Chapter 2. Prehension

Chapter 6 - During Contact 235
Figure 6.10 Hand about to grasp object. Two virtual fingers each
apply a force in opposition to each other in order to hold the
object. In order to lift the object, they must counteract the weight
of the object. The object rests on a table, supporting the weight
of the object.
and the conditions for a stable grasp. In this section, these topics are
explained.
6.3.1 Force transmittal in robotics
When two surfaces come into contact, they may move in order to
maintain or break the contact. If there is sufficient friction between the
two contact areas, then motion in directions resisted by the friction
forces will not be possible. Formally, a contact is a collection of
adjacent points where touching occurs over a contiguous area
(Salisbury, 1985). A contact point is characterized by having a
position (coordinates of the point), orientation (direction of the surface
normal), and curvature (maximum and minimum radii of curvature).
A contact normal is an outward-pointing surface normal tangent to the
plane at the point of contact. Salisbury (1985) noted that different
types of contacts are possible. Point contacts occur when the radii of
curvature are very small, such as when two real fingers are used to
grasp a basketball. Assuming that contact is maintained and assuming
there is friction between the fingerpad and basketball, then each
contact can be modelled as three different twists to specifv the relative
motion of that fingerpad against the basketball. A twist is a six vector