Chapter 2. Prehension

Chapter 6 - During Contact 265
Internal forces: what kinds of internal grasp forces can the hand
apply to the object?
Manipulability: can the fingers impart arbitrary motions to the
object?
Resistance to slipping: how large can the forces and moments on
the object be before the fingers will start to slip?
Stability: will the grasp return to its initial configuration after
being disturbed by an external force or moment?
These quality measures can be applied to the human hand
(Cutkosky & Howe, 1990). All grasps except the gravity dependent
ones satisfy force closure, in that they can control internal forces and
apply positive and negative forces to counteract external forces. The
non-clamping ones satisfy force closure as long as external forces do
not cause the fingers to detach from the object (e.g:, a tray does not
start rising against gravity). Many grasps do not samfy form closure
without friction, meaning that they do not form a complete kinematic
constraint around the object. Instead, the direction of task forces are
anticipated and postures are chosen to counteract only those.
Examples of force closure without form closure include pulling on a
large wrench to loosen a bolt with a hook grasp or holding a
screwdriver in pad opposition while pushing down on it.
Assumptions are made that the wrench will not pull away from the
hand and that the screw will never pull on the screwdriver.
Table 6.5 summarizes Cutkosky and Howe’s analysis of pad
versus palm opposition. Pad opposition has a high manipulability
measure, isotropy, high compliance, low stability, low resistance to
slipping, satisfies force closure, and a connectivity of at least three
degrees of freedom. In English, this means that the fingers can impart
arbitrary motions to the object and accurately apply the forces and
moments, along at least three degrees of freedom. No expected
external force and moment will move the object out of the posture, but
large external forces will cause the object to slip, and if disturbed, the
posture will not return to its initial configuration. With high
compliance (low stiffness), fingers can move rapidly and still
minimize any potential danger to the fingers or object; colliding with
surfaces will cause deflection, instead of damage. A compliant grasp
is more sensitive to small changes in force. In contrast, palm
opposition satisfies form and force closure, is less compliant, more
stable and has a larger resistance to slipping than precision grasps. As
well, palm opposition has a connectivity of zero and a low
manipulability measure. In English, this means that the object is fixed