## Structure Design of the Wearable Upper Limb Rehabilitation Robot  
### Shoulder Joint Design  
From the anatomical point of view, the upper limb of human body mainly includes three joints: shoulder, elbow and wrist [25; 26]. The shoulder joint is the most flexible ball and socket joint in the whole body, which can perform flexion/extension, internal rotation/external rotation, abduction/adduction and other movements. The maximum range of flexion/extension angle of shoulder joint is 135\({}^{\circ}\), abduction/adduction is 135\({}^{\circ}\), internal rotation/external rotation is 110\({}^{\circ}\), and the range of motion is largee [27]. At the same time, since the three rotating pairs are at the beginning of the human upper limb motion chain, the operation space of the terminal effector (hand) is most affected by the motion of the joint at the beginning of the motion chain. Considering that any one degree of freedom will cause the hand to fail to complete some routine actions, the three degrees of freedom of the shoulder joint are reserved [28]. The shoulder joint is composed of the glenoid of the humeral head and scapula, which is connected by the joint capsule. It belongs to the ball and socket joint. It rotates around three mutually vertical axes respectively. At the same time, the shoulder joint is simplified as a connecting rod to connect the shoulder joint and the elbow joint. The degrees of freedom of shoulder joint motion is illustrated in Figure 1.  
The overall structure of the rehabilitation robot depends on the movement of the shoulder joint and the characteristics of different body shapes. The structure of the wearable upper limb rehabilitation robot shoulder joint is depicted in Figure 2. There are three degrees of freedom in the shoulder structure, in which the flexion/extension and abduction/adduction of the shoulder joint are driven and connected by harmonic motor. The in-swing/out-swing degree of freedom with the arm as the axis cannot be used due to the need to wear. The arc-shaped rack on the arc-shaped slide rail is meshed with the gear of the output shaft of the reducer, and the servo motor is used as a driving force to transmit the movement of the input shaft of the reducer to the arc-shaped rack, so as to realize the internal/external rotation of the shoulder joint. In order to ensure a certain motion accuracy, two sets of pulley blocks are installed on both sides of the arc-shaped rack. The function is to restrict the movement of the arc-shaped guide  
Figure 1: Shoulder joint freedom of motion. (**a**) Flexion/extension; (**b**) abduction/adduction; (**c**) internal rotation/external rotation. rail along the direction of the arc, and play the role of limit. Since the position of the shoulder joint internal/external rotation mechanism is far away from the base, the quality of the mechanism will have a great impact on the control during the movement. After repeated experiments, it is determined that the arc joint rack is used to achieve the shoulder joint internal/external rotation movement. Because the curved rack needs to cooperate with the pulley set, the driving gear is not allowed to directly mesh with the curved rack in the assembly space. To this end, we have added a passive gear between the active gear and the curved rack, which can reduce the output torque of the motor reduces the quality of the mechanism and the structure is more compact and beautiful. The transmission mechanism of the shoulder joint internal/external rotation mechanism is active gear-passive gear-arc rack, where both ends of the arc racks are provided with shoulders. The rack gear meshes. Once it exceeds the rehabilitation range, the passive gear will be blocked by the shoulder and cannot continue to move, which ensuring the safety of the patient and avoiding secondary injuries to the patient. The structure uses gear transmission instead of belt transmission, with high transmission accuracy, fast response speed, stable transmission and strong bearing capacity.