Chapter 2. Prehension

Chapter 4 - Planning of Prehension 83
this is the relevant frame of reference.
In summary, the CNS seems to store knowledge about how ob-
jects behave. This knowledge is accessible to the task plan, and it can
only be assumed that it is part of the plan. Planning the precise
location for finger placement is not needed, and humans can anticipate
object behavior during the interaction between hand and object.
4.5 Selecting a Grasp Strategy
A prehensile posture must be selected that satisfies the object and
task requirements. This choice is made based on the human hand ca-
pabilities described in Chapter 2, taking into account the precision and
power capabilities and specialized features of the hand. The goal is to
find a posture that can gather the sensory information, effect a stable
grasp and produce motions as needed for the object and given task.
The grasp strategy, then, refers to selecting a grasp posture (Le., se-
lecting oppositions, virtual finger mappings, and hand opening size)
and a grasp position and orientation on the object.
4.5.1 Selecting oppositions
One way to make explicit the relationship between object and task
requirements and the oppositions needed to satisfy them is to develop
a set of mapping rules. This can be done with an Gxuert svste m which
is a computer program that mimics human intelligence by focusing on
domain knowledge. In an expert system, knowledge and control
structures are separated, unlike more standard programming styles, so
that the amassed knowledge of an expert is contained in an easy to
analyze, modifiable module. Cutkosky (Cutkosky, 1989; Cutkosky
and Howe, 1990) wrote an expert system ‘GRASP-Exp’ for selecting
a grasp posture using observations of one-handed operations by ma-
chininists working with metal parts and hand tools. Using the pre-
hensile classification in Figure 2.3, task requirements (forces and
motions that must be imparted) and object properties (shape, size,
surface geometry) were placed on a continuum and could be used to
select a grasp. A set of grasp attributes (sensitivity, precision,
dexterity, stability, and security) described the conditions under which
each posture can be used. A posture is chosen by GRASP-Exp using
a hierarchical analysis of the task, gathering information about the task
requirements (e.g., dexterity requirements, clamping requirements)
and then the object properties (e.g., thickness, size, shape). To gather
this information, GRASP-Exp asks the user questions. In order for