Biomedical Engineering Reference
In-Depth Information
FIGURE 3-74
Using shape
memory alloy to
actuate a prosthetic
hand. (a) Schematic
diagram. (b)
Photograph of hand.
(Courtesy Robotics
and Mechatronics
Laboratory, Rutgers
University.)
Many different solutions have been proposed for these problems, including using “mus-
cles” controlled by air pressure, piezoelectric materials, or SMAs.
3.7.1 Shape Memory Alloys
Shape memory alloys mimic human muscles and tendons quite well. They are strong and
compact so that large groups of them can be used for prosthetic applications. In addition,
the motion with which they contract and expand is very smooth, producing a lifelike
movement unavailable in other systems.
Creating human motion using SMA wires is a complex task, but the basics are straight-
forward, as illustrated in Figure 3-74. For example, to create a single direction of move-
ment, a bias spring shown in the upper portion of the finger holds the finger straight,
stretching the SMA wire. When the wire on the bottom portion of the finger is heated,
it shortens, and the joint will be bent downward. The heating takes place by passing an
electric current through the wire.
Some challenges must still be overcome before prosthetic hands can become more
commonplace: (1) generating the computer software used to control the artificial muscle
systems within the robotic limbs; (2) creating sufficient movements to emulate human
flexibility (i.e., being able to bend the joints as far as human beings can); and (3) repro-
ducing the speed and accuracy of human reflexes.
3.7.2 Electric Motors
A combination of microelectronics and micromechanics has provided the means to produce
prosthetic hands with separately controllable fingers and joints based on human hands. One
example is the device developed by the German Aerospace Centre (DLR), in cooperation
with the Harbin Institute of Technology (HIT). This prototype prosthetic hand, shown in
Figure 3-75, uses miniature actuators and high-speed bus technology.
Constructing a prosthetic hand with the strength and dexterity of a human hand requires
at least four fingers: three fingers to allow the hand to grip conically shaped parts; and a
thumb in opposition. Consequently, the DLR hand consists of three fingers, each containing
four joints with three degrees of freedom. The fourth finger, designed as a thumb, has four
degrees of freedom.
Tiny but powerful motors and shaft encoders are fitted directly in the finger. Each
finger joint also houses a contactless angle sensor and a torque sensor. Since both sensors
provide extremely high-resolution feedback, a high-speed, three-wire serial connection
