Chapter 2. Prehension

332 CONSTRAINTS AND PHASES
b) impart motion to the object (manipulate) or transport the object as
necessary
c) gather sensory information about the state of the interaction with
the object during the task in order to ensure grasping and
manipulative stability
In the definition of prehension, the words ‘functionally effective’ are
used to highlight the fact that the forces must be applied within the
functional constraints of the task; i.e., while forces can be used to
effect a stable grasp and impart motions as needed in a task, there are
functionally specific demands on how this is accomplished, such as
the precision requirements or stability needs.
Much work has gone into characterizing tasks, without much
agreement even within fields. For example, roboticists have
developed analytic measures, many of which are listed and explained
in Chapter 6. These include compliance (Mussa-Ivaldi, 1986;
Salisbury, 1985), connectivity (Salisbury, 1985), force and form
closure (Nguyen, 1986a, Salisbury, 1985), grasp isotropy (Li &
Sastry, 1990; Salisbury, 1985; Yoshikawa & Nagai, 1990), stability
and resistance to slipping (Fearing, 1986; Nguyen, 1986b, 1987a).
Yet, there isn’t agreement which should be included in an objective
function for choosing a grasp.
These analytic measures can be used as a start in being explicit
about task requirements. In terms of the application of forces, there is
a tradeoff between force and position control in response to
perturbations. This is the issue of compliance. When choosing a
prehensile posture, humans make assumptions about the external
forces that could arise during the task. This is the issue of force
closure. The degree to which these external forces must be
counteracted is the issue of force and form closure, and it also refers
to the issue of resistance to slipping. The direction and magnitude of
the applied forces required in the task is the issue of internal forces.
Whether forces have to be applied accurately is the issue of isotropy.
Finally, stability refers to the needed response to perturbations. (See
Fearing, 1986; Nguyen, 1986a,b, 1987a,b; Salisbury, 1985).
In terms of imparting motion, there is the question of how many
degrees of freedom are needed. This is the issue of connectivity.
Whether arbitrary motions must be imparted to the object is the issue
of manipulability. A finer point is whether the motions have to be
imparted accurately, and this is the issue of isotropy. (See Li &
Sastry, 1990; Salisbury, 1985; Yoshikawa & Nagai, 1990).