Chapter 2. Prehension

Appendix D - Prosthetic and Robotic Hands 403
certain restrictions apply, particularly at the physical level in terms of
dynamics and kinematics, even though they are different realizations
(see Chapter 8). While a robot controller will not necessarily use the
same algorithms that the central nervous system does (since we don't
know those!), behaviors can be modelled and tested on a mechanical
hand, with success or failure being seen immediately. High level
control variables, such as opposition space parameters, could be used
to drive the artificial device. Such an abstraction at the functional level
is typical of the general progression toward machine-independence
seen in computer systems, and would allow a variety of hand designs
to be developed for different purposes but still be able to share a
common control language.
D.2 Prostheses and Their Control
Objectives for the design of future prosthetic devices (Childress,
1974) include subconscious control, comfort, cosmetics, reliability,
esthetics, and improvement over existing prosthetic devices. The
control system should be (Jacobsen et al., 1982) reasonable to learn,
natural, graceful, reliable, flexible, and include feedback. In terms of
the fingers themselves (Law 198l), they should be light, robust, have
consistent joint frictional forces, be compatible with socket and palmar
materials, and provide an accurate fill for the cosmetic covering,
maintaining a good cosmetic profile throughout the full range of
movement. The needs of the amputee have been divided into two basic
requirements: the desire for the best aesthetic replacement, and the
restoration of specific hand functions (Baumgartner, 198 1, Peizer,
1981). Currently, no one prosthetic device can accomplish all of
these. The devices available include body-powered split hook or hand,
electrically powered hand controlled by myoelectrically, and cosmetic
passive hand. Studies have shown that amputees have a variety of
functional needs (c.f., Millstein et al., 1986), and thus they may have
one or more devices. In fact, Therapeutic Recreation Systems
(Boulder, Colorado) has introduced a family of designs called the
Super Sport series, shaped specifically just for catching a ball.
D.2.1 Control of prosthetic devices
Body powered prostheses use power transmitted in the most direct
way for maximum indirect sensory feedback. Four different body
sources are used (Baumgartner, 1981): the wrist joint, the forearm,