Page 275 - Control Theory in Biomedical Engineering

Exoskeletons in upper limb rehabilitation 247

fit a shaft along the axis of rotation of above cases (axis of humerus and
radius), the developer of ETS-MARSE used an antibacklash spur gear
meshed with open-type semicircular gear and bearing assembly.
Harmony (Kim and Deshpande, 2015, 2017) as shown in Fig. 2C, a
recent robotic exoskeleton in the field of upper limb rehabilitation, has been
developed intending to enable the patient to do bilateral arm training. This
system is comprised of a dual arm with four-bar linkage, which makes it
capable of providing naturalistic shoulder movement. Unlike, ARMin-III
(where shifting of shoulder CR was considered only for vertical flexion-
extension), here the four-bar linkage mechanism moved shoulder CR dur-
ing either shoulder abduction-adduction or vertical flexion-extension,
which made it more anatomical-like (Kim and Deshpande, 2017). The
range of motion of the robot differs based on the way its other joints are con-
figured. For instance, ROM of shoulder abduction increased when it was
performed simultaneously with shoulder external rotation.

3 Design requirements and challenges

Unlike industrial and other genres of robots, upper limb exoskeletons by
their nature are complex in structure, requiring more sophistication in
design. Researchers have used different measures and features in upper limb
exoskeleton design to enhance functionality. However, there are still limi-
tations that need to be addressed.

Safety

Since upper limb exoskeletons have close interactions with wearers, safety is
paramount. Human-exoskeleton interaction (HEI) must be designed so as to
ensure safe operation. For safe running of exoskeletons, a HEI should
include safety measures in mechanical, electronic, and control design.
Mechanically, safety is ensured by placing physical stoppers in the exoskel-
eton’s structure to prevent it going beyond natural ROM; safety can also be
ensured by designing links and exoskeleton robot parts in a way where adja-
cent links act as physical stoppers in extremes. Electronically, by setting cur-
rent and voltage limits in motors, exoskeleton joints can be refrained from
going beyond permissible ROM. In control design, saturation can be set for
torque, force, velocity, and position to ensure the wearer’s safety during
exoskeleton failure.