## 2 Related Work
(2023). The singular kinematic screw was derived by using the reciprocal product notion in screw theory when the manipulator is in singularity. The singular configurations are determined to be a part of the singular linear complexes. There are several other parallel manipulators (PMs) and serial manipulators (SMs) employed for performing rehabilitation therapy. Some of them are discussed in Table 1. In these devices, the power is transmitted by a linkage mechanism (LM), gear drive (GD), or cable drive (CD). These devices use an electric motor (EM), pneumatic actuator (PA), or hydraulic actuator (HA) as actuators and perform basic movements such as flexion/extension (F/E), adduction/abduction (AD/AB), internal/external rotation (I/E), pronation/supination (P/S), and ulnar deviation/radial deviation (UD/RD) motions for shoulder (SR), elbow (EW), and wrist (WT) joints. These devices can also perform additional movements other than the above-mentioned basic movements and therefore have additional DOFs. In recent years, many exoskeleton robots have been developed for upper limb rehabilitation and researchers have reviewed all of them, but focus has not been given to the mechanical design of active exoskeleton robots performing passive therapy. Active robots are those robots which are powered using an external source such as an electric motor. They can be used for performing passive therapy, a therapy where human effort is not needed. Hence, in Table 1, the proposed robot is compared with existing active robots which perform passive therapy in stroke patients. The comparison is based on the active degree of freedom (DOF), kinematic structure, power transmission method, type of actuator used, and rehabilitation applied on selected joints and supported movements. From Table 1, the following can be concluded: The contemporary mechanical designs, especially the kinematic structure of upper limb exoskeleton robots, have unique features and are manifested in the fact that almost all of them are based on serial manipulators (SMs), and only a few others use parallel manipulators (PMs). Almost all exoskeleton robots employ electric motors or pneumatic actuators. The majority of the robots have less than seven DOFs. Comparing the proposed exoskeleton robot with the existing devices in Table 1, it can be seen that the proposed system has only three DOFs. It can be used only for rehabilitating the wrist joint. Other devices perform additional movements for elbow and shoulder joints. The proposed exoskeleton robot has a parallel kinematic structure. Parallel manipulators are rarely used for wrist joint rehabilitation, whereas serial manipulators are mostly used. In this device, the power is transmitted using a linkage mechanism. The mechanical linkage is more reliable, very long-lived, and inexpensive compared to gear drives and chain drives. The proposed exoskeleton robot device uses an electric motor to perform basic wrist movements such as flexion/extension and ulnar deviation/radial deviation. The electric motors have advantages over the pneumatic and hydraulic actuators such as low maintenance cost, low operating cost, and high efficiency. In 2022, [PERSON] et al. conducted a thorough review [2] with an emphasis on mobile hand rehabilitation robots, encompassing both end-effector and exoskeleton models. Exoskeleton-style devices are ones whose joints match the patient's anatomical joints. This allows for direct control over specific joints, lowering the possibility of unnatural postures. An end-effector-style device applies force to the farthest portion of the finger, enabling a straightforward initial setup at the expense of restricted control over the proximal finger joint. Twelve exoskeleton hand rehabilitation robots have been identified. Almost all of them use motor drives as driving modes except one device which uses series-elastic