On the synthesis of feasible and prehensile robotic grasps

Trabajo presentado al ICRA celebrado en Minnesota del 14 al 18 de mayo de 2012.This work proposes a solution to the grasp synthesis problem, which consist of finding the best hand configuration to grasp a given object for a specific manipulation task while satisfying all the necessary constraints. This problem had been divided into sequential sub-problems, including contact region determination, hand inverse kinematics and force distribution, with the particular constraints of each step tackled independently. This may lead to unnecessary effort, such as when one of the problems has no solution given the output of the previous step as input. To overcome this issue, we present a kinestatic formulation of the grasp synthesis problem that introduces compliance both at the joints and the contacts. This provides a proper framework to synthesize a feasible and prehensile grasp by considering simultaneously the necessary grasping constraints, including contact reachability, object restraint, and force controllability. As a consequence, a solution of the proposed model results in a set of hand configurations that allows to execute the grasp using only a position controller. The approach is illustrated with experiments on a simple planar hand using two fingers and an anthropomorphic robotic hand using three fingers.This work was partially supported by the CICYT projects DPI2010-18449, DPI2010-15446 and DPI2011-22471, and by the European Commission under CP grants no. 248587, “THE Hand Embodied”, and no. 270350, “ROBLOG”, within the FP7-ICT-2009-4-2-1 program “Cognitive Systems and Robotics”.Peer Reviewe

Path planning for active tensegrity structures

This paper presents a path planning method for actuated tensegrity structures with quasi-static motion. The valid configurations for such structures lay on an equilibrium manifold, which is implicitly defined by a set of kinematic and static constraints. The exploration of this manifold is difficult with standard methods due to the lack of a global parameterization. Thus, this paper proposes the use of techniques with roots in differential geometry to define an atlas, i.e., a set of coordinated local parameterizations of the equilibrium manifold. This atlas is exploited to define a rapidly-exploring random tree, which efficiently finds valid paths between configurations. However, these paths are typically long and jerky and, therefore, this paper also introduces a procedure to reduce their control effort. A variety of test cases are presented to empirically evaluate the proposed method. (C) 2015 Elsevier Ltd. All rights reserved.Peer ReviewedPostprint (author's final draft

Output-only modal identification of tensegrity structures

Tensegrity systems are a special class of spatial reticulated structures that are composed of struts in compression and cables in tension. In this paper, the performance of stochastic subspace algorithms for modal identification of complex tensegrity systems is investigated. A sub-class algorithm of the Stochastic Subspace Identification family: the Balanced Realization Algorithm is investigated for modal identification of a tripod simplex structure and a Geiger dome. The presented algorithm is combined with a stabilization diagram with combined criteria (frequency, damping and mode shapes). It is shown that although the studied structures present closely spaced modes, the Balanced Realization Algorithm performs well and guarantees separation between closely-spaced natural frequencies. Modal identification results are validated through comparisons of the correlations (empirical vs. model based) showing effectiveness of the proposed methodology

Soluciones estructurales con mallas de aspensión : pequeño pabellón desmontable

El propósito de este documento es realizar un estudio comparativo entre una estructura autotensada y una estructura de barras trianguladas con condiciones de partida similares, a fin de extraer conclusiones sobre las ventajas en la aplicación de las estructuras tensegrity para proyectos como los que se estudian. En este trabajo se introduce el concepto básico de Tensegrity, su definición, clasificación y las realizaciones e investigaciones realizadas anteriormente. Más adelante se explica y estudia la tipología de estructura de aspensión, analizando mediante fichas proyectos realizados a fin de conocer la escala de la aplicación de estas estructuras en el marco arquitectónico. Como metodología de estudio se propone la definición de una serie de estructuras de pequeñas luces que serán analizadas con programas de cálculo de elementos finitos a fin de extraer datos sobre el diseño y funcionamiento de estas. Se documentará el proceso de generación de los modelos así como las conclusiones del análisis de cada uno de ellos para al final realizar la comparación de uno de estos tres pabellones con una estructura de barras trianguladas y observar las diferencias entre ellos. Como ampliación del trabajo se propondrá la investigación de soluciones constructivas concretas para el modelo de pabellón obtenido y para el apoyo con el terreno.Universidad de Sevilla. Grado en Fundamentos de Arquitectur

