Chapter 2. Prehension

284 THE PHASES OF PREHENSION
perceive task-
specific object
select a grasp
into rest position
or open posture
plan a location
and orientation
transport hand
away from object
Orient
Palm
grasping
*capture object
hto stable grasp;
. maintain stable grasp
while lifting, holding,
and
surface patches contour follow
preshape with OPPW- if necessary
band into ition vector
posture . I .' .,*[
enclose
transport and fingers
orient palm
to object
replacing object
Figure 7.1 Phases in Opposition Space Analysis of Prehension.
Prior to grasping, there is a planning phase for choosing an
opposition space, based on motivations and task context. The
choice of an opposition space includes: the opposition vector, the
opposition type(s), virtual fingers, and the parameterization for
these. Setting up an opposition space requires activity of the
proximal and distal muscles for locating and orienting the palm,
and distal muscles for posturing the fingers. After peak aperture,
the enclosing is a guarded motion anticipating contact. This
bridges the free, unrestrained motion with the compliant motion.
Using an opposition space involves the application and modulation
of functionally effective forces to stably grasp and release the
object. With a magnet reaction, the gripping forces decrease and
the hand releases opposition space, by letting go. For setting up
and releasing an opposition space, only the arm and hand are
controlled; in contrast, when using an opposition space, the object
also becomes a part of the controlled system.