Page 315 - Handbook of Biomechatronics

Upper and Lower Extremity Exoskeletons 307

the system; and mechanical domain that involves how mechanical power is
transported and how the different joints are supported (e.g., cables, linkages,
transmission).

4.2.1 Signal Domain
In order to enhance cHRI of users controlling exoskeletons using bioelectric
signals, multiple-source signal fusion is an emerging approach. Signal fusion
permits that multimodal signals be combined to provide sufficient informa-
tion for motion intention decoding.
Thus, sEMG plays an important role in the control of exoskeletons tak-
ing into account its relative ease of acquisition and abundant content of neu-
ral information; however, implementation of the EMG-based pattern
recognition algorithms is not easy to be accomplished due to some difficul-
ties, such as EMG signals are time varying and highly nonlinear. Further-
more, the activity level of each muscle for a certain motion is different
between each person. A trend for EMG-based controlled exoskeleton relies
on using non-EMG signals that are combined with sEMG signals to realize a
more precise extraction of motor commands. Furthermore, acquisition of
information using high-density sensors array provide more information to
improve control.

4.2.2 Energy Domain
In several applications, the exoskeleton must be able to generate high forces
to sustain, assist, and/or perturb the motor capabilities of the user. Thus, tak-
ing into account of current actuator technologies with characteristic of size,
weight, and torque, it is limited to power multiple joints.
A trend for actuator technologies is muscle-like actuators, which are
built using soft materials that have good properties, and they behave like
human muscles. Most of them are made of elastomers, including silicon
and rubber, and so they are inherently safe. This technology enables the
development of “soft exoskeletons” (Majidi, 2014). Active polymers appear
promising, being thin, lightweight, compliant and able to perform both
sensing and actuation. However, fundamental enhancements would be
required for the feasible use in exoskeletons. Similar to shape-memory
alloys, forces are generally low and take time to build up (i.e., low band-
widths), which results in the need for large stacked configurations
(Villoslada et al., 2015).