Biomedical Engineering Reference
In-Depth Information
FIGURE 10-29
Low-cost prosthetic
arm developed by
Simón Guerrero
Castillo of the
Instituto Politécnico
Nacional in Mexico.
(a) Photograph of the
prosthesis hardware.
(b) Relationship
between hardware
and a human arm.
(Castillo 2007).
Not all prosthetic arms are so costly, as has been demonstrated by Simon Guerrero
Castillo, a student from the Instituto Politecnico Nacional of Mexico. He developed a
myoelectric-controlled prosthesis at a total cost of 18,000 pesos that mimics the natural
motions of an arm including wrist flexion, forearm rotation, and movement of the elbow
and the shoulder. The device, shown in Figure 10-29, does not look like a conventional
arm, but it works.
10.9.2 Hand Mechanisms
Two classes of hands are available: (1) anthropomorphic, with many degrees of freedom
and mostly prototypes or in limited production for use as research tools; and (2) terminal
devices (TDs), which are much lighter and generally simpler devices with fewer degrees of
freedom for use as prosthetics. Specifications of some of the research hands under devel-
opment, or in limited production, are listed in Table 10-4. Hands of various complexities
are available from a number of manufacturers, including Motion Control Inc., Otto Bock,
RSL Steeper, Liberating Technologies Inc., and Touch Bionics. The more primitive hands
contain a small geared motor that drives the fingers to produce pinch forces of up to 10
kg at the fingertips, which allows them to grip most objects securely even without the
compliance of a natural hand. However, without the appropriate feedback such hands tend
to crush delicate objects, so most modern active TDs include force sensors or motor drive
current monitoring with some form of feedback to the user.
The earliest devices had digital control (they were either on or off), but the introduction
of proportional control, in which the rate of closure is proportional to the actuation signal
amplitude, has given these TDs more precision. Actuation signals could be provided by
myolectrics, force sensors, or linear potentiometers.
Most TDs come with quick-release attachments so that the correct hand for the job
can be fitted easily and quickly. In the past these included cosmetic hands with limited
movement, power grippers, and hooks, as shown in Figure 10-30. However, in the past
decade or so, hands with individually articulated fingers and opposable thumbs have
become available.
