Dexterous In-Hand Manipulation

Turning an object over in your fingers without putting it down is called in-hand manipulation. One way robots do it is finger gaiting, like walking with fingers: at each step two fingers hold the object while the third lifts, moves to a new spot, and grips again. Then the object can be rolled a little further.

Rotation per step Finger friction Speed

Finger gaiting

Each cycle has two phases. In the rolling phase, all fingers hold the object and turn it a little. In the regrasp phase, one finger lifts and moves to a new contact while the other two must keep a stable grip alone, which needs enough friction:

$$\text{two-finger hold: } \tan^{-1}\mu \ \ge\ \text{angle between the line joining the fingers and each surface normal}$$

What to notice

Lower the friction and, during the regrasp phase, the two remaining fingers can no longer hold the object, so it slips. Larger rotation steps finish faster but move the contacts further around the object before each regrasp, which makes the two-finger hold harder. Dexterous hands like the Shadow Hand and OpenAI's Dactyl, and newer learned controllers, do far more fluid versions of this, often relying heavily on touch sensing.