Chapter 2. Prehension

196 THE PHASES OF PREHENSION
collapses the CCP’s ‘ballistic movement’ schema (or Movefast
Schema) and ‘hand rotation’ schema. The Orient Palm Schema
generates necessary commands for the six spatial degrees of freedom
specifying the position and orientation of the palm. Importantly, the
Drive Fingers Guarded Schema encloses the fingers around the object
which entails motor commands to the fingers, palm, wrist, and arm.
The Drive Fingers Guarded Schema replaces the CCP’s ‘adjustment’
(or Moveslow Schema) and differentiates the enclose action from the
preshape action in the ‘finger adjustment’ schema. During the acti-
vation of the Drive Fingers Guarded Schema, guarded movements of
the fingers, palm, wrist, arm and pectoral girdle bring the fingers into
contact with the object. Fine adjustments at all joints can be used for
final alignment of the contacting hand surfaces with the opposition
vector(s) seen in the object. This guarded move, programmed in terms
of sensory consequences, activates the Actual Grasp Schema at
contact. The Actual Grasp Schema is detailed in Chapter 6.
In Figure 5.33, the behavior of the palm-centered model can be
observed. The Orient Palm Schema is used to position and orient the
palm in Figure 5.33b within some ballpark on a desired palm position
and orientation. At the same time, the Reshape Schema opens the
fingers into some virtual finger configuration within the ballpark of the
desired VF configuration. In Figure 5.33c, the Drive Fingers
Guarded Schema closes the fiigers into the object and brings the palm
in closer. The goal driving the finger trajectories (and the arm) is the
alignment of the grasping surface patches of the hand with the seen
opposition vector, given the opposition space parameterization
appropriate for the task. Actual contact stops the fingers and arm.
In summary, the palm-focused model suggests that the palm is the
interface between the hand and the arm, not the wrist. During the first
phase of the movement, the focus is to align the palm to the object. In
parallel, the fingers open or close into a posture based on that palm
alignment. During the second phase of the movement, the focus is to
make contact with the object, thus enslaving the arm to the hand.
5.6 Summary of Setting up an Opposition Space
The black box introduced in Chapter 1 mapping object and task
characteristics to prehensile behaviors can be looked at in terms of
observable kinematic features (see Figure 5.34). From the work cited
in Chapter 4 and also here, we extend the description of objects to
have both intrinsic and extrinsic properties. Intrinsic properties
include surface spatial density, fragility, weight, size, shape, center of