Fast Grasp Planning Using Cord Geometry

International audienceIn this paper, we propose a novel idea to address theproblem of fast computation of stable force-closure grasp configurationsfor a multifingered hand and a 3-D rigid object representedas a polygonal soup model. The proposed method performsa low-level shape exploration by wrapping multiple cords aroundthe object in order to quickly isolate promising grasping regions.Around these regions, we compute grasp configurations by applyinga variant of the close-until-contact procedure to find thecontact points. The finger kinematics and the contact informationare then used to filter out unstable grasps. Through many simulatedexamples with three different anthropomorphic hands, wedemonstrate that, compared with previous grasp planners such asthe generic grasp planner in Simox, the proposed grasp plannercan synthesize grasps that are more natural-looking for humans(as measured by the grasp quality measure skewness) for objectswith complex geometries in a short amount of time. Unlike manyother planners, this is achieved without costly model preprocessingsuch as segmentation by parts and medial axis extraction

On Neuromechanical Approaches for the Study of Biological Grasp and Manipulation

Biological and robotic grasp and manipulation are undeniably similar at the level of mechanical task performance. However, their underlying fundamental biological vs. engineering mechanisms are, by definition, dramatically different and can even be antithetical. Even our approach to each is diametrically opposite: inductive science for the study of biological systems vs. engineering synthesis for the design and construction of robotic systems. The past 20 years have seen several conceptual advances in both fields and the quest to unify them. Chief among them is the reluctant recognition that their underlying fundamental mechanisms may actually share limited common ground, while exhibiting many fundamental differences. This recognition is particularly liberating because it allows us to resolve and move beyond multiple paradoxes and contradictions that arose from the initial reasonable assumption of a large common ground. Here, we begin by introducing the perspective of neuromechanics, which emphasizes that real-world behavior emerges from the intimate interactions among the physical structure of the system, the mechanical requirements of a task, the feasible neural control actions to produce it, and the ability of the neuromuscular system to adapt through interactions with the environment. This allows us to articulate a succinct overview of a few salient conceptual paradoxes and contradictions regarding under-determined vs. over-determined mechanics, under- vs. over-actuated control, prescribed vs. emergent function, learning vs. implementation vs. adaptation, prescriptive vs. descriptive synergies, and optimal vs. habitual performance. We conclude by presenting open questions and suggesting directions for future research. We hope this frank assessment of the state-of-the-art will encourage and guide these communities to continue to interact and make progress in these important areas

