Chapter 2. Prehension

Chapter 8 - Constraints on Human Prehension 319
asked to pick up the dowel and either to place it on a marker or to
shake it. It was observed that in the less forceful, more precise
placing task, the subjects used precision grasps. In the more forceful,
less precise shaking task, the subjects initially used a precision grasp
to lift the cylinder, and then adjusted their posture into a more
powerful posture once the cylinder was lifted. The posture chosen
appeared to match the forces currently acting in the task. During the
lifting phase in both tasks, only the light weight of the cylinder was in
effect, and therefore the subject used a precision grasp. The subjects
did not use the more powerful posture before it was necessary.
In summary, people grasp objects with specific objectives. Once
the objective is established, such as to achieve some functional goal, to
convey some information, or to satisfy some internal motivations, the
movement can be carried out within socio-cultural guidelines. While
goals in the strictest sense are not constraints (people do drop objects),
their effect on human prehensile behavior can be seen in the selection
of movement parameters and the choice of prehensile postures.
Because multiple grasping solutions are possible, the controller is
faced with the question of how best to use hand features, in relation to
the anticipated object properties and predicted interaction outcome, in
order to achieve the goals of the task.
8.4. Mapping Opposition Space into Human
and Robot Hands
An opposition space describes a hand posture as a collection of
virtual fingers able to apply functionally effective forces against an
object for a task. Real fingers group together into a virtual finger to
apply an opposing force against other VFs or against task torques.
Selecting an opposition space identifies a goal posture that closely
resembles the posture taken on in contacting the object (Marteniuk,
Leavitt, MacKenzie, & Athenes, 1990). The ‘preshape posture’
consists of combinations of oppositions and virtual-to-real-finger
mappings that are to be used in the task. As detailed earlier in Chapter
2, each VF has state variable that describes it:
a) VF length (from the center of the grasping surface patch to the
joint where it connects to the palm)
b) VF orientation relative to the palm
c) VF width (number of real fingers mapped into the VF)
d) orientation of the grasping surface patch (the orientation of the
applied force)