Adaptive and Resilient Soft Tensegrity Robots

International audienceLiving organisms intertwine soft (e.g., muscle) and hard (e.g., bones) materials, giving them an intrinsic flexibility and resiliency often lacking in conventional rigid robots. The emerging field of soft robotics seeks to harness these same properties to create resilient machines. The nature of soft materials, however, presents considerable challenges to aspects of design, construction, and control—and up until now, the vast majority of gaits for soft robots have been hand-designed through empirical trial-and-error. This article describes an easy-to-assemble tensegrity-based soft robot capable of highly dynamic locomotive gaits and demonstrating structural and behavioral resilience in the face of physical damage. Enabling this is the use of a machine learning algorithm able to discover effective gaits with a minimal number of physical trials. These results lend further credence to soft-robotic approaches that seek to harness the interaction of complex material dynamics to generate a wealth of dynamical behaviors

Exploring the Behavior Repertoire of a Wireless Vibrationally Actuated Tensegrity Robot

Soft robotics is an emerging field of research due to its potential to explore and operate in unstructured, rugged, and dynamic environments. However, the properties that make soft robots compelling also make them difficult to robustly control. Here at Union, we developed the world’s first wireless soft tensegrity robot. The goal of my thesis is to explore effective and efficient methods to explore the diverse behavior our tensegrity robot. We will achieve that by applying state-of-art machine learning technique and a novelty search algorithm

Modelling, design and control of a bird neck using tensegrity mechanisms

International audienceIn birds, the neck exhibits remarkable performances and serves as a dextrous arm for performing various tasks. Accordingly, it is an interesting bioinspiration for designing new manipulators with enhanced performances. This paper proposes a preliminary bird neck model using several stacked tensegrity crossed bar mechanisms. It addresses several issues regarding kinetostatic and dynamic modelling, design and control

Bayesian Optimization with Automatic Prior Selection for Data-Efficient Direct Policy Search

One of the most interesting features of Bayesian optimization for direct policy search is that it can leverage priors (e.g., from simulation or from previous tasks) to accelerate learning on a robot. In this paper, we are interested in situations for which several priors exist but we do not know in advance which one fits best the current situation. We tackle this problem by introducing a novel acquisition function, called Most Likely Expected Improvement (MLEI), that combines the likelihood of the priors and the expected improvement. We evaluate this new acquisition function on a transfer learning task for a 5-DOF planar arm and on a possibly damaged, 6-legged robot that has to learn to walk on flat ground and on stairs, with priors corresponding to different stairs and different kinds of damages. Our results show that MLEI effectively identifies and exploits the priors, even when there is no obvious match between the current situations and the priors.Comment: Accepted at ICRA 2018; 8 pages, 4 figures, 1 algorithm; Video at https://youtu.be/xo8mUIZTvNE ; Spotlight ICRA presentation https://youtu.be/iiVaV-U6Kq

Prototype of a tensegrity manipulator to mimic bird necks

International audienceThis paper deals with the building of a 2D tensegrity mechanism. The considered mechanism is derived from the Snelson's X-shape mechanism and is used as an elementary part of the bird neck modelling. Indeed, an n-dof manipulator can be obtained by stacking in series n X-shape mechanisms. This paper explains the design and building process of a 1-dof prototype, both on hardware and software aspects, and will be used further to have experimental results on the dynamic modelling, control laws and ac-tuation strategy.Une structure de tenségrité est un assemblage d'éléments en compression (barres) et d'éléments en traction (câbles, ressorts) maintenus ensemble en équilibre [1],[2]. La tenségrité est connue en architecture et en art depuis plus d'un siècle [3] et est adaptée à la modélisation des organismes vivants [4]. Les mé-canismes de tenségrité ont été étudiés plus récemment pour leurs propriétés prometteuses en robotique telles que la faible inertie, la souplesse naturelle et la capacité de déploiement [5]. Un mécanisme de tenségrité est obtenu lorsqu'un ou plusieurs éléments sont actionnés. Ces travaux s'inscrivent dans le cadre du projet AVINECK, auquel participent des biologistes et des roboticiens dans le but principal de modéliser et de concevoir des cous d'oiseaux. En conséquence, une classe de manipulateurs de tenségrité planaire composée d'un assemblage en série de plusieurs mécanismes en X de Snelson [6], c'est-à-dire des mécanismes à quatre barres croisées avec des ressorts sur leurs côtés latéraux, a été choisie comme candidat approprié pour un modèle préliminaire plan d'un cou d'oiseau. Le prototype consiste en un mecanisme en X de Snelson. Les barres sont assemblées selon différents plans pour éviter les collisions internes. Le manipulateur est entraîné par des câbles parallèles aux res-sorts et traversant les axes grâce à des perçages. Chaque câble est attaché à un tambour. Le manipulateur est actionné par deux câbles, ce qui en fait un mécanisme antagoniste, dont on peut contrôler la raideur. Les pièces structurelles (barres, supports, tambours) sont imprimées en 3D en ABS. Chaque liaison pivot entre les barres et les axes est construite avec deux roulements qui assurent un centrage long, et toutes les pièces sont arrêtées axialement avec des colliers d'arbre. Nous avons décidé d'avoir une lon-gueur de barre transversale de 100 mm et une longueur de barre supérieure de 50 mm. Ces dimensions sont adaptées à plusieurs jeux de ressorts disponibles, c'est-à-dire que les ressorts considérés sont tou-jours en tension et ne sont pas trop étendus pour toutes les positions accessibles du manipulateur. Une fois la longueur et la raideur du ressort définies, le modèle statique est calculé afin d'obtenir la force d'entrée maximale pour les câbles. Cette force doit être suffisante pour actionner le mécanisme dans un grand espace de travail et pour résister aux chargements externes. La force appliquée par les câbles est directement liée au rayon du tambour et au couple du moteur. Le rayon du tambour influe également sur la vitesse de translation du câble. Un compromis est fait pour avoir des efforts et vitesses de câbles suffisants. Deux variateurs interagissent avec un microprocesseur sur lequel est programmé la loi de commande. Chaque moteur est équipé d'un codeur pour connaître la position réelle du mécanisme. Le bon compor-tement du mécanisme est assuré par une commande dynamique

