Controllo robusto di forza per una struttura robotica articolata di tipo Body Extender

La tesi affronta la progettazione e la sintesi di un controllo robusto per un amplificatore di forza ad elevate prestazioni denominato Body Extender. Il Body Extender è un esoscheletro corpo intero a 22 gradi di libertà progettato e costruito dal laboratorio PERCRO della Scuola Superiore Sant'Anna di Pisa. Il sistema è concepito per essere indossato da un operatore umano e muoversi in coerenza con i suoi movimenti, sgravando i pesi e le inerzie della sua struttura e dei carichi trasportati. Il livello di compensazione del carico viene inoltre ridotto a una percentuale arbitraria, in modo da consentire una manipolazione naturale per l'operatore. Per tali motivi nella progettazione del controllo si è dovuto tenere conto della presenza dell'uomo che causa notevoli variazioni dei parametri (massa e rigidezza), della elasticità della trasmissione, e delle differenti situazioni di esercizio della macchina, quali variazioni di carico e interferenza con l'ambiente. Il funzionamento della macchina deve garantire una adattività a tutti i suddetti effetti. Il controllo realizza un comportamento dell'esoscheletro pari ad un amplificatore di forze; nel caso in cui il movimento dell'operatore sia libero risulta invece trasparente per l'utilizzatore. La struttura del controllo presenta due anelli principali: uno più interno che si occupa dell'inseguimento delle traiettorie, implementato con un controllo a dinamica inversa adattivo; mentre l'anello esterno genera il riferimento simulando la pura massa virtuale agli organi di presa. L'anello interno di controllo compensa inoltre l'elasticità concentrata ai giunti annullandone gli effetti dinamici nell'intorno della traiettoria. L'anello esterno utilizza la forza misurata dal sensore per generare un riferimento in accelerazione che viene opportunamente integrato per determinare i riferimenti di velocità e posizione. Un ultimo modulo si occupa della stima delle forze esterne per la corretta amplificazione di forza e per il funzionamento del controllo in posizione. Considerando la natura non stazionaria del modello da identificare, la stima dei carichi viene effettuata con un operatore a media mobile basato su un algoritmo di regressione non lineare. L'analisi della stabilità del sistema è stata effettuata tramite il luogo delle radici, e quindi verificata nei parametri incerti con il software Mathematica. E' stata inoltre programmata un interfaccia in python per il log e la visualizzazione realtime delle variabili di stato del sistama. L'implementazione del controllo progettata in ambiente Simulink di MATLAB usa il real-time Workshop opportunamente configurato, e può essere eseguire gli schemi direttamente nel robot. Sono state effettuate prove sperimentali su un modello di ridotta complessità per verificare le proprietà dinamiche degli algoritmi sviluppati. Sono infine stati effettuati test sulla struttura completa. I risultati delle prove sperimentali hanno evidenziato un accordo con l'analisi teorica realizzata sia in termini di stabilità sia di prestazione

Reduced Model and Application of Inflating Circular Diaphragm Dielectric Elastomer Generators for Wave Energy Harvesting

Dielectric elastomers (DE) are incompressible rubberlike solids whose electrical and structural responses are highly nonlinear and strongly coupled. Thanks to their coupled electromechanical response, intrinsic lightness, easy manufacturability, and low-cost, DEs are perfectly suited for the development of novel solid-state polymeric energy conversion units with capacitive nature and high-voltage operation, which are more resilient, lightweight, integrated, economic, and disposable than traditional generators based on conventional electromagnetic technology. Inflated circular diaphragm dielectric elastomer generators (ICD-DEG) are a special embodiment of polymeric transducer that can be used to convert pneumatic energy into usable electricity. Potential application of ICD-DEG is as power take-off system for wave energy converters (WEC) based on the oscillating water column (OWC) principle. This paper presents a reduced, yet accurate, dynamic model for ICD-DEG that features one kinematic degree of freedom and which accounts for DE visco-elasticity. The model is computationally simple and can be easily integrated into existing wave-to-wire models of OWCs to be used for fast analysis and real-time applications. For demonstration purposes, integration of the considered ICD-DEG model with a lumped-parameter hydrodynamic model of a realistic OWC is also presented along with a simulation case study

Analysis and design of an oscillating water column wave energy converter with dielectric elastomer power take-off

