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Exoskeletons in upper limb rehabilitation 253

muscle activity. Researchers have also been working on using brain signals
(EEG) to detect user intention (Bhagat et al., 2016). In a rehabilitative
exoskeleton, unidirectional sensors are generally used. However, bidirec-
tional sensors can be used to optimize the exoskeleton’s maneuvering.
The sensors of exoskeletons should have appropriate bandwidth, enough
resolution, and high accuracy to be operated on a real-time controller.

4 Control approaches
The use of rehabilitation robots in the medical rehabilitation field has proved
to be of great ability to improve patient quality of life, enhancing practical
motions and assisting the patient in daily exercises. The exoskeleton robot is
an articulated mechanical structure with several DOFs having the same anat-
omy of the human arm or leg. Unlike prostheses that replace a limb of the
body, the exoskeleton robot clings to it externally and acts in parallel. This
fixation allows the robot’s wearer to move his/her arm in the workspace.
The reachable workspace envelope depends on the number of DOFs avail-
able in the exoskeleton robot. It can be dedicated to a specific part of the
body, such as the hand, arm, leg, or several limbs at the same time. Equipped
with sensors and actuators, it measures the movements and forces of the
user that allow the physiotherapist to accurately evaluate the patient’s
performance.
Usually, the design of these kind of robots is based on the anatomy of the
human upper limb and is developed to faithfully represent the joints and
movements of the upper limb movements. This robot system is able to pro-
vide the different levels of robotic assistance strategies used after neurological
accidents. The most urgent, usually the first 6 weeks after the accident, is
passive physical therapy (Sidney et al., 2013; Xie et al., 2016). In this type
of therapy, the exoskeleton brings the patient’s limb, which is completely
passive, to realize a therapy task. Its advantage lies in the robot’s ability to
provide intensive therapy over a long period of time (Brahim et al.,
2016a, b). The next types of therapy, active-assisted and active modes, allow
the patient to voluntarily initiate movement. Then, the exoskeleton’s
wearer can perform a free motion (active mode) or an active-assisted move-
ment where the robot corrects or guides this movement. In the latter case,
the robot limits the tremors or corrects the trajectory. After detecting the
initiation of a motion, usually voluntary, the exoskeleton will guide the
achievement of the activity, often using an impedance and/or admittance
control (Li et al., 2017; Ochoa Luna et al., 2015; Liu et al., 2020).