control and its application to robotic and electromechanical systems. The second volume is dedicated to special topics in robust control and problem specific solutions. Recent Advances in Robust Control will be a valuable reference for those interested in the recent theoretical advances and for researchers working in the broad field of robotics and mechatronics

Guaranteed safe switching for switching adaptive control

Adaptive control algorithms may not behave well in practice due to discrepancies between the theory and actual practice. The proposed results in this manuscript constitute an effort in providing algorithms which assure more reliable operation in practice. Our emphasis is on algorithms that will be safe in the sense of not permitting destabilizing controllers to be switched in the closed-loop and to prevent wild signal fluctuations to occur. Coping with the connection or possible connection of destabilizing controllers is indeed a daunting task. One of the most intuitive forms of adaptive control, gain scheduling, is an approach to control of non-linear systems which utilizes a family of linear controllers, each of which provides satisfactory control for a different operating point of the system. We provide a mechanism for guaranteeing closed-loop stability over rapid switching between controllers. Our proposed design provides a simplification using only finite number of pre-determined values for the controller gain, where the observer gain is computed via a table look-up method. In comparison to the original gain scheduling design which our procedure builds on, our design achieves similar performance but with much less computational burden. Many multi-controller adaptive switching algorithms do not explicitly rule out the possibility of switching a destabilizing controller into the closed-loop. Even if the new controller is ensured to be stabilizing, performance verification with the new controller is not straightforward. The importance of this arises in iterative identification and control algorithms and multiple model adaptive control (MMAC). We utilize a limited amount of experimental and possibly noisy data obtained from a closed-loop consisting of an existing known stabilizing controller connected to an unknown plant-to infer if the introduction of a prospective controller will stabilize the unknown plant. We propose analysis results in a nonlinear setting and provide data-based tests for verifying the closed-loop stability with the introduction of a new nonlinear controller to replace a linear controller. We also propose verification tools for the closed-loop performance with the introduction of a new stabilizing controller using a limited amount of data obtained from the existing stable closed-loop. The simulation results in different practical scenarios demonstrate efficacy and versatility of our results, and illustrate practicality of our novel data-based tests in addressing an instability problem in adaptive control algorithms

Characterising pattern asymmetry in pigmented skin lesions

Abstract. In clinical diagnosis of pigmented skin lesions asymmetric pigmentation is often indicative of melanoma. This paper describes a method and measures for characterizing lesion symmetry. The estimate of mirror symmetry is computed first for a number of axes at different degrees of rotation with respect to the lesion centre. The statistics of these estimates are the used to assess the overall symmetry. The method is applied to three different lesion representations showing the overall pigmentation, the pigmentation pattern, and the pattern of dermal melanin. The best measure is a 100% sensitive and 96% specific indicator of melanoma on a test set of 33 lesions, with a separate training set consisting of 66 lesions