Chapter 2. Prehension

84 THE PHASES OF PREHENSION
the machinists to understand these questions, Cutkosky discovered
that they had to be hand-related (e.g., Is the object smaller than your
fist?) and analagous task-related (e.g., Is the task like a prying task?).
While asking the user questions, GRASP-Exp looks for supporting
evidence that a particular grasp wil satisfy the given requirements.
It does this using a rule-base, the collection of rules that map task requirements,
object properties, and grasp attributes to postures. This
method of looking for supporting evidence is called backward chain-
Examples of rules are as follows:
a.
RULE 1: IF the task requires dexterity > 75%
AND
THEN
the task requires sensitivity> 75%
the grasp is a “precision grasp”
RULE 2: IF the grasp is a “precision grasp”
AND the object-size is not small
AND the object-global-shape is compact
THEN the grasp is a “precision circular grasp”
RULE 3: IF the grasp is a “precision circular grasp”
AND the object-shape is spherical
THEN the grasp is a “precision sphere circular grasp”
If GRASP-Exp was in the process of determining whether the grasp is
a “precision sphere circular grasp” (Grasp 13 in Figure 2.3), RULE 3
would be used to search for supporting evidence. The controller
would search in a backward chaining style, looking for true values for
the rule’s antecedents or else looking for rules that can be used to de-
termine their true value. In this case, RULE 2 would be triggered be-
cause it could be used to prove that the grasp is a “precision circular
grasp”. However, for RULE 2 to be true, the grasp must first be a
“precision grasp”. This triggers RULE 1, using backward chaining.
But there is no rule that has RULE 1’s antecedal data as a consequent,
and so the controller would ask the user “WHAT IS THE
DEXTERITY REQUIREMENT?” If the dexterity requirement is less
than or equal to 75%, this line (whether the grasp is a “precision
sphere circular grasp”) is abandoned, and another posture selected as
the hypothesized answer. As might be seen, the advantage of using
backward chaining is that it creates a set of highly directed questions
The advantage of an expert system approach is that it is useful in
testing a framework, making explicit the mapping from inputs (object
and task requirements) to outputs (hand posture). Such an explicit