# Hardware Development and Safety Control Strategy Design for a Mobile Rehabilitation Robot
###### Abstract
However, this kind of body-weight-supported treadmill gait rehabilitation is non-ecological training. Patients can only perform gait training at a controlled speed on the treadmill and cannot move freely. Therefore, posture, dynamic balance ability, and proprioception are not adequately trained in these devices. The bodyweight-supported treadmill gait rehabilitation requires that all patients receive the same gait training and does not satisfy the gait pattern of individual patients [1]. These drawbacks have resulted in an alternative approach to over-ground gait training. Over-ground and treadmill walking differ with respect to joint kinematics, muscle activation, gait parameters, and body coordination [11]. [PERSON] et al. showed that patients recover quickly if they frequently train in their daily environment [12]. Integrating a lower limb exoskeleton and a mobile robot can mimic a natural walk on the over-ground [13; 14; 15; 16]. The integration of over-ground gait training with the function of obstacle avoidance provides balance exercises to users [17]. However, most current gait training systems must use a treadmill for training, although it cannot simulate a functional environment. Over-ground natural gait training systems (ONGS) allow patients to walk over-ground and give patients the confidence and ability to perform daily activities. A BSS is essential for the ONGS. The adjustable bodyweight unloading force of the BSS increases the complexity and intensity of gait training. However, few mobile-type BSSs are designed explicitly for over-ground natural gait training. Therefore, over-ground walking rehabilitation systems were developed [18; 19; 20; 21] for lower-limb rehabilitation that allows patients to have higher freedom of locomotion. [PERSON] et al. developed a gait training system, \"NaTUre-gails\", which comprises a mobile vehicle, a lower extremity exoskeleton, a BSS, and a pelvic adjustment mechanism [18]. The \"Andago\" robot consists of two units [19], a four-wheel mobile vehicle and an independent passive body support mechanism. However, Andago does not have a lower extremity exoskeleton to aid over-ground gait training. [PERSON] et al. developed a differentially steered mobile vehicle called \"Where Robot\", which follows the subject's motion and adjusts the subject's center of gravity to reduce the bodyweight [20]. [PERSON] et al. developed the \"WalkTrainer\", which applies a lower-extremity exoskeleton to control motion for the subject's hip and knee, allowing people with paraplegia to walk on the ground [21]. However, the systems mentioned above still present several drawbacks. For example, these mobile platforms cannot perform functions of obstacle avoidance and navigation and cannot walk along ground corridors. In addition, in this training type, the lower limbs of the patient must be tightly connected to the robotic system for rehabilitation training. However, the conventional position controller, i.e., the proportional-integral-derivative (PID) one, widely used in limb rehabilitation systems, cannot adequately guarantee safety during the training. Therefore, an MRR prototype was developed in this study for passive over-ground gait training and to present a safety controller to compensate for various uncertainties of MRR and external load disturbances, thus providing safety to the patient. In addition to control accuracy, the control of human-centered limb rehabilitation robots also needs to focus on human safety and compliance. The patient may feel unsafe or may hurt themselves again if vibration or transient overshoot happen during the tracking response. Therefore, the controller of the rehabilitation robot needs to have over-damped characteristics to increase the shock resistance ability and avoid harmful situations occurring. With several differences among patients, such as weights, heights, and body appearance, the controller design needs to be capable of adapting to these variations.