# Design of an Active and Passive Control System of Hand Exoskeleton for Rehabilitation
## 1 Introduction  
Stroke often results in a combination of cognitive, sensory, and motor impairments. Now, it has become one of the main diseases threatening human survival and health [1; 2]. The most common impairment of stroke is hemiparesis, which results in dyskinesia of some parts of the body. It reflects not only in the upper and lower limbs, but also in the loss of motor function of the hand [2]. It was verified by rehabilitation medicine that exercises with high strength and repeatability can stimulate cortical layer to recombine to help the stroke patients learn to control the motion again [3]. The application of robotics and related technologies has greatly promoted the development of clinical techniques [4; 5; 6]. Recent researches have shown that exoskeleton devices are feasible and effective for hand rehabilitation [7; 8]. Hand is one of the most important limbs of humans, and the rehabilitation of hand motor function can be assisted by exoskeleton robots [9; 10; 11]. Many dexterous and advanced mechanisms of hand exoskeletons have been studied and developed. But not all of them are developed for hand rehabilitation. For example, some of them are designed for master-slave systems [12; 13] and some others are designed as force-feedback devices [14]. They are limited in the number of independently actuated degrees of freedom. Their mechanism design ideas can be used as a reference for the study of the hand rehabilitation exoskeleton [15; 16; 17; 18; 19]. However, due to the special rehabilitation application, we need to modify the mechanism to make sure that exoskeleton and each finger joint have the same center of rotation, which avoids secondary injuries when the exoskeleton has direct contact with the hand [20]. Control system and methods study of the exoskeleton manipulator are also important research categories. The most popular control system of rehabilitation training is a closed loop control system with sensors [20], such as force sensors [21; 22] and electromyography (EMG) sensors [23]. The most commonly reported rehabilitation modes provided by developed rehabilitation exoskeletons are the continuous passive motion (CPM) and the active assisted movement (AAM). Some exoskeleton systems are designed for passive rehabilitation mode based on CPM [24]. The passive mode is useful for preventing muscle contractures in early rehabilitation stage. But its effectiveness in the mid-to-late stage is limited when patients can initiate movement but have difficulty in completing it. Some other exoskeleton systems are designed for active rehabilitation control mode based on AAM [25]. The active mode could make up the shortcomings of passive mode in mid-to-late rehabilitation stage. Nowadays, most systems have only one of the modes mentioned above, which is not suitable for the whole scale of rehabilitation, especially the middle rehabilitation stage when patients have some residual motion capability but have not yet reached the normal level. Therefore, if there were two kinds of rehabilitation modes that could switch actively according to patients' rehabilitation status, it can promote hand rehabilitation more effectively. In addition, the rehabilitation exercises in a virtual environment will be more interesting. It will induce the patients to take the exercise more actively [26; 27]. The patients will be more desired to take the exercise so the effect of rehabilitation will be better, such as Hand Mentor \(\text{Pro}^{\text{TM}}\) (Motus Nova, Atlanta, GA, USA) [28]. But because the rehabilitation interface of this kind of device mainly adopts hardware system based on PC or dedicated closed hardware system, it is too expensive, not open and not portable [29]. In this paper, a new kind of hand exoskeleton system has been introduced. The introduced mechanism can adapt to fingers with different thicknesses and lengths and prevent the secondary injuries. Hardware system and human-robot interactive rehabilitation software based on Android system are also designed.