# Configuration Design of an Upper Limb Rehabilitation Robot with a Generalized Shoulder Joint
## 1 Introduction  
Neuromuscular injury can lead to disability or inconvenient movements, such as stroke and spinal cord injury, which have become important problems worldwide [1]. Limb motor dysfunction is caused by a wide range of nervous system diseases, and both patients and communities bear a huge economic burden [2]. Currently, there are more than 33 million stroke patients in the world [3]. Robots play a key role in the development of rehabilitation technology [4]. The human arm can complete complex movements, such as grasping, lifting, touching the eyes and touching the back of the head, in a complex environment. This ability is due to the complex bones and abundant muscle groups of the arm [5]. Since the 21 st century, some studies on the anatomy of the human shoulder joint have shown that the rotation axis of the human glenbummeral joint drifts when it moves [6, 7]. The joint movements of the scapula, clavicle and humerus are called the shoulder humperal rhythm [8, 9]. Because the axis drift of the glenohumeral joint only occurs in the range of motion of the shoulder joint, if the motion of the shoulder blade belt is considered, it increases the complexity of the shoulder joint of the rehabilitation robot, so at present, most upper limb rehabilitation robots widely use six degrees of freedom. The representative product is Armeo Power developed by HOCOMA [10; 11]. On this basis, HOCOMA developed the Armeo Spring training evaluation device [12], which is currently the most commonly used upper limb rehabilitation robot in clinics. Additionally, the 7-DOF (Degree-of-Freedom) upper limb rehabilitation robot, CADEN-7, was developed by the University of Washington [13]. The team of [PERSON] in Italy developed the ARAMIS dual arm rehabilitation robot system [14], and the representative product in China is the SFRobot developed by Anyang Co. Shen Fang. However, 6-DOF upper limb rehabilitation robots ignore the movement of the scapula, and the joint axis does not match the movement of the upper limbs of the human body, which may lead to a joint compensation function of the patient and affect the reconstruction of the normal movement function of the upper limb of the patient. In research results involving the movement of the scapula, various types of mechanism configurations have been proposed to realize the drift of the axis of the shoulder joint movement. Canada Queen's University designed MEDARM [15], which has a 5-DOF shoulder joint composite structure. In MEDARM, the 2-DOF sternoclavicular joint and the 3-DOF shoulder joint can track the movement of the robot's shoulder joint rotation to the human body's generalized shoulder joint rotation. The dual-arm rehabilitation robot Harmony [16] developed by the University of Texas adds 2-DOF of shoulder girdle raising/lowering and extension/retraction in the shoulder joint and can adjust the swing centre of the boom on a spherical surface. LIMPACT [17], developed by the University of Twente in the Netherlands, adds two passive DOFs at the shoulder joint of the mechanism, which can realize movement of the centre of the ball and socket joint at any position on the horizontal plane. The Intelli-Arm [18], developed by Northwestern University in the United States, adds three mutually perpendicular mobile joints to the shoulder joints of the organization; of these joints, two are passive and one is active, and they can adapt to changes in the scapulohumeral rhythm. Others, including Arizona State University [19], Columbia University [20], Wonkwang University [21], Southeast University [22], North University of China [23], Beijing University of Technology [24], Shanghai JiaoTong University [25; 26], Hubei University of Technology [27], etc., have conducted research in this area. In existing research, the mechanical configuration of shoulder rehabilitation has been more in-depth, and most of the upper limb rehabilitation robots with bionic shoulder strap structure are relatively large, which is due to the introduction of the driving scapula structure.