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Exoskeletons in upper limb rehabilitation 249

tradeoff of limited ROM and complex design. Apart from the shoulder
joint, misalignment may occur at the elbow and wrist joints (Rocon
et al., 2008). Gunasekara et al. (2015) in their 6-REXOS exoskeleton has
proposed two additional DOFs to compensate for elbow and wrist joint mis-
alignment. An exoskeleton that has a cable-driven parallel mechanism, such
as CAREX-7 (Cui et al., 2017), is inherently aligned and has great kinematic
matching with humans. A review on cable-driven exoskeletons was recently
published (Sanjuan et al., 2020). Li et al. (2019) made an exoskeleton com-
prised of a novel mechanism that makes the exoskeleton self-aligning during
its maneuver (Li et al., 2019). Furthermore, to align axes of an exoskeleton’s
joint for upper arm internal/external rotation and forearm pronation/supi-
nation is a challenging task as those axes are longitudinal axes of correspond-
ing limb segments; therefore, placing motors along those axes is impossible.
Perry et al. (2007) used curve rail bearing to provide rotational motion along
the upper arm and forearm. Rahman et al. (2014) developed an innovative
gear mechanism where motion is transmitted from an antibacklash gear
(mounted on a motor shaft) to an open-type, custom-made meshing ring
gear attached rigidly to the upper arm cup. In harmony robot, a parallelo-
gram mechanism along with belt and pulley drive has been used to produce
forearm pronation/supination (Kim and Deshpande, 2017). Though pro-
gress has been made, designing an exoskeleton that is ergonomic with full
natural ROM and capable of evading misalignment in joints is still a great
challenge for researchers. To provide better HEI, kinematic matching
between the exoskeleton and the human is a must in upper limb rehabilita-
tive exoskeletons.

Actuation
Actuators are the main elements that contribute more to exoskeleton weight
than any other elements. Various kind of actuators have been used in upper
limb exoskeletons, such as electric actuators (Kiguchi et al., 2008; Nef et al.,
2009a; Gopura et al., 2009; Rahman et al., 2014; Cui et al., 2017; Xiao et al.,
2018; Islam et al., 2019), pneumatic actuators (Song et al., 2007; Sutapun
and Sangveraphunsiri, 2015; Balasubramanian et al., 2008), hydraulic actu-
ators (Stienen et al., 2009; Otten et al., 2015; Liu et al., 2016), pneumatic
muscle actuators (Tsagarakis and Caldwell, 2003; Tu et al., 2017; Irshaidat
et al., 2019; Liu et al., 2020), and series elastic actuators (Kim and Desh-
pande, 2017). In electrical actuation, DC brushed, DC brushless, and servo-
motors are frequently used to generate required joint torques in exoskeleton.