In this paper, we present a concept of near/off-shore Oscillating Water Column (OWC) Wave Energy Converter (WEC) that is equipped with a Power Take Off (PTO) unit based on Dielectric Elastomer Generators (DEGs). DEGs are soft/deformable generators with variable capacitance able to directly convert the mechanical energy that is employed for their deformation into electrostatic energy. The proposed WEC is based on an existing tubular collector chamber of an OWC system designed by the company Sendekia, that is combined with an Inflatable Circular Diaphragm (ICD) DEG. This simplified design presents a very reduced number of moving parts showing potentially high efficiency, reliability and noise-free operation. A multi-physics dynamic model of the system is built using time domain linear hydrodynamics coupled with an analytical non-linear electro-hyperelastic model for the DEG-based PTO. The power matrix of the system is calculated for both regular and irregular waves. Some design issues are introduced showing that the electro-elastic response of the DEG provides the system with an additional stiffness that adds up to the hydrostatic stiffness and affects the resonance of the WEC. As a consequence, the geometric shape/dimensions of the OWC chamber and the layout of the DEG diaphragm should be chosen using an integrated procedure aimed at tuning the overall response of the WEC to the spectra a reference wave climate

CACTO: Continuous Actor-Critic with Trajectory Optimization -- Towards global optimality

This paper presents a novel algorithm for the continuous control of dynamical systems that combines Trajectory Optimization (TO) and Reinforcement Learning (RL) in a single framework. The motivations behind this algorithm are the two main limitations of TO and RL when applied to continuous nonlinear systems to minimize a non-convex cost function. Specifically, TO can get stuck in poor local minima when the search is not initialized close to a "good" minimum. On the other hand, when dealing with continuous state and control spaces, the RL training process may be excessively long and strongly dependent on the exploration strategy. Thus, our algorithm learns a "good" control policy via TO-guided RL policy search that, when used as initial guess provider for TO, makes the trajectory optimization process less prone to converge to poor local optima. Our method is validated on several reaching problems featuring non-convex obstacle avoidance with different dynamical systems, including a car model with 6D state, and a 3-joint planar manipulator. Our results show the great capabilities of CACTO in escaping local minima, while being more computationally efficient than the Deep Deterministic Policy Gradient (DDPG) and Proximal Policy Optimization (PPO) RL algorithms.Comment: 8 pages, 8 figures. Submitted to IEEE RA-

Modelling and testing of a wave energy converter based on dielectric elastomer generators

This paper introduces the analysis and design of a wave energy converter (WEC) that is equipped with a novel kind of electrostatic power take-off system, known as dielectric elastomer generator (DEG). We propose a modelling approach which relies on the combination of nonlinear potential-flow hydrodynamics and electro-hyperelastic theory. Such a model makes it possible to predict the system response in operational conditions, and thus it is employed to design and evaluate a DEG-based WEC that features an effective dynamic response. The model is validated through the design and test of a small-scale prototype, whose dynamics is tuned with waves at tank-scale using a set of scaling rules for the DEG dimensions introduced here in order to comply with Froude similarity laws. Wave-tank tests are conducted in regular and irregular waves with a functional DEG system that is controlled using a realistic prediction-free strategy. Remarkable average performance in realistically scaled sea states has been recorded during experiments, with peaks of power output of up to 3.8 W, corresponding to hundreds of kilowatts at full-scale. The obtained results demonstrated the concrete possibility of designing DEG-based WEC devices that are conceived for large-scale electrical energy production

Resonant wave energy harvester based on dielectric elastomer generator

Dielectric elastomer generators (DEGs) are a class of capacitive solid-state devices that employ highly stretchable dielectrics and conductors to convert mechanical energy into high-voltage direct-current electricity. Their promising performance in terms of convertible energy and power density has been mostly proven in quasi-static experimental tests with prescribed deformation. However, the assessment of their ability in harvesting energy from a dynamic oscillating source of mechanical energy is crucial to demonstrate their effectiveness in practical applications. This paper reports a first demonstration of a DEG system that is able to convert the oscillating energy carried by water waves into electricity. A DEG prototype is built using a commercial polyacrylate film (VHB 4905 by 3M) and an experimental campaign is conducted in a wave-flume facility, i.e. an artificial basin that makes it possible to generate programmed small-scale waves at different frequencies and amplitudes. In resonant conditions, the designed system demonstrates the delivery of a maximum of 0.87 W of electrical power output and 0.64 J energy generated per cycle, with corresponding densities per unit mass of dielectric elastomer of 197 W kg-1 and 145 J kg-1. Additionally, a notable maximum fraction of 18% of the input wave energy is converted into electricity. The presented results provide a promising demonstration of the operation and effectiveness of ocean wave energy converters based on elastic capacitive generators