Chapter 2. Prehension

106 THE PHASES OF PREHENSION
planning process. For example, the thumb and index finger do not
have to grasp the object at precisely the same time. One can anticipate
how to ‘push’ the object into the grasp. If the goal is to impart motion
to the object, anticipation of those translations and rotations is part of
the planning process. Planning a posture includes the anticipated
motions in multi-operation tasks.
ch
Opposable surfaces
radius of cwahue
osition Vector
Antiapated Forces
Functional Degrees of Freedom
directionof movement
range of movement
repolution of movement
Figure 4.18. Approach vector for the hand, relative to the
opposition vector seen in the object.
A grasp strategy must be chosen in accordance with the task, so
that functionally effective forces of a given direction and magnitude
may be applied. In order to do this, one first perceives the opposition
vector. The term ‘grasp strategy’ refers to selecting appropriate
opposition types, mapping virtual fingers into real anatomical fingers,
and determining opposition space parameters. Mappings between
object and task characteristics and hand postures have been modelled
using expert systems and using neural networks. Expert systems
make explicit the rules of these mappings; neural networks can learn
the rules. Both depend on experimental data that demonstrates how
virtual finger mappings depend on object properties.
Besides choosing a grasp strategy, a location and orientation in
space for the hand must be planned, given the opposition vector and