Page 53 - Handbook of Biomechatronics

Actuator Technologies 47

include changing the wire shape to improve heat dissipation, dividing wires
into individually controllable segments (Selden et al., 2006), and active
cooling. Other challenges of SMAs are low-energy efficiencies (10%–
15%) and high costs.

3.1.4 Electroactive Polymers
EAPs are another category of smart materials that have been called “artificial
muscles.” Of all current actuator technologies, EAPs are the most function-
ally similar to natural muscle. EAPs are polymer materials that transduce
electrical energy into mechanical energy (and vice versa). Similar to the
above smart metal alloys, they have high power-to-weight ratios, but also
have the benefits of lower costs, inherent compliance, and much larger strain
capabilities. For a great introduction to EAPs for bioinspired applications,
see Bar-Cohen (2001).
There are two categories of EAPs: ionic EAPs, which respond to ion
flow, and electronic EAPs, which respond to electrostatic forces. Ionic EAPs
require wet environments; so, electronic EAPs are generally more appropri-
ate for biomechatronic applications. Of the electronic EAPs, a recently
developed but highly promising type of EAP is the dielectric elastomer.
A dielectric elastomer is composed of two compliant electrodes that
sandwich an insulative polymer film. When voltage is applied across the
electrodes, electrostatic forces squeeze the dielectric film, causing a decrease
in thickness and increase in area (Fig. 9). This dielectric behavior enables
capabilities that approach that of natural muscle: strains of 10%–100% and
stress levels of 0.1–9MPa (Carpi et al., 2008). The advantages of dielectric
elastomers are many: they are compliant, lightweight, inexpensive, quiet,
and have high power densities. The major disadvantage is that they must
be prestrained to reach full performance, which requires mechanisms that
increase weight and packaging size. However, dielectric elastomers are in
the early stages of commercialization, and show promise for further
improvements.

3.2 Transmissions
A motor outputs mechanical energy to the overall system through some
coupling or transmission. The transmission may be as simple as a clamp that
connects a motor shaft to the load, or as complex as a system of variable
springs and dampers. The transmission is often designed to have some
dynamic behavior that improves the overall system safety or performance.