Simulations of manipulation tasks performed by an anthropomorphic robotic hand equipped with soft fingertips
As dynamic simulation systems become more accurate at modeling the real world, the number of possible applications for such systems increases. One example is the dynamic simulation of grasping and manipulation tasks, enabling interaction with virtual objects. However, accurate grasping simulation requires modeling subtle physical phenomena not taken into account by traditional multi-rigid-body dynamic systems. Here we present examples of simulated manipulation tasks using an analytical model for soft fingertip contacts. This allows fingertips to apply frictional torque, a key factor in the ability to create stable grasps using only two fingers. Our approach also captures subtle phenomena, such as the effect of local object geometry on the space of frictional forces that can be applied at the contact, as well the coupling between tangential force and frictional torque.

Simulation is performed at interactive rates, allowing the user to send force control commands to the motors controlling the hand and also change camera position during the simulation. These movies run at approximately twice the speed of the actual simulation.
Stable pinch grasp example
In this example, the hand successfully picks up a rectangular object and holds it in a vertical position. This is possible because the frictional forces applied at the contacts are able to counter the effects of gravity. Since the grasped object is locally planar, the fingertips create large contact areas which helps increase the magnitude of frictional torque supported at the contacts.

Complete movie ( 1.5 Mb mpeg4 avi )
Unstable pinch grasp example
The cylindrical object used in this case has similar material properties (friction coefficient, mass, inertia tensor) with the block used in the previous example, and the fingertips are applying identical normal forces. However, local curvature at the contact locations leads to smaller contact areas with the fingers, which in turn limits the magnitude of frictional torque. The fingers are unable to counter the effect of gravity, and the cylinder rotates around the axis of the contacts, eventually falling into the robotic palm.

Complete movie ( 1.8 Mb mpeg4 avi )
Complex manipulation example
The goal of the simulated task is to obtain a stable grasp of the glass-shaped object by using finger contacts around its circumference as well as contacts on the palm. However, in the initial position (top left image) the surface of the table prevents the execution of such a grasp. We have simulated a control algorithm that uses two fingers to pick up the object and rotate it above the palm (top row). This is possible only in the presence of frictional torque applied by soft finger contacts. With the palm facing up (top right image), the force applied by the fingers is decreased allowing the glass to rotate while maintaining contact with the fingertips until it hits the rigid palm of the robot (bottom left image). The ring and little finger can now be closed around the glass, creating a stable grasp (bottom right image).

Complete movie ( 5.4 Mb mpeg4 avi)
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