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Base Reaction Control of Space Manipulators
Full text linkIn this thesis a research activity is presented, concerning matters of dynamics and control of robot manipulators for space robotics applications. In particular, kinematic control principles suitable for the realization of trajectory-tracking manoeuvres are considered for manipulators in the kinematic redundancy condition, that is typically available on actual space robotic systems. A well known characteristic of space manipulators is due to the dynamic coupling that arises between the manipulator motion and the motion of the base spacecraft on which the manipulator is mounted, due to action-reaction exchanges between the subsystems, and to the characteristics of their momentum equations that determine the non-holonomic nature of the system. This coupling determines dynamic disturbances generated by the manipulator during operation. It reveals important to control these disturbances in order to respect the pointing requirements of the spacecraft, for what concerns communications, on-board instrumentation, and solar panels orientation, and also for what concerns restrictions on the admissible acceleration disturbances exerted on the base platform, that in particular can be imposed during experimental micro-gravity activities onboard the space station.
Fundamental objective of the kinematic control schemes developed in this work is to achieve an optimization of the possible joint trajectories that command the movement of the arm, in order to minimize the dynamic disturbances exerted on the platform, which is possible thanks to a balanced coordination of the arm internal motions, that compensates for its momentum variations that are produced during a trajectory-tracking manoeuvre. Original formulations for the base reaction control are presented and analyzed, and the problem is set in the mathematical framework of constrained least squares methods, while the kinematic control is resolved at the joint acceleration level, in order to attain an effective expressions of the kinematic and dynamic variables involved. The proposed principles reveal suitable for real-time space applications, thanks to the local formulation of the optimization problems and to the use of stable and consolidated solution routines. Analysis and validation of the proposed laws have been developed be means of an experimental test campaign on a planar robot manipulator prototype with three degrees-of-freedom, suspended by air bearings on a flat granite plane, in order to simulate the microgravity environment. In particular a series of trajectory-tracking tests have been performed with dynamic measurements of the resultant base reactions. The analysis is completed by means of a robot simulator system, that has been developed by reproducing the geometrical and inertial characteristics of the experimental prototype. The performance of the control laws have been evaluated both in the fixed base condition, and in the free-floating base condition, and in this case an evaluation on the influence of the inertial parameters involved have been carried out. An independent research activity was related to the application of optimization methods for contact forces control of a bio-inspired climbing robots with dry adhesive pads. Control principles are presented, and their performances evaluated by means of a robot simulator and validated through an experimental robot prototype.In questa tesi è presentato un lavoro di ricerca sulla dinamica ed il controllo di bracci robotici per applicazioni spaziali. In particolare, sono proposti dei principi di controllo cinematico adatti all'inseguimento di traiettorie nello spazio operativo del manipolatore in condizioni di ridondanza cinematica, le quali sono tipicamente disponibili negli attuali sistemi robotici spaziali. Caratteristica meccanica peculiare delle applicazioni di robotica orbitale è l'accoppiamento dinamico che si verifica tra il moto del braccio ed il moto della piattaforma satellitare sulla quale è montato, dovuto agli scambi di azione e reazione che i due sottosistemi si scambiano tra di loro ed alla natura non-olonomica del sistema. Tale accoppiamento è causa di disturbi dinamici esercitati dal manipolatore in fase di operazione, che è necessario controllare in modo da ottemperare sia ai requisiti di puntamento del satellite per quanto riguarda le comunicazioni, la strumentazione di bordo, e l'orientazione dei pannelli solari, sia alle restrizioni sui disturbi di accelerazione impartiti alla piattaforma, che in particolare possono essere imposte durante attività sperimentali in microgravità a bordo della stazione spaziale. Obiettivo fondamentale degli schemi di controllo cinematico sviluppati in questo lavoro, è quello di realizzare una ottimizzazione delle possibili traiettorie di giunto che comandano il movimento del braccio, in modo da minimizzare il disturbo dinamico esercitato sulla piattaforma, reso possibile attraverso la coordinazione dei movimenti interni del braccio, i quali compensino le variazioni di momento che si producono in questo durante l'inseguimento di traiettoria. Formulazioni originali di controllo delle reazioni sono presentate ed analizzate, attraverso il supporto matematico dei metodi ai minimi quadrati vincolati, mentre il controllo cinematico è risolto al livello delle accelerazioni di giunto, in modo da poter esprimere in maniera efficace le grandezze cinematiche e dinamiche coinvolte. I principi proposti si rivelano adatti per l'implementazione in tempo reale in applicazioni spaziali, grazie all'impostazione dei problemi in forma locale ed alla possibilità di utilizzo di algoritmi numerici stabili e consolidati. L'analisi e la validazione delle leggi proposte è stata effettuata attraverso prove sperimentali su un manipolatore planare sperimentale a tre gradi di libertà, sospeso su cuscinetti d'aria in modo da simulare l'ambiente di microgravità, con il quale sono state effettuate prove di inseguimento con misure dinamiche dei disturbi di reazione. L'analisi sperimentale è accompagnata dallo sviluppo di un ambiente di simulazione, il quale riproduce le caratteristiche geometriche ed inerziali del robot sperimentale. Le prestazioni delle leggi di controllo sono state valutate sia per le condizioni di vincolo a base fissa, che di base libera, ed in quest'ultimo caso sono state effettuate valutazioni rispetto all'influenza dei parametri inerziali coinvolti. Una parte indipendente del lavoro, riguarda infine l'applicazione di metodi di ottimizzazione per il controllo delle forze di adesione, adatti al controllo di robot arrampicatori, i quali sfruttano l'utilizzo di sistemi di adesione secca in modo da aumentare l'aderenza alla superficie. I principi di controllo sono testati attraverso un simulatore ed i risultati validati in un robot prototipo sperimentale
