Slip detection for general objects
The motion of the markers on the GelSight surface help to detect slip states, which can hardly be inferred from contact geometry. We try to detect slip or incepient slip states when grasping general objects. Here we roughly devide the objects in 2 categories: objects with flat and smooth sufaces, and objects with textures or curved surfaces.
For objects with flat and smooth sufaces, it is hard for GelSight to directly trac the translational motion of the object, because the geometry features within the contact area are not significant. However, we found that, within the contact area, the motion of the markers makes some special patterns in the slip or incipient slip states. That is, when the shear force is large, and slip is going to occur soon, the motion of the markers are not evenly distributed. Compared to the markers in the central area, the markers in the peripheral contact area tend to have much smaller shear displacement. That is because the slip occurs from the peripheral area of the contact; and before the overall slip occurs, the elastomer on GelSight has already got some relative displacement with the object. Here is an example of the marker motion when a flat GelSight contacts a flat surface:
For objects with flat and smooth sufaces, it is hard for GelSight to directly trac the translational motion of the object, because the geometry features within the contact area are not significant. However, we found that, within the contact area, the motion of the markers makes some special patterns in the slip or incipient slip states. That is, when the shear force is large, and slip is going to occur soon, the motion of the markers are not evenly distributed. Compared to the markers in the central area, the markers in the peripheral contact area tend to have much smaller shear displacement. That is because the slip occurs from the peripheral area of the contact; and before the overall slip occurs, the elastomer on GelSight has already got some relative displacement with the object. Here is an example of the marker motion when a flat GelSight contacts a flat surface:
The inhomogeneity degree of the marker motion distribution can describe the likeliness of slip occurance. To measure the inhomogeneity degree, we use entropy. During the experiment, as the shear force increases and the partial slip grows, The entropy of the marker motion field increases. For grasping tasks, we can set a safe threshold for the entropy value to prevent slip.
The rule works for objects with other shapes as well. The following figures and video shows the slip growing experiment with general objects:
The rule works for objects with other shapes as well. The following figures and video shows the slip growing experiment with general objects:
The rule also applies to rotational load: when the rotational torque is large, rotational slip would occur, and the border area slips first. The inhomogeneity of the rotation angle distribution is a good indication of how rotational slip is likely to occur. The following figures shows the marker displacement fields when the rotational load is small and large:
In the following figure, we draw the heat map of the magnitude distribution: the figures in the left column compares the motion magnitude of the markers, the figures in the middle column compares the rotational angles of the markers around the rotational center, the figures in the right column shows the subtraction result of the rotation angle of the contact surface and the angle of the markers.
For objects with rich textures or small curvetures, we measure slip directly by comparing the motion of the object and the marker. If there is an obvious relative displacement between the object and the marker, which represents the deformation of the sensor surface, slip occurs. The method works for both translational and rotational slip. Examples are shwon in the following figures:
By combining the slip detection in the 2 situations, we could predict slip or incipient slip state of a category of 37 common objects in 84% of the cases.
