Chapter 2. Prehension

Chapter 4 - Planning of Prehension 81
environments, since it eliminates uncertainty without the use of
sensors. Mason calls operations that eliminate uncertainty funnels,
and argues that manipulation funnels can be used in grasping objects
because they eliminate position uncertainty. The use of a manipulation
funnel is seen in the way humans let a resting pencil rotate into their
grasp by sweeping it up with their fingers.
Is it possible to combine perceived object properties and task in-
formation into a summary representation that the CNS might have ac-
cess to during planning? In Figure 4.7, a process is suggested. For
regularly shaped objects (such as cylinders, blocks and spheres) with
an equal distribution of mass, the specific location on the object to
grasp is not as important as the fact that two (in general) locations are
chosen that are generally parallel to each other so that an opposition
between two virtual fingers can be applied. We call those two surfaces
the ormosable surfaces. If they are not parallel, then due to task
mechanics, the object will be either pushed into or away from the
grasp (see Chapter 6). The surfaces have visible characteristics, such
as length, spatial density, and a radius of curvature, all available to the
CNS during the planning process. Klatzky, Lederman, and Reed
(1987) showed that surface spatial density (an aspect of texture) is an
characteristic accessible by vision, as is size. Surface texture relates to
the forces needed (rough surfaces are easier to get a grip on).
Cutkosky and Newell (Cutkosky, 1989 Cutkosky and Howe 1990;
Newell, Scully, Tenenbaum & Hardiman, 1989) demonstrated that
size is thought about in terms relative to the hand. The critical hand
planning parameters about object size are the object width (how wide
does my hand have to open?) and the object length (how many fingers
can I use in VF2?). An opposition vector can be drawn between the
two surfaces with its magnitude being the width of the object between
the two surfaces. Jeannerod and Decety (1990) and Chan et al.( 1990)
demonstrated that object size can be judged in terms of hand opening
size, and Marteniuk et al. (1990) showed that the opening of the hand
relates to object size. For irregularly shaped objects (such as mugs,
flashlights, tools, etc), Klatzky, Lederman, and Reed (1987) have
shown that global shape is accessible to the vision system, thus
making it possible to plan without specific details being known.
Again, knowledge about task mechanics is relevant here, because it
provides a mechanism for anticipating the location of the center of
mass. Handles are easily perceivable aspects of objects, because they
are regularly shaped and in hand-related sizes. As Napier (1980)
pointed out, handles lend themselves to a posture relevant to the task
(e.g., power tasks need power grasps which need larger dimensions