## Structure Design of the Wearable Upper Limb Rehabilitation Robot
### Wrist Joint Design
The driving rope and spring represent the muscles and ligaments around the wrist respectively, which providing kinetic energy and support for the motion of the radial and middle wrist joints. The parallel mechanism uses three servo motors to drive three ropes, which realize the wrist flexion/extension and ulnar/radial movement of the robot. It can not only reduce the flexible degree of freedom, but also enhance the stability of the mechanism, and make the mechanism satisfy the motion amplitude of the wrist under different angles when the mechanism is in retraction and abduction, flexing and stretching. Therefore, the mechanism can achieve the wrist joint adduction and abduction, flexion straight action. The flexible wrist joint driven by rope is mainly composed of three parts: adjustable grip, flexible parallel mechanism and forearm fixation. According to above analyses of the motion freedom of each joint of the upper limb, a six degrees of freedom wearable upper limb rehabilitation robot with the characteristics of tension mechanisms is proposed in this paper. The six degrees of freedom of the rehabilitation robot include three degrees of freedom of the shoulder joint: flexion/extension, internal/external rotation, abduction/adduction; one degree of freedom of the elbow joint: flexion/extension; one degree of freedom of the forearm: internal/external rotation; two degrees of freedom of the wrist joint: flexion/extension and abduction/adduction. The 3D structure of the rehabilitation robot is shown in Figure 5. The wearable upper limb rehabilitation robot designed in this article is wearable, which consists of a vertical wearable rehabilitation robot and a seat: the patient sits on the seat during the rehabilitation process; the patient's arm passes through the traction of the rehabilitation robot institutional contacts. The arm passively performs full-circle rotation by driving the curved rack and provides resistance during active movements of the shoulders. The seat height adjustment mechanism can be adjusted according to the height of the upper limb and the body shape of the human body, adapt to different treatment environment and the difference of the patient's body shape, and ensure that the affected limb is trained on the sagittal plane during the rehabilitation process. Figure 4: Wrist joint structure diagram. (**a**) Wrist joint freedom of motion; (**b**) 3D model of wrist joint. Each joint mechanism of the wearable upper limb rehabilitation robot is closely combined with the upper arm, forearm and wrist of the human body, and the rehabilitation training of multiple joints of the upper limb is realized by using the freedom of the rehabilitation robot hinge type rotating pair, arc rack, tower spring, etc. On the sagittal plane, the rehabilitation robot can realize the flexion/extension of the shoulder 0\({}^{\circ}\)-100\({}^{\circ}\), abduction/adduction 0\({}^{\circ}\)-120\({}^{\circ}\), internal/external rotation out 0\({}^{\circ}\)-110\({}^{\circ}\); the flexion/extension of the elbow 0\({}^{\circ}\)-105\({}^{\circ}\), internal/external rotation out 0\({}^{\circ}\)-90\({}^{\circ}\); the flexion/extension 0\({}^{\circ}\)-90\({}^{\circ}\) joint motion analysis angle range of the wrist meets the angle requirements of the upper limb rehabilitation training process [33]. The free distribution of human upper limbs and rehabilitation robot degrees of freedom are shown in Table 1.