Robustness for Free: Quality-Diversity Driven Discovery of Agile Soft Robotic Gaits

Soft robotics aims to develop robots able to adapt their behavior across a wide range of unstructured and unknown environments. A critical challenge of soft robotic control is that nonlinear dynamics often result in complex behaviors hard to model and predict. Typically behaviors for mobile soft robots are discovered through empirical trial and error and hand-tuning. More recently, optimization algorithms such as Genetic Algorithms (GA) have been used to discover gaits, but these behaviors are often optimized for a single environment or terrain, and can be brittle to unplanned changes to terrain. In this paper we demonstrate how Quality Diversity Algorithms, which search of a range of high-performing behaviors, can produce repertoires of gaits that are robust to changing terrains. This robustness significantly out-performs that of gaits produced by a single objective optimization algorithm.Comment: 6 pages, submitted to IEEE RoboSof

Theoretical considerations on 3D tensegrity joints for the use in manipulation systems

This paper presents a comprehensive analysis of a three-dimensional compliant tensegrity joint structure, examining its actuation, kinematics, and response to external loads. The study investigates a baseline configuration and two asymmetric variants of the joint. The relationship between the shape parameter and the parameters of the tensioned segments is derived, enabling the mathematical description of cable lengths for joint actuation. Geometric nonlinear static finite element simulations are performed to analyze the joint's response under various load conditions. The results reveal the joint's range of motion, the effect of different stiffness configurations, and its deformation behavior under external forces. The study highlights the asymmetric nature of the joint and its potential for targeted motion restriction. These findings advance the general understanding of the behavior of the considered tensegrity joint and provide valuable insights for their design and application in soft robotic systems

TWrist: An agile compliant 3-DoF tensegrity joint

Tensegrity structures, with their unique physical characteristics, hold substantial potential in the field of robotics. However, the very structures that will give tensegrity robots potential advantages over traditional robots also hold long term challenges. Due to the inherent high redundancy of tensegrity structures and the employment of tension elements, tensegrity robots exhibit excellent stability, compliance, and flexibility, although this also results in lower structural deformation efficiency. Existing research has endeavoured to enhance the motion performance of tensegrity robots, exploring diverse approaches such as actuation schemes, structure design, aligned with control algorithms. However, the physical constraints of the elements in such structures and the absence of suitable controllers impede further advancements in the usefulness of tensegrity robots. This paper presents a novel design based on an under constrained transition region design and a tailored control approach based on inverse kinematics, improving the motion performance of the proposed novel tensegrity joint. Through this approach, the tensegrity joint, while preserving the advantages of compliance and flexibility expected from tensegrity structures, offers three degrees of rotational freedom, mirroring the controllability of conventional rigid-body joints. The results demonstrate the capability of tensegrity- based robotic joints to provide flexible actuation under situations demanding high compliance. The integration of structure design with a tailored control approach offers a pioneering model for future development of tensegrity robots, underscoring the practical viability of tensegrity structures in the realm of robotics

Towards an ontology for soft robots: What is soft?

The advent of soft robotics represents a profound change in the forms robots will take in the future. However, this revolutionary change has already yielded such a diverse collection of robots that attempts at defining this group do not reflect many existing ‘soft’ robots. This paper aims to address this issue by scrutinising a number of descriptions of soft robots arising from a literature review with the intention of determining a coherent meaning for soft. We also present a classification of existing soft robots to initiate the development of a soft robotic ontology. Finally, discrepancies in prescribed ranges of Young’s modulus, a frequently used criterion for the selection of soft materials, are explained and discussed. A detailed visual comparison of these ranges and supporting data is also presented