Model Based Tracking for Slippage Detection

The ability of a robotic system to generate both a feasible and a stable grasp adds to its robustness. By a feasible grasp, a kinematically feasible grasp is considered. By a stable grasp, a grasp for which the object will not twist or slip relative to the end-effector. Here, the latter issue is considered. Aside to pick up the object, a task for the robot may be to also place the object to some desired position in the workspace. If that is the case, after the object is grasped, a task monitoring step may be initiated. The basic idea is that, even if the planed grasp was considered stable, when the manipulator starts to move the object may start to slide. If the grasp is stable, the relative transformation between the manipulator (gripper) and the object frames should be constant. Since tactile sensing is not available to us at this stage, our vision system is used to track the object held by the robot and estimate its pose during the placement task. The estimated pose of the object is then used to estimate the change between the object and hand coordinate frames. For a stable grasp this change should be ideally zero or very small.

In Fig.  and Fig. , the variation in the hand/object transformation is presented. Two cases obtained during a stable grasp and a grasp where the object slid from the hand are shown. Comparing the figures, we see a significant difference in the transformation plots. The most significant change is observed for $dX$ and $dZ$ which is for $dX$ approximately 100mm and for $dZ$ almost 150mm. These changes are caused by the object being removed from the gripper in Fig. , see Fig. .

Figure: Removing the object from the gripper - the object is tracked during the whole sequence. The pose of the object is used to estimate the change in object/gripper transformation. The results are presented in Fig. .

Figure: Change in translation between the gripper and object frames during a stable grasp. The change in $dX,dY$ and $dZ$ is very small and mostly less than 10 mm.

Figure: Change in translation between the gripper and object frames when the object was removed from the gripper. Compared to the plot for a stable grasp (Fig. ), the change is approximately 100mm for $dX$ and 150mm for $dZ$ component.