## 1 Introduction  
The number of young patients with functional impairment of the upper limbs caused by stroke has increased rapidly, as influenced by accelerated pace of life, poor lifestyles and environmental factors [1, 2]. Limb movement disorder, which is caused by hemiplegia after stroke, not only reduces the quality of life of patients, but also brings great pain to their physiology and psychology. Effective rehabilitation training can improve the defect of patients' nerve function and maintain the degree of joint activity; it also prevents joint spasms and enhances the final rehabilitation degree of patients' motor functions significantly [3]. The traditional rehabilitation training is one-to-one auxiliary exercise for patients by therapists. This method is difficult to develop an effective treatment plan, and it is tough to control accurately [4]. With the development of rehabilitation robot technology and rehabilitation medicine, the rehabilitation robot has become a novel motor nerve rehabilitation treatment technology. It is of great significance to take advantage of rehabilitation robot technology for rehabilitation training to the recovery of limb function of stroke patients [5]. The traditional methods of treatment, which are based on the therapist's clinical experience, have the problems of large staff consumption, long rehabilitation cycles, limited rehabilitation effects, and so on. The research and application of rehabilitation robot system is expected to alleviate the contradiction between supply and demand of rehabilitation medical resources effectively, and improve the quality of life of stroke patients [6; 7]. The upper limb rehabilitation robot can be divided into two types according to the structural form: terminal traction and exoskeleton type. The terminal traction type mainly provides the rehabilitation training of plane movement. However, the exoskeleton type extends the rehabilitation training range from plane to three-dimensional (3D) space, which can assist the affected limb to complete the rehabilitation training in 3D space. The exoskeleton rehabilitation robot generally drives the movement of the patient's limbs through the auxiliary device (also known as exoskeleton mechanical structure). The structure of the auxiliary device is similar to the skeleton structure of the human limbs. During the training, the patient's limbs and the corresponding parts of the auxiliary device are bound together, and the connecting rod of the auxiliary device swings around the corresponding joint, so as to bring the moving limbs into motion. It can make the patient's limbs train in different postures through controlling the trajectories of power-assist device. At present, the structural design method of exoskeleton rehabilitation robot is one of the hot issues in the research of rehabilitation robot. Owing to different mechanical structures and rehabilitation principles, a variety of exoskeleton rehabilitation robots are developed, e.g., a dynamic exoskeleton system ADEN-7 robot with 7 degrees of freedom [8], an ARMIN robot with six degrees of freedom (four active and two passive) semi exoskeleton structure [9], an ARMEO robot providing arm weight reduction support system training, enhancing performance feedback and evaluation tools [10], etc. In addition, the pneumatic muscle is used as a driver to realize four degrees of freedom active auxiliary motion RUPERT robot [11], hydraulic drive robot LIMPACT [12], suspended rope drive robot CAREX [13]. After that, researchers developed and designed the upper limb rehabilitation robot based on pneumatic muscle drive, unpowered upper limb rehabilitation robot, hybrid drive upper limb rehabilitation robot and under drive exoskeleton upper limb rehabilitation robot [14; 15; 16; 17; 18; 19; 20; 21; 22].