How Slippery Is Viscous Friction? [Lecture Notes]

One of the many things that humans and many animals are very good at is picking up objects. Teaching robots to pick up objects is not so easy, however, for at least two reasons. First, the coefficient of friction may be unknown, and therefore, an object may slip through the robot's grasp. Although this uncertainty can be overcome by using a tighter grip, another issue arises, namely, that excessive force may crush the object. Furthermore, the length of the object must also be accounted for since the ultimate goal is to achieve a slip distance that is less than the length of the object. Consequently, reliable gripping is a control problem that requires a careful tradeoff between the possibility of dropping an object and the potential for crushing it.