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Mechanical Sciences An open-access journal for theoretical and applied mechanics
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Volume 2, issue 1
Mech. Sci., 2, 1–8, 2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

Special issue: Underactuated Grasping

Mech. Sci., 2, 1–8, 2011
© Author(s) 2011. This work is distributed under
the Creative Commons Attribution 3.0 License.

  08 Feb 2011

08 Feb 2011

A simple design rule for 1st order form-closure of underactuated hands

S. Krut1 and V. Bégoc1,* S. Krut and V. Bégoc
  • 1LIRMM (CNRS & Univ. Montpellier 2), Montpellier, France
  • *now at: B+ Equipment, Gemenos, France

Abstract. The property of form-closure of a grasp, as generally defined in the literature, is based on the assumption that contact points between the hand and the object are fixed in space. However, this assumption is false when considering a grasp exerted by an underactuated hand, since in this case, it is not possible to control the position of each phalanx independently. In spite of researchers' interest in studying form-closure, none of the available published work on this subject takes into consideration the particular kinematics of underactuated hands. Actually, there are few available tools to qualify or quantify the stability of a grasp exerted by an underactuated hand, thus the design of underactuated hands mostly results from an intuitive approach. This paper aims to reduce this gap.

A classification of underactuated hands is proposed, based on the expression of contact forces. This highlights the influence of non-backdrivable mechanisms introduced in the transmission of the closing motion of the hand on the stability of the grasp. The way to extend the original definition of form-closure to underactuated grasps is illustrated. A more general definition is formulated, which checks the stability of the set "object + hand". Using this new definition, a simple rule is proposed for designing a hand capable of achieving 1st order form-closed grasps.

This paper was presented at the IFToMM/ASME International Workshop on Underactuated Grasping (UG2010), 19 August 2010, Montréal, Canada.

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