  Ensuring stable object placement is crucial to prevent objects from toppling
over, breaking, or causing spills. When an object makes initial contact to a
surface, and some force is exerted, the moment of rotation caused by the
instability of the object's placing can cause the object to rotate in a certain
direction (henceforth referred to as direction of corrective rotation).
Existing methods often employ a Force/Torque (F/T) sensor to estimate the
direction of corrective rotation by detecting the moment of rotation as a
torque. However, its effectiveness may be hampered by sensor noise and the
tension of the external wiring of robot cables. To address these issues, we
propose a method for stable object placing using GelSights, vision-based
tactile sensors, as an alternative to F/T sensors. Our method estimates the
direction of corrective rotation of objects using the displacement of the black
dot pattern on the elastomeric surface of GelSight. We calculate the Curl from
vector analysis, indicative of the rotational field magnitude and direction of
the displacement of the black dots pattern. Simultaneously, we calculate the
difference (Diff) of displacement between the left and right fingers'
GelSight's black dots. Then, the robot can manipulate the objects' pose using
Curl and Diff features, facilitating stable placing. Across experiments,
handling 18 differently characterized objects, our method achieves precise
placing accuracy (less than 1-degree error) in nearly 100% of cases. An
accompanying video is available at the following link:
https://youtu.be/fQbmCksVHlU
