Role of softness in object grasping

Relationship between surface softness and friction

In this study, we experimentally investigated the effect of robot fingertip stiffness on friction during grasping of an object. To make robots more human-friendly, robotic hands with soft surfaces have been developed. A soft fingertip, i.e., one with low stiffness, is considered desirable because it produces high friction. However, in our experiments, we were able to obtain high friction from a stiff fingertip under a certain condition. We initially investigated the maximum resistible force when solid objects with different angled surfaces were grasped by spherical fingertips of different stiffness. When the contact surface was flat, a stiffer fingertip produced larger frictional force. When the contact surface was highly convex, the maximum frictional force increased with decreasing fingertip stiffness. Secondly, we examined the relationships among the contact area, the load, and the maximum frictional force. We reformulated the relationship between the load and the maximum frictional force and, together with our experimental results, used it to determine the factor that increased the maximum frictional force.

Softness effects on manipulability and grasping stability

The stability for grasping can be regarded that how much magnitude of external wrench we can balance. We formulate manipulability and the set of generable object wrenches for grasping system, taking deformation of the fingertips into consideration, and show that the increase of the softness decreases the manipulability while it increases generable object wrench.

Related works

  • Yoshinori Fujihira and Tetsuyou Watanabe, Experimental Investigation of Effect of Fingertip Stiffness on Resistible Force in Grasping, Proceedings of the IEEE International Conference on Robotics and Automation (ICRA), pp.4334-4340, 2015.[PDF File]
  • Tetsuyou Watanabe, Yoshinori Fujihira, Experimental investigation of effect of fingertip stiffness on friction while grasping an object, Proceedings of the IEEE International Conference on Robotics and Automation (ICRA), 2014.[PDF File]
  • Tetsuyou Watanabe, Softness Effects on Manipulability and Grasp Stability, Proceedings of the IEEE/RSJ International Conference on Intelligent Robots and Systems (IROS), pp.1398-1404, 2011.