Investigation of the Coin Snapping Phenomenon in Linearly Compliant Robot Grasps

Compliant grasping systems offer a wide range of robot hand designs. Understanding the stability behavior of compliant grasps can enhance the reliability and security of such hands. A classical result in compliant grasp mechanics states that a stable multifinger grasp can suddenly lose its stability when the finger force magnitudes exceed a critical threshold determined by the grasp's geometry. This event is known as coin snapping. This paper provides a full analysis of the coin snapping phenomenon for planar grasps governed by linear compliance laws. The analysis leads to important insights concerning compliant grasp security. For instance, does a grasping system give warning signs before an object snaps out of the fingers' grip? Is this an inevitable phenomenon in linearly compliant grasps? By systematically studying the bifurcation patterns of compliant multifinger grasps, this paper provides analytic characterization of the stability behavior of these systems, as well as answers to the mentioned questions under certain simplifying assumptions. Graphical examples and experimental measurements illustrate and validate the results.