Chapter 2. Prehension

Appendix D - Prosthetic and Robotic Hands 417
considerations and has a rotary thumb. As it rotates, it provides side
opposition either to the flat side or else to the more curved side of the
larger jaw. Prehensor C was based on functional considerations, and
has 3 jaws. Pad opposition can occur in two different ways: first, it
can occw between small pads on the left (VF1) and middle jaw (VF2),
and secondly, it can occur between the middle jaw (VF1) and the right
jaw (VF2). Due to the bowed space between the jaws, this second
way allows a cylindrical object, such as a soda can, to be grasped.
D.3 Dextrous Robot Hands
A simple two-fingered gripper is similar to a prosthetic split hook
in that there is only one degree of freedom. As a functinal unit, it can
grasp a variety of objects and it is easy to control. However, it is
limited in the tasks that it can perform. One industrial solution has
been to develop special purpose end effectors. The end of the
manipulator is a uniform clamping mechanism, such as a bayonet
mount or magnet, and devices that can be attached to this include
powered screwdrivers, drills, paint cans, etc. When one task is
finished, the computer replaces the current end-effector to some
known location, and picks up another end-effector, specific to the task
at hand. This is both costly and time-consuming.
An alternative has been to develop general purpose end-effectors,
called dextrous robot hands. While these are also costly and still
research tools, to date each device built has been designed specifically
to study one aspect of grasping.
As mentioned in the introduction to this chapter, a major
stumbling block is the control problem of coordinating multiple
degrees of freedom in light of the complex interaction of forces in
prehension. Adding more degrees of freedom complicates the standard
control equations used today; reducing them makes the equations
simpler but one loses versatility. One solution has been to add
sensors to the hands. This causes its own problems, of course, in
terms of physical layout, transducing mechanisms, and interpretation
of the signals.
In the next sections, we describe various multi-fingered robot
hands.