A constrained least squares approach for reaction torque control in spacecraft/manipulator systems
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Novel Reaction Control Techniques for Redundant Space Manipulators: Theory and Simulated Microgravity Tests
No full textThis paper presents two novel redundancy resolution schemes aimed at locally minimizing the reaction transferred to the spacecraft during manipulator manoeuvres. The subject is of particular interest in space robotics because reduced reactions result in reduced energy consumption and longer operating life of the Reaction Control System. The first presented solution is based on a weighted Jacobian pseudoinverse and is derived using Lagrangian multipliers. The weight matrix is defined by means of the generalized inertia matrix, which appears in the spacecraft reaction dynamics. The second one is based on a Least Squares formulation of the minimization problem. In this formulation the linearity of the forward kinematics and of the reaction dynamics equations with respect to the joint accelerations is used, since the joint variables and their derivatives are considered as state variables. A weighting matrix is introduced in both the formulations in order to take into account the relative importance of reaction forces and torques in the specific task. A closed-form solution is derived for both the presented methods, and the equivalence of these two solutions is analytically demonstrated. Two very important characteristics of the presented methods are that they are suitable for real time implementation and that they can take into account the robot physical/mechanical constraints in term of joint angle, velocity and acceleration limits directly inside the solution algorithm. A software simulator has been developed in order to verify the equivalence of the presented inverse kinematics solutions and to simulate their performance for the selected test cases. The proposed solutions have then been experimentally tested using a 3D free-flying robot previously tested in an ESA Parabolic Flight Campaign. In the test campaign the robot has been converted in a planar robot taking advantage of its modular structure, suspended by means of air-bearings on a granite plane, and fixed on ground by means of a custom design dynamometer in order to measure the reaction forces and torques. In this way it is possible to perform simulated microgravity tests without time constraints. The experimental validation of the presented inverse kinematics solutions has been carried out, and the experimental results confirmed the good performance of the proposed methods. In particular, two test cases have been analyzed in order to validate and evaluate the performances of both the unconstrained solution and the solution which takes into account the robot physical limits
Dynamic Coordination Principles for Multiple Spacecraft Mounted Manipulators
No full textThe motion of spacecraft mounted manipulators generates an interaction with the spacecraft dynamics during manoeuvres, while the attitude parameters are actively controlled in order to maintain communication links of the antennas, and the orientation of solar panels and pointing devices. When more manipulators are supported by the same spacecraft, the composition of the base reaction forces and torques generated by each robotic arm can represent an advantage in terms of the resultant dynamic effect. This paper presents original coordination principles that can be used in order to achieve a mutual compensation and minimization of the reaction loads transferred to the base spacecraft. The reaction-tracking concept for a single manipulator is here proposed, aimed at driving the robotic joints in order to realize a desired base reaction profile, and the associated acceleration-level kinematic control is derived. Subsequently, the coordination problem of two separate manipulators is considered, according to different levels of coordination. The first analyzed configuration consists of an operative manipulator, with a second non-operative manipulator in charge of the compensation of the base reactions. Afterwards, the configuration with two manipulators in operative mode is considered, in which the coordination is undertaken thanks to the availability of a kinematic redundancy. Kinematic control schemes are derived for the different operating conditions in terms of pseudoinverse formulations, and the dynamic problems are set in local optimization forms, leading to the definition of constrained least squares problems. Closed form solutions are given at the joint acceleration level, and simulations are provided for a system of two planar manipulators fixed to the same supporting base, in order to validate the proposed concepts and give an insight on their potentials and limits. Experimental tests have been carried out on a three degrees-of-freedom planar robot prototype, suspended by air bearings over a flat granite plane in order to simulate the microgravity environment, and a custom designed load cell provided dynamic measurements of the base reactions
Control of dynamic attitude disturbances on spacecrafts equipped with robotic systems for orbital maintenance
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Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
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