## 1 Introduction  
The high incidence of stroke [1] and the recent trend toward developing rehabilitation robots have led to the development of several types of rehabilitation robots [2]. Over the past few years, there have been considerable improvements in neurorehabilitation robotics. However, not many types of neurorehabilitation robots have entered the developmental stage for large-scale randomized controlled clinical trials, nor have they been widely commercialized. Part of the reason for this would be the regulation issues pertaining to medical devices; however, a more important factor could be a lack of sustainable motivation to practically achieve sufficient \"task-specific high repetition\" throughout the long-term rehabilitation process [3; 4; 5]. From a clinical perspective, it is undeniable that more task-specific repetition of the paralyzed extremity would lead to better recovery in patients with limb paralysis caused by central nervous system (CNS) injuries or disorders. In fact, most of the rehabilitation robots focus on providing high repetitions and maintain motivation by applying virtual reality or gamification factors, such as providing disturbance during desired movements [6; 7; 8; 9; 10]. Robots that are developed as assistive devices, which interact with real objects, would primarily provide ultimate task-specific movements [11; 12; 13; 14]. In general, electromechanical devices and robots for upper limb rehabilitation are classified into three major categories: Grounded end-effector robots, grounded exoskeletons, and wearable exoskeletons [8; 15]. Grounded end-effector type robots are relatively simple in structure. These robots usually let the patients hold a handle with their hands and the handle generates power according to its trajectory and direction. The joint of the robot does not correspond with human anatomical structure; therefore, there are various types of end-effector robots [16]. InMotion is the most representative type of the end-effector type robot [17]. End-effector robots are generally not in a wearable form; therefore, they are mostly fixed or grounded in one space and are mainly used in the clinics. Grounded exoskeletons have a structure in which the robot joints correspond to human joints [16]. These types of robot are generally large and expensive, and they are usually fixed in space, which makes them only usable in occupational therapy rooms in hospitals. Armeo series exoskeleton robots are the most representative exoskeletons [18]. Wearable exoskeletons are similar to grounded exoskeletons in structural concept, however, they require a light-weight to be portable. This type usually falls into the assistive device category and include a robotic interface, mostly in hand exoskeleton form, for interaction with real objects. Examples of this type are Gloreha and Hand of hope [13; 19]. Another type of upper limb rehabilitation robot could be arm support devices [10; 20; 21]. While it is not clear whether either type of rehabilitation robot is better than another, numerous systematic reviews and meta-analyses showed conflicting results on the efficacy of robot-assisted arm rehabilitation [2; 15]. There is growing evidence that robot-assisted training improves both muscle strength and functional abilities, however, whether the amount of increase in outcome measures is clinically significant remains questionable, especially when considering cost-effectiveness. Regardless of the robot type, the most important reason that the efficacy of robot-assisted training is currently not as high as expected [2] seems to be that it simply does not provide a sufficient amount of task-specific repetition [22]. Many factors contribute to this limitation, including the patient's medical status, functional status, socioeconomic factors, hospital accessibility, insurance policies, etc.