Chapter 2. Prehension

Chapter 2 - Prehension 41
and then pad opposition. In terms of sensory information, high den-
sities of mechanoreceptors have been noted in the finger pads, as also
will be seen in Chapter 6.
VF state variables must obey their own constraints. For a task
using an object of width w, height h, and with a minimum force to be
applied p, the following constraints must be obeyed:
a) Finger Position constraint. The distance between the grasping
surface patches of the two VFs must be w (the object width).
b) Force Magnitude constraint. The minimum force, p, necessary
in the task must be smaller than the maximum force that can be
applied by the VFs. The magnitude of the force applied by the
two VFs must be equal.
c) Finger Width constraint. The object length h can represent an
upper limit on the width of a VF (e.g., coffee mug handle vs.
coin held in palm).
Within an opposition, state variables must also obey constraints.
Given an opposition, the following constraint must be obeyed:
d) Force Orientation constraint. The two force orientations y1 and
a must align with each other during the opposition. From
Figure 2.9, y1 = 2~.
Setting up an opposition space in effect fixes certain degrees of
freedom while making others the variables for the task. The useful-
ness of this model is that it reduces the complexity of the sensorimotor
problem. The style of posture chosen matches the task requirements
with the hand’s capabilities. In pad opposition, the hand can exert
small forces, impart fine motions, and gather precise sensory informa-
tion to match the accuracy and manipulation requirements of the task.
In palm opposition, the hand can match or create larger anticipated
forces while still ensuring a stable grasp, using the arm and wrist to
provide grosser motions. Side opposition is a bridge that offers a
medium range of forces while still offering some availability of sen-
sory information due to the thumb pad being in contact with the object
and some ability to impart motions to the object. Using the mathemat-
ical description provided here, a more quantifiable notion of power
and precision capabilities is possible. As a parameterized view of hu-
man prehension, this model provides components for modeling a bio-
logical subsystem for the control of natural, prosthetic, and robotic
hands. The abstracted description of opposition space in terms of re-