# Hardware Development and Safety Control Strategy Design for a Mobile Rehabilitation Robot
## 2 System Design for Mobile Rehabilitation Robot (MRR)  
In this study, we present a novel compact MRR for over-ground natural gait rehabilitation for low muscle tone and disabled lower limb muscle. The overall design concept and the photos of the MRR are shown in Figure 1. The system prototype photo of MRR shown in Figure 1 was taken with healthy subjects wearing it. This MRR comprises a pair of lower-limb exoskeleton systems (LES) and an autonomous mobile bodyweight support system (AMBSS). MRR provides two kinds of assistance solutions, including (a) an AMBSS and (b) an AMBSS and LES. These two independent devices can work in unison. AMBSS is a differential drive vehicle whose movement speed is regulated according to the gait training cycle set by the LES. A LiDAR is located at the front of AMBS. The LiDAR can detect obstacles within an angular range of 240\({}^{\circ}\) at a distance of 4 m with a resolution of 0.36\({}^{\circ}\). AMBS is designed with three user-facing ultrasonic sensors, each covering an angular range of 15\({}^{\circ}\). As mentioned above, the sensor measures the distance between the user and AMBS and the feedback to the embedded system to keep a safe distance of 0.2-0.4 m. The participants' weight, percentage of weight offload, gait training cycles, and data stored can be entered using the human user interface (HUI). Embedded systems enable the coordination of information from peripherals, such as LES, ultrasonic sensors, motors, linear actuators, and load cells. Two buttons on the AMB control the movement of the AMB. The left and right wheels of the AMBS are governed by the left and right armrest buttons, respectively. If both buttons are pressed at the same time, the AMBS will move forward according to the gait speed set by the LES. The emergency button is installed above the right armrest. When this button is pressed, the AMBS and LES stop immediately. The LES provides drive assistance for hip and knee joint movement. Its principal function is additional support for the subject's lower limbs and provides extra torque to their joints. The LES also leads the swinging lower limb through a specified cycle time of joint trajectory and avoids flexing the lower limb in standing. The LES uses four brushless motors and four harmonic drives to track gait trajectory, including knee and hip joint motions in the sagittal plane. When a participant wears the LES, the harness, and grips the handles on the armrest, the AMBSS controls the static unloading force to the desired level. Subsequently, the MRR performs obstacle avoidance, man-machine safe distance maintenance, and dynamic weight unloading force control through AMBSS. Simultaneously, the LES guides the patient to walk in the environment according to the set gait training cycle. Allowing patients to wander around in the environment can avoid loss of adherence due to the boredom of walking on a static treadmill. Therefore, our MRR can provide \"corrective, not simple repetition\" training according to gait with different cycles because it allows gait training to be set with different gait cycles. The LES is a module in MRR; it provides actuated assistance to perform the extension and flexion of human hip and knee joints in the sagittal plane and implement the gait locomotion of the lower limb. It can also perform a prerecorded walking pattern while recording the angular position of four actuated joints (knees and hips). The LES can be adjusted with respect to width and height to match the user's characteristics (1.55 m to 1.85 m for altitude and up to 120 kg for weight), covering more than 95% of the target population [13]. The limiter switches are designed for each joint of LES to limit the range of motion for safety. The LES is walking assistance that provides the patient with CPM of the natural gait on the over-ground; it can generate different cycles of gait training modes according to the gait trajectory of healthy people.