Chapter 2. Prehension

Chapter 4 - Planning of Prehension 101
Chapter 5).
Since the two excitatory networks computed a wrist location, they
had to agree on a location. A good location for the wrist was one
where: 1) the VFs could reach the ends of the dowel, 2) the pads were
aligned with each other (as a rough approximation of how precision
grasping occurs), and 3) the pads were aligned with the dowel.
Weights were tuned in such a way that the computed response of a
node was active when an input was presented and inactive when the
input died out. The input, I, to the field was a map of the solution
space, indicating all the locations (r, f) that were reasonable places to
put the wrist. Reasonable is used here in the sense that kinematic so-
lutions exist for both VF1 and VF2. This input represents a very large
solution space, because it could be defined over all possible kinematic
solutions for all object sizes that could be grasped.
4.6.2 Orienting the palm
While the arm transports the hand to the object, the hand is ori-
ented. This can be done from the shoulder joints through all the more
distal joints to the grasping surfaces of the hand. With respect to wrist
angle, static isometric force studies have shown that the tendons of the
finger extensor muscles are too short to allow maximum grasping
force when the wrist is fully flexed. For a forceful grasp, it has been
noted that optimal wrist angles exist (Hazelton et al., 1975, Bunnell,
1942).
Constraints on orientation, such as biomechanical ones and in-
tended actions, can affect grasp planning. Rosenbaum, Vaughan,
Barnes, Marchak and Slotta (1990) examined the planning of complex
movements in terms of awkwardness of postures. Subjects were
asked to pick up a horizontal two-colored bar using either an overhand
grasp (coming down on the bar, as seen in Figure 4.16a) or an under-
hand grasp (coming up on the bar, as seen in Figure 4.16~). They
then had to place the bar vertically either on its gray (see Figure 4.16b)
or black end (Figure 4.16d) either to the left or right. All subjects
used a grasp that left their wrist, elbow, and shoulder joints in a com-
fortable posture at the end of the place movement. In some of these
conditions, it meant the initial posture was more awkward. For ex-
ample, it is initially awkward for a right-handed subject to use an un-
derhand grasp to place the black side on the right side of the table.
Initially it would be less awkward to use an overhand grasp for the