## 2 Related Work
These devices use either linear electric actuators or pneumatic linear actuators. Five of these devices are modelled using inverse kinematics and the remaining three are modelled using both forward and inverse kinematics. Challenges in the existing devices are the following:  
* All existing devices can be used for only one of the upper limbs (right or left limb). * All devices are attached to the limb. The user is not free from wearing it. * The weight of the device acts on the limb. * The devices do not perform any prescribed exercises suggested by clinicians. * The devices do not perform rehabilitation for all the fingers simultaneously. * The cost of the device is very high. * The users (stroke patients) cannot set up the device by themselves. * The existing devices cannot be used for all age groups (including children). * Additive manufacturing technology is not adopted. It saves time for manufacturing. * There is no single device which performs rehabilitation for the fingers, wrist, elbow, and shoulder joints together. * The majority of the devices are serial manipulators. * Most devices use linkage mechanisms, which are rigid links, and do not use complaint mechanisms. * Most devices use electric motors for actuation and do not use flexible actuators. * Existing devices do not perform wrist joint motions and finger motions together. In this context, a 3 UPS/S robot which can be set up to function as an SPM for wrist rehabilitation by executing three rotational DOFs is introduced. The exoskeleton and its parts are depicted in Figure 1. The 3 UPS/S mechanism is suitable for executing the required movements in the rehabilitation process. The 3 UPS/S exoskeleton robot has three rotational DOFs according to Grubler-Kutzbach criterion. The human wrist joint has two rotational DOFs. Therefore, the 3 UPS/S exoskeleton robot can be used to rotate the wrist joint about two axes (roll and pitch), thereby performing wrist motions such as flexion/extension and ulnar deviation/radial deviation. Using the 3 UPS/S parallel mechanism leads to the lightweight design of the exoskeleton robot with high stiffness. The exoskeleton robot, presented in this paper, has many features/uniqueness when compared to other existing systems. Some of them are discussed below. * This exoskeleton robot is unique since it is one of the first to use a parallel wrist manipulator for upper limb rehabilitation. Given that the wrist joint has two rotational degrees of freedom, the proposed device can be used to rehabilitate the wrist after a stroke. * The 3 UPS/S manipulator has good stiffness because it has a central leg connecting the base and top platforms. * This exoskeleton has a stand which is attached to a table. This stand reduces the weight of the exoskeleton acting on the patient. * The exoskeleton could be utilized to accommodate patients with the same average body size. * This exoskeleton can be used on both the hands. * This exoskeleton technology operates without the use of real-time computers or software. * It is an inexpensive rehabilitation system that may be used at home. * If one of the upper limbs is functional, the patients can set up the device on their own. * The only attachment point is at the hand. Some devices use multiple attachment locations. This exoskeleton robot has few disadvantages when compared to other existing systems. Figure 1: (**a**) Exoskeleton and (**b**) parts of the exoskeleton. * The robot does not perform a complete range of motion during wrist flexion and extension motions. * The top platform does not stay parallel to the base platform at the home position when the exoskeleton is idle. In this case, the central leg is an imaginary one and it is substituted by the human arm and wrist in the rehabilitation process. * This robot has singular configurations within its workspace. * This robot has more degrees of freedom than required to perform wrist motions. * This robot cannot be used for all age groups. It needs to be customized for different groups.