Page 279 - Control Theory in Biomedical Engineering

Exoskeletons in upper limb rehabilitation 251

exoskeleton joints. Many researchers have used cable-driven pulley reduc-
tion in power transmission mechanisms of their exoskeletons. Tsagarakis and
Caldwell (2003) have used cable-driven pulley reduction for pneumatic
muscle actuators. In the CADEN-7 exoskeleton, Perry et al. (2007) have
used a cable-driven pulley reduction for brushed motors. Some research
groups have used a hybrid approach such as CAREX-7 (Cui et al., 2017)
and SUFUL-7 (Gopura et al., 2009), both of which a cable-driven pulley,
spur gear, and bevel gear, and the CABXLexo, which uses cable drive with
epicyclic gear train for power reduction (Xiao, 2019; Xiao et al., 2017,
2018). The main advantage of cable drive is that it can be fitted easily at spots
within the exoskeleton, whereas it would be hard to place other drives.
In addition, cable drives are beneficial in terms of low inertia, simplicity,
and long range transmission. However, the main disadvantage of cable drive
is that it can easily stretch and slip, leading to improper tension in the cable,
and cause different joint movement than what is desired (Laschi and Cian-
chetti, 2014). Researchers also used gear drive in exoskeleton systems, such
as 6-REXOS (Gunasekara et al., 2015), which embodied spur and bevel
gears in its design for motor speed reduction. Such gear drive inherently
contains some clearance between meshed teeth, causing friction and back-
lash (Walha et al., 2006; Zhou et al., 2019). Backlash is undesirable in exo-
skeleton design. Furthermore, gear drive requires lubrication, significant
space for the gear box, and regular maintenance. In contrast, because of zero
backlash, low maintenance, and smooth transmission, harmonic drive/strain
wave gear reduction has gained popularity among exoskeleton researchers
(Kiguchi et al., 2008; Nef et al., 2009a; Rahman et al., 2013a; Kim and
Deshpande, 2017; Islam et al., 2019). The only limitation of such drives
is their weight, as they need adapter parts to connect with both the motor
output shaft and joint input shaft. An exoskeleton’s power transmission
mechanism should be (a) lightweight, (b) transmit quick, smooth, and
backlash-free power, and (c) compact.

Singularity

Mechanical singularity occurs in an exoskeleton when any two joint axes are
aligned with each other. As a result, in this scenario, a DOF is lost and ideally
requires infinite torque to move further from this configuration. In upper
limb, there are three joints (i.e., shoulder vertical flexion/extension, upper
arm internal/external rotation, and forearm pronation/supination) whose
axes could be aligned. For example, a singular position is seen when the