A method to generate stable, collision free configurations for tensegrity based robots

Tensegrity structures appeared in the science community about half a century ago, but they have already been applied to several heterogeneous research fields, such as architecture, civil engineering, space and even biology. Such structures keep a stable volume in space due to an intricate balance of forces between a disjoint set of rigid elements (bars) and a continuous set of tensile elements (cables). The use of tensegrity structures in robotics is still new and there exist only a handful of works about this subject. Some of their main features such as light weight, flexibility, energetic efficiency and redundancy, make them interesting candidates for both mobile robots and manipulators. In this paper, a new method to detect and avoid both internal collisions between the structure members and external collisions with the environment is presented. In this way, we are providing a fundamental tool to develop more complete form-finding procedures and path-planning strategies for tensegrity structures.Peer Reviewe

Pretensado de columnas Tensegrity para el incremento de rigidez axil

El propósito de este documento es estudiar la aplicación de estructuras Tensegrity en el marco arquitectónico. Este trabajo estudia el concepto básico Tensegrity, su definición, su clasificación de acuerdo a anteriores investigadores (Motró, Skelton, Pugh…), su comportamiento no-lineal y la propia aportación centrada en nuevos elementos arquitectónicos basados en este principio. A través de la geometría y programas informáticos, se plantea una nueva tipología y el desarrollo de un método constructivo, teniendo en cuenta, algunos aspectos tan importantes como el sistema de pretensado para encontrar el equilibrio. El principal objetivo es contribuir de una manera particular a la aplicación de sistemas Tensegrity en espacios arquitectónicos, en este caso con la modelización columnas trabajando a bajo carga axil, analizando la forma y escala de la unidad geométrica básica y planteando la superposición de módulos donde los elementos a compresión son discontinuos y los cables a tracción continuos. A lo largo del estudio analizaremos diferentes modelos de mástiles sometidos a compresión, con la ayuda del software WinTess (desarrollado por Ramón Sastre), donde se verifica que la aportación de pretensado incrementa la rigidez axil de la columna y se garantiza su equilibrio estructural

Grasp plannind under task-specific contact constraints

Several aspects have to be addressed before realizing the dream of a robotic hand-arm system with human-like capabilities, ranging from the consolidation of a proper mechatronic design, to the development of precise, lightweight sensors and actuators, to the efficient planning and control of the articular forces and motions required for interaction with the environment. This thesis provides solution algorithms for a main problem within the latter aspect, known as the {\em grasp planning} problem: Given a robotic system formed by a multifinger hand attached to an arm, and an object to be grasped, both with a known geometry and location in 3-space, determine how the hand-arm system should be moved without colliding with itself or with the environment, in order to firmly grasp the object in a suitable way. Central to our algorithms is the explicit consideration of a given set of hand-object contact constraints to be satisfied in the final grasp configuration, imposed by the particular manipulation task to be performed with the object. This is a distinguishing feature from other grasp planning algorithms given in the literature, where a means of ensuring precise hand-object contact locations in the resulting grasp is usually not provided. These conventional algorithms are fast, and nicely suited for planning grasps for pick-an-place operations with the object, but not for planning grasps required for a specific manipulation of the object, like those necessary for holding a pen, a pair of scissors, or a jeweler's screwdriver, for instance, when writing, cutting a paper, or turning a screw, respectively. To be able to generate such highly-selective grasps, we assume that a number of surface regions on the hand are to be placed in contact with a number of corresponding regions on the object, and enforce the fulfilment of such constraints on the obtained solutions from the very beginning, in addition to the usual constraints of grasp restrainability, manipulability and collision avoidance. The proposed algorithms can be applied to robotic hands of arbitrary structure, possibly considering compliance in the joints and the contacts if desired, and they can accommodate general patch-patch contact constraints, instead of more restrictive contact types occasionally considered in the literature. It is worth noting, also, that while common force-closure or manipulability indices are used to asses the quality of grasps, no particular assumption is made on the mathematical properties of the quality index to be used, so that any quality criterion can be accommodated in principle. The algorithms have been tested and validated on numerous situations involving real mechanical hands and typical objects, and find applications in classical or emerging contexts like service robotics, telemedicine, space exploration, prosthetics, manipulation in hazardous environments, or human-robot interaction in general