Chapter 2. Prehension

266 THE PHASES OF PREHENSION
Table 6.5 Summary of Cutkosky 8z Howe (1990) description of pad
and palm oppositions in terms of analytic measures from the field
of robotics.
Grasp Value of English
type analytic measure Meaning
Pad opposition High manipulability Fingers can impart arbitrary motions to
High isotropy
High compliance
Connectivity 2 3 dfs
Satisfies force closure
Low stability
Low resistance to
the object
Fingers can accurately apply forces and
moments to object
Minimize danger to fingers and object
by using force control in response to
perturbations
At least 3 dfs exist between object and
hand
Assuming fingers will maintain
contact with object, fingers can resist
arbitrary forces and moments
After disturbance, fingers will not
return to initial position
Fingers cannot resist external forces
slipping witiout slipping
Palm opposition Satisfies form closure No external arbitrary forces and
moments can move object
Satisfies force closure Assuming hand surfaces will maintain
contact with object, fingers can resist
Low compliance
High stability
High resistance to
slipping
Connectivity = 0
Low manipulability
arbitrary forces and moments
Minimize ‘give’ in collisions with
environment, using position control in
response to perturbations
After disturbance, fingers will return to
initial position
Large forces and moments are needed in
order to make object slip
No degrees of freedom between object
and hand
Fingers cannot impart arbitrary
motions to object
in the hand and it cannot be moved. Very large forces are needed to
make the object slip, and if external forces try to move it, the posture
will return to its configuration. With low compliance (high stiffness),
colliding with external surfaces will have little ‘give’, thus providing a
method for transmitting forces from the arm through the object (e.g.,
hitting a nail with a hammer, etc).