Theory and Applications for Control and Motion Planning of Aerial Robots in Physical Interaction with particular focus on Tethered Aerial Vehicles

Marco Tognon 1
1 LAAS-RIS - Équipe Robotique et InteractionS
LAAS - Laboratoire d'analyse et d'architecture des systèmes
Abstract : This thesis focuses on the study of autonomous aerial robots interacting with the surrounding environment, and in particular on the design of new control and motion planning methods for such systems. Nowadays, autonomous aerial vehicles are extensively employed in many fields of application but mostly as autonomously moving sensors used only to sense the environment. On the other hand, in the recent field of aerial physical interaction, the goal is to go beyond sensing-only applications and to fully exploit aerial robots capabilities in order to interact with the environment, exchanging forces for pushing/pulling/sliding, and manipulating objects. However, due to the different nature of the problems, new control methods are needed. These methods have to preserve the system stability during the interaction and to be robust against external disturbances, finally enabling the robot to perform a given task. Moreover, researchers and engineers need to face other challenges generated by the high complexity of aerial manipulators, e.g., a large number of degrees of freedom, strong nonlinearities, and actuation limits. Furthermore, trajectories of the aerial robots have to be carefully computed using motion planning techniques. To perform the sough task in a safe way, the planned trajectory must avoid obstacles and has to be suitable for the dynamics of the system and its actuation limits. With the aim of achieving the previously mentioned general goals, this thesis considers the analysis of a particular class of aerial robots interacting with the environment: tethered aerial vehicles. The study of particular systems, still encapsulating all the challenges of the general problem, helps on acquiring the knowledge and the expertise for a subsequent development of more general methods applicable to aerial physical interaction. This work focuses on the thorough formal analysis of tethered aerial vehicles ranging from control and state estimation to motion planning. In particular, the differential flatness property of the system is investigated, finding two possible sets of flat outputs that reveal new capabilities of such a system. One contains the position of the vehicle and the link internal force (equivalently the interaction force with the environment), while the second contains the position and a variable linked to the attitude of the vehicle. This shows new control and physical interaction capabilities different from standard aerial robots in free-flight. In particular, the first set of flat outputs allows realizing one of the first “free-floating” versions of the classical hybrid force-motion control for standard grounded manipulators. Based on those results we designed two types of controllers. The first is an easyto- implement controller based on a hierarchical approach. Although it shows good performance in quasi-static conditions, actually the tracking error increases when tracking a dynamic trajectory. Thus, a second controller more suited for tracking problems has been designed based on the dynamic feedback linearization technique. Two observers, for the 3D and 2D environments, respectively, have been designed ii in order to close the control loop using a minimal sensorial setup. We showed that the tether makes possible to retrieve an estimation of the full state from only an IMU plus three encoders for the 3D case, while from just an IMU for the 2D case. Parts of those results were extended to a novel and original multi-robots case as well. We considered a multi-tethered system composed of two aerial robots linked to the ground and to each other by two links. The theoretical results on generic tethered aerial vehicles were finally employed to solve the practical and challenging problem of landing and takeoff on/from a sloped surface, enhancing the robustness and reliability of the maneuvers with respect to the free-flight solution. In addition, moved by the interest on aerial physical interaction from A to Z, supplementary problems related to the topic have been addressed as: i) Design of new omnidirectional-thrust aerial vehicles more suited for physical interaction: we proposed an algorithm to obtain an optimal design that is omnidirectional-thrust using only fixed unidirectional thrusters. We also designed a controller for such vehicle that respects the unidirectionality of the thrusters; ii) Cable suspended load manipulation: we proposed a communication-less control strategy for a team of two robots manipulating an object that guarantees the stability and passivity of the system; iii) Control for unidirectional-thrust aerial manipulators: we proposed a flatnessbased decentralized controller for protocentric unidirectional-thrust aerial manipulators endowed with any number of articulated arms; iv) Motion planning for aerial manipulators: we proposed a control-aware motion planner based on the paradigm of control and planning tied together, for aerial manipulators in interaction with the environment; v) Push-and-slide tasks with an aerial manipulator: considering a truly redundant aerial manipulator based on a multidirectional-thrust aerial vehicle, we designed a controller that, together with the previously mentioned planner, allows the operation of push-and-slide tasks. Such a complete aerial system, result of a wise design of the mechanical system and its controller and motion planner, has been integrated with a sensory suit and used for a real contactbased inspection of a metallic pipe.
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Marco Tognon. Theory and Applications for Control and Motion Planning of Aerial Robots in Physical Interaction with particular focus on Tethered Aerial Vehicles. Automatic. Institut national des sciences appliquées de Toulouse, 2018. English. ⟨tel-02003048⟩

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