Page 209 - Handbook of Biomechatronics

206 Georgios A. Bertos and Evangelos G. Papadopoulos

shown great promise due to their low cost, lightweight, simple actuating
structure, and good performance in low frequencies with large deformation
(Yuan et al., 2016). However to fully deploy their capabilities, DEAPs
require very high voltages, of the order of 2kV DC. Despite the small cur-
rents and compact amplifiers, these voltages are not human friendly.
Current DEAP challenges were reviewed with respect to durability, pre-
cision control, energy consumption, and anthropomorphic implementation
(Biddiss and Chau, 2008). DEAP actuators in powered upper-limb prosthet-
ics is impeded by poor durability and susceptibility to air-borne contami-
nants, unreliable control owing to viscoelasticity, hysteresis, stress
relaxation and creep mechanisms, high voltage requirements, and insuffi-
cient stress and strain performance within the confines of anthropomorphic
size, weight, and function (Biddiss and Chau, 2008). Although this technol-
ogy is currently infeasible for upper-limb prosthetics, research continues,
aiming at reducing the voltage required and increasing their overall potential
(Bar-Cohen et al., 2018).
Shape memory alloys are alloys that convert heat into mechanical displace-
ment through thermo-elastic transformations, passing from martensite to
austenite when heated; when cooled, the material returns to austenite.
SMAs exhibit shape memory, that is, they return to a predetermined shape
when heated ( Jani et al., 2014). In practice, this actuator is made by a num-
ber of SAM wires in parallel, which can be heated by current passing through
the strained wires. Usually the heat is produced by the alloy’s own resistance,
causing it to contract and return to its original shape, producing large forces.
When these alloys are used in the form of wires, they present a good
strength/weight ratio, and high strength/area ratio, rendering this material
appropriate for application in upper-limb prostheses. The most common
SMA, Nitinol, is composed of nickel and titanium (NiTi). This SMA dis-
3
plays one of the highest work density at 10 J/cm , which is 25 times greater
than that of electric motors and is able to lift >100 times of its weight.
Furthermore, the NiTi SMA is biocompatible, exhibits high wear resistance,
and is highly corrosion resistant ( Jani et al., 2014).
However, SMAs require high temperatures (up to 100°C) to develop
their maximum force and have slow response since it takes time to cool
the wires. As their strain is 4%–8.5%, they need either special mechanisms
or long lengths to achieve useful displacements. Although recent advance-
ments in SMAs have produced strains of up to 32% using a braided coil
design, additional shortcomings including high hysteresis, short service life,
and high-energy consumption, still limit their applicability to practical
prostheses.