Chapter 2. Prehension

Chapter 6 - During Contact 275
a grain of sand). For example, there is uncertainty in using pad
opposition to grasp a pencil resting on a table; i.e., what will happen if
one finger contacts the pencil before the other? Instead of trying to
make contact exactly, it is possible to use controlled slip. The pencil is
grasped between two fingerpads, applying a force in a direction
nonorthogonal to the long axis of the pencil. One finger applies a
torque, rotating the pencil into stable grasp. Absolute precision is not
needed, since a funnel will eliminate position uncertainty despite
variations in the initial location, shape and orientation of the pencil. A
funnel uses constraints (usually kinematic ones) to limit the possible
end configurations of objects. Using an arm to squeeze a pile of parts
into the center of a table gets all the parts into the center, independent
of friction; dropping a pen onto a table gets the pen flat on the table.
Figure 6.23 Inset shows a geometric interpretation of the cone of
friction. The task mechanics of grasping a glass with uncertainty
as VF2 makes contact with glass before VF1. The thin arrowed
line is the line of pushing, drawn parallel to the motion of the
fingers, through the contact point. (a) When it is on the near side
of the center of mass, the glass will rotate out of grasp. (b) When
it is through the center of mass, the glass will translate towards
VF1. (c) When it is to the far side of the center of mass, the glass
will rotate into VF1. Example based on Mason (1985).