Page 208 - Handbook of Biomechatronics

Upper-Limb Prosthetic Devices 205

or the slider (for linear displacement). As the mechanical vibrations are in the
ultrasonic region, that is, above 20kHz, they are silent. These motors can be
very small in size, they exhibit high power density, high torque and low
speed, low moment of inertia, fast response, noiseless operation, self-braking
drive, and generate no magnetic fields (Cura et al., 2003; Pons et al., 2002).
Its disadvantages include its need for a high-frequency energy source, its
short service life due to stator/rotor contact, variations in speed and low
efficiency compared to electromagnetic motors, and its requirement for
complicated control.
Piezoelectric motors. A piezoelectric or piezo motor is an electric motor,
which is based on the change of shape of piezoelectric materials when an
electric field is applied. This change of shape, and combined with the
stick-slip phenomenon produces mechanical displacements in the form of
linear of rotary motion. Compared to dc motors, piezo motors are small
and produce large torques, but they are relatively expensive (Da Cunha
et al., 2000).
Artificial muscles can be built in principle using pneumatics or dielectric
electroactive polymers. This idea is very attractive, because such muscles
can fit well in a prosthetic arm, and the load-length curve produced resem-
bles that of the actual limb.
Pneumatic artificial muscles (PAMs) consist of an inflatable inner bladder
inside a braided mesh, clamped at the ends, that contracts or extends when
supplied with high/low pressure, respectively. As they can only pull, PAMs
are applied in agonist and antagonist pairs. This technology was invented in
the 1940s and developed in the 1950s as McKibben artificial muscles (Chou
andHannaford,1996).PAMsarelightweight,failsafe,andcompliant.Exper-
imental results indicate that accurate position control is feasible, with power/
weight outputs in excess of 1kW/kg at 200kPa (Caldwell et al., 1995). How-
ever, to operate them, one needs a compressor, which tends to be bulky and
noisy, or an external pressurized gas (CO2, air) tank. It also requires solenoid
valves, driver electronics, and a battery. Recently, PAMs are of renewed
interest due to applications in soft robotics (Greer et al., 2017).
Electroactive polymers were discovered in 1880. They are also known as
compliant capacitors, as they have similar behavior to capacitors. These
polymers, when stimulated by an electric field, exhibit a change in size or
shape; if constrained, they apply large forces (Kim and Tadokoro, 2007).
The concept of using dielectric electroactive polymers (EAPs or DEAPs)
as artificial muscles was revived recently as it has been demonstrated that
some EAPs can exhibit up to a 380% strain (Bar-Cohen, 2001). They have