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Exoskeletons in upper limb rehabilitation 257

mechanism should also be novel with light weight, compactness, high gear-
ratio, friction free, zero backlash, and quick responsiveness. One of the key
novelties that researchers should include in exoskeleton design is modularity
(also known as reconfigurability). A modular exoskeleton can be reconfi-
gured by adding or removing parts/modules to or from it. For instance, a
7-DOF exoskeleton is redundant for an individual who has disability only
in the shoulder. In this case, the elbow and wrist motion support parts could
be removed from it, reducing the 7-DOF exoskeleton to a 3-DOF exoskel-
eton. Having quick attachable-detachable links in exoskeletons would allow
it to be modular.
Despite the fact that an enormous amount of research has been done,
development of a control strategy to provide effective rehabilitation is still
evolving. To provide efficacious rehabilitation to upper limb-impaired
patients, first, we must determine the patient’s safe extreme ROM before
starting therapy. This may also help to select appropriate and safe rehabili-
tation protocols for patients. For example, it is essential to know the safe
ROM of a patient having muscle tone. In conventional therapy, this is done
manually by a therapist who observes the patient. Second, the therapist can
adjust his/her approach if the patient feels any pain or something unwanted
happens. This is something exoskeletons should include in their control
design. Third, to the best of the authors’ knowledge, there is no such a con-
troller that automatically selects a rehabilitation protocol for the patient and
changes it depending on how the patient is doing exercise. Fourthly, when
the patient starts sharing control, he/she has to move his/her impaired limb
as well as exoskeleton before getting assistance. This seems to be a burden
because the patient has already lost mobility. In such a case, it is worth com-
pensating a portion of torque required to move the exoskeleton only.

6 Conclusion
In this chapter, the HEI, hardware design, safety, compactness, control tech-
nique, actuation, and power transmission mechanism of existing upper limb
rehabilitative exoskeletons were reviewed. We found that most research
prototypes of existing exoskeletons were not transferred into a commercial
product, and perhaps the reason behind it is the lack of clinical testing.
Therefore, in this review, the challenges that need to be addressed for
improved functionality have been identified and discussed. Addressing these
challenges in the development of exoskeletons will make them more