Kinematics and control of precision grip grasping

This thesis is about the kind of signals used in our central nervous system for guiding skilled motor behavior. In the first two projects a currently very influential theory on the flow of visual information inside our brain was tested. According to A. D. Milner and Goodale (1995) there exist two largely independent visual streams. The dorsal stream is supposed to transmit visual information for the guidance of action. The ventral stream is thought generate a conscious percept of the environment. The streams are said to use different parts of the visual information and to differ in temporal characteristics. Namely, the dorsal stream is proposed to have a lower sensitivity for color and a more rapid decay of information than the ventral stream. In the first project the role of chromatic information in action guidance was probed. We let participants grasp colored stimuli which varied in luminance. Criti- cally, some of these stimuli were completely isoluminant with the background. These stimuli thus could only be discriminated from their surrounding by means of chro- matic contrast, a poor input signal for the dorsal stream. Nevertheless, our partici- pants were perfectly able to guide their grip to these targets as well. In the second project the temporal characteristics of the two streams were probed. For a certain group of neurological patients it has been argued that they are able to switch from dorsal to ventral control when visual information is re- moved. These optic ataxic patients are normally quite bad at executing visually guided movements like e.g. pointing or grasping. Different researchers, however, demonstrated that their accuracy does improve when there is a delay between tar- get presentation and movement execution. Using different delay times and pointing movements Himmelbach and Karnath (2005) had shown that this improvement in- creases linearly with longer delay. We aimed at a replication of this result and a generalization to precision grip movements. Our results from two patients, however, did not show any improvement in grasping due to longer delay time. In pointing an effect was found only in one of the patients and only in one of several measures of pointing accuracy. Taken together the results of the first two projects don´t support the idea of two independent visual streams and are more in line with the idea of a single visual representation of target objects. The third project aimed at closing a gap in existing model approaches on pre- cision grip kinematics. The available models need the target points of a movement as an input on which they can operate. From the literature on human and robotic grasping we extracted the most plausible set of rules for grasp point selection. We created objects suitable to put these rules into conflict with each other. Thereby we estimated the individual contribution of each rule. We validated the model by predicting grasp points on a completely novel set of objects. Our straightforward approach showed a very good performance in predicting the preferred contact points of human actors.Diese Dissertation handelt von den Mechanismen mit denen unser Zentralnerven- system menschliche Feinmotorik koordiniert. Gegenstand der ersten beiden Projekte ist die Theorie von A. D. Milner und Goodale (1995). Laut diesen Autoren gibt es im visuellen System zwei unabhängige Verarbeitungspfade. Der dorsale Pfad verarbeitet visuelle Information zum Zweck der Handlungssteuerung. Der ventrale Pfad vermittelt bewusste visuelle Wahrnehmung. Beide Pfade verfügen uber teils unterschiedliche Anteile der gesamten visuellen Information. So soll der dorsale Pfad gegenüber dem ventralen zum Beispiel durch geringere Farbsensitivität sowie einen schnelleren Zerfall der Information gekennzeichnet sein. Im ersten Projekt wurde die Eignung von Farbinformation zur Handlungskontrolle getestet. Teilnehmer der Studie griffen nach farbigen Stimuli deren Helligkeit variiert wurde. Einige der Stimuli hatten die gleiche Helligkeit wie der Hintergrund vor dem sie präsentiert wurden. Diese Stimuli hoben sich also nur durch ihre Farbe vom Hintergrund ab. Trotz der angenommenen Farbinsensitivität des dorsalen Pfades konnten unsere Teilnehmer auch diese Stimuli problemlos greifen. Gegenstand des zweiten Projektes waren die Unterschiede beider Pfade im zeitlichen Verfall der visuellen Information. Einigen Patienten mit speziellen Hirn- schädigungen soll es möglich sein zwischen den Repräsentationen beider Pfade zu wechseln. Diese optischen Ataktiker zeigen starke Unsicherheit bei visuell geführten Bewegungen wie Zeigen oder Greifen. Wiederholt wurde jedoch gezeigt, dass ihre Bewegungen genauer werden wenn die Ausführung einige Zeit nach der Zielpräsentation erfolgt. Himmelbach und Karnath (2005) berichten, dass diese Verbesserung beim Zeigen linear mit der Länge des zwischengeschalteten Intervalles zunimmt. Wir versuchten dieses Ergebnis zu reproduzieren und auf das Greifen zu generalisieren. Die zwei von uns gemessenen Patienten zeigten beim Greifen jedoch keinen Effekt. Beim Zeigen zeigte sich eine Verbesserung nur bei einem Patienten und nur in einem von mehreren Maßen für die Zeigegenauigkeit. Insgesamt betrachtet widersprechen die Ergebnisse des ersten und zweiten Projektes der Vorstellung zweier getrennter visueller Pfade. Die hier präsentierten Daten lassen sich ebenso effektiv, aber deutlich effizienter, durch die Verarbeitung in einem einzelnen visuellen Verarbeitungspfad erklären. Das dritte Projekt soll eine Lücke in bestehenden Modellen zur Beschreibung der Kinematik des Greifens schließen. Alle diese Modelle sind darauf angewiesen, dass ihnen die Zielpunkte der Bewegung vorgegeben werden. Aus der Literatur zu menschlichem und maschinellem Greifen extrahierten wir die plausibelsten Regeln zur Auswahl dieser Zielpunkte. Wir brachten diese Regeln experimentell in Konflikt zueinander und schätzten auf diese Weise ihren relativen Einfluss. Das Modell wurde anschließend validiert indem wir die besten Greifpunkte für einen neuen Satz von Objekten vorhersagten. Mit wenigen Regeln konnten wir so sehr erfolgreich im Vorhinein die vom Menschen präferierten Greifpunkte bestimmen

Neural bases of hand synergies

abstract: The human hand has so many degrees of freedom that it may seem impossible to control. A potential solution to this problem is “synergy control” which combines dimensionality reduction with great flexibility. With applicability to a wide range of tasks, this has become a very popular concept. In this review, we describe the evolution of the modern concept using studies of kinematic and force synergies in human hand control, neurophysiology of cortical and spinal neurons, and electromyographic (EMG) activity of hand muscles. We go beyond the often purely descriptive usage of synergy by reviewing the organization of the underlying neuronal circuitry in order to propose mechanistic explanations for various observed synergy phenomena. Finally, we propose a theoretical framework to reconcile important and still debated concepts such as the definitions of “fixed” vs. “flexible” synergies and mechanisms underlying the combination of synergies for hand control.View the article as published at http://journal.frontiersin.org/article/10.3389/fncom.2013.00023/ful

Incrementality and Hierarchies in the Enrollment of Multiple Synergies for Grasp Planning

Postural hand synergies or eigenpostures are joint angle covariation patterns observed in common grasping tasks. A typical definition associates the geometry of synergy vectors and their hierarchy (relative statistical weight) with the principal component analysis of an experimental covariance matrix. In a reduced complexity representation, the accuracy of hand posture reconstruction is incrementally improved as the number of synergies is increased according to the hierarchy. In this work, we explore whether and how hierarchy and incrementality extend from posture description to grasp force distribution. To do so, we study the problem of optimizing grasps w.r.t. hand/object relative pose and force application, using hand models with an increasing number of synergies, ordered according to a widely used postural basis. The optimization is performed numerically, on a data set of simulated grasps of four objects with a 19-DoF anthropomorphic hand. Results show that the hand/object relative poses that minimize (possibly locally) the grasp optimality index remain roughly the same as more synergies are considered. This suggests that an incremental learning algorithm could be conceived, leveraging on the solution of lower dimensionality problems to progressively address more complex cases as more synergies are added. Second, we investigate whether the adopted hierarchy of postural synergies is indeed the best also for force distribution. Results show that this is not the case