Chapter 2. Prehension

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satisfy active life-styles of the differently abled. Reduced voltage
requirements, battery saving features, light weight alloys, modular
designs, use of reinforced silicones for cosmetic gloves, and space age
miniaturized motors and circuits have led to light weight, electrically
powered prosthetic hands for adults and children. Users can
comfortably wear these during an eight hour day. However,
Baumgartner (198 1) argued that no upper limb prosthesis will be able
to do everything a hand does. He suggested that the functional
prosthesis must be compared to a tool rather than to the human hand.
As it is, tools already seem to be treated as an extension of the human
arm (see Law, 1981). Proprioception of joint angles, muscle length,
and muscle force seems to be extended to include clubs and tools.
A parallel engineering problem has been the development of
functional end effectors for robots. Early robot end effectors, built and
designed in the 1960’s and 1970’s, were simple two fingered
grippers. Much like the split hook, these have only one degree of
freedom. More recently, multi-fingered hands have been developed,
such as the three fingered StanfordJPL hand (Salisbury, 1985), the
four fingered Belgrade/USC hand (Bekey, Tomovic, & Zeljkovic,
1990), and the five fingered UtahNIT hand (Jacobsen, Iversen,
Knuti, Johnson, & Biggers, 1986). Besides more fingers and thus
more degrees of freedom, these devices are equipped with various
sensors, such as tactile, force, pressure, slip, and joint sensors.
Besides the difficult mechanical problems of actuator design and
packaging, a major stumbling block in both prosthetic and robot hand
development is the control problem of coordinating and controlling
multiple degrees of freedom when interacting with the environment.
Adding more degrees of freedom complicates the control equations.
Reducing the number makes the equations simpler but at the expense
of versatility. Adding sensors provides the opportunity for triggered
responses and feedback control, creating the problem of transducing
sensory information accurately. The control problem in prosthetics
takes on the additional complexity of identifying control sites. When a
human hand is amputated, lost are all the intrinsic and many or most of
the extrinsic muscles in the forearm (see Appendix A). So, even
though a prosthetic hand might some day be built that parallels the
appproximately 27 degrees of freedom of the human hand, a method
for actuating those degrees of freedom must also be developed.
Prosthetic and robot hand designs provide a unique opportunity
for understanding prehension. As alluded to in Figure 1.2, these
devices and their controllers provide a realized or implemented model.
Whether the hand to be driven is a mechanical hand or a human one,