# Hardware Development and Safety Control Strategy Design for a Mobile Rehabilitation Robot
## 2 System Design for Mobile Rehabilitation Robot (MRR)
The LES adopts a forced standing posture, which forces the knee and hip joints to be at 0 degrees, for the start, stop, and end motion during the gait trajectory. The AMBSS must support patients of similar size and weight to those allowed by the LES. To increase safety, the AMBSS uses a medical-grade harness (from the waist to the perineum) to take the weight of the user and the LES. This configuration increases the balance and stability of the lower body of the patient during gait training. The AMBSS allows users to walk around freely with reduced weight; it integrates with the LES and also serves as the carrier of the controller, power source, and any other MRR electronic components. It is actuated by DC 24 V motors and linear actuators to perform the functions of obstacle avoidance navigation and dynamic bodyweight unloading. The AMBSS has a wheeled base with two frontal driving wheels and uses two DC motors to drive two active wheels and uses two passive universal wheels to maintain balance. Each active wheel is independently controlled. Therefore, it can move following a straight or curved path. The maximum speed of AMBSS is designed to be 0.7 m/s, in which the operating speed of AMBSS can be set to be consistent with LES at 0.2-0.5 m/s according to the gait cycle requirements of rehabilitation. The embedded system is used to implement the control algorithm and signal processing. The software of AMBSS runs in a real-time environment on an embedded system and is programmed using LabVIEW. To match the propulsion speed of AMBSS with LES, it is important to measure the distance between the patient and the AMBSS. The AMBSS uses three ultrasound sensors to measure the distance between the user and the AMBSS, and each sensor covers an angular range of 15 degrees with a resolution of 0.3 cm. The ultrasound sensors are placed in the middle of the central crossbar, pointing to the user's waist. The measured distances are sent as feedback to the embedded system to regulate the AMBSS speed. AMBSS acts as a slave subsystem to the LES (master), adapting the AMBSS's speed to the user's pace and maintaining a safe distance of 200-400 mm. A webcam mounted in front of the AMBSS captures the front view. The AMBSS exchanges messages with a human user interface (HUI) over Wi-Fi. A LIDAR is set in front of the AMBSS. It detects objects within 4000 mm with an angular range of 240\({}^{\circ}\) and a resolution of 0.36\({}^{\circ}\). Besides the automatic obstacle avoidance navigation mode, AMBSS can also be controlled by two buttons on the armrest; the button on the right handle controls the right wheel motor, and the button on the left handle controls the left wheel motor. Simultaneously pressing both buttons, the AMBSS continuously moves forward at a set speed. When the emergency button is pressed, the AMBSS stops immediately. The LES can be removed from the AMBSS, resulting in a more gradual bodyweight unloading and greater freedom of possible locomotion. The AMBSS can support up to 40% of the user's weight. For MRR operations, communication  
Figure 1: The overall design concept and prototype photos of the MRR. is performed through connections to an embedded system. The control system for the two modules (LES and AMBSS) was developed separately. The AMBSS moves according to the gait speed set by the LES and uses ultrasonic sensors to measure the user's distance to achieve collaborative control between the LES and AMBSS [4,11]. The MRR defines three possible regions within the acceptable range of 0 to 600 mm (near 300 mm, optimal 400 mm, and far 500 mm). The MRR-provided human-machine interface allows the users to input the desired bodyweight unloading percentage and gait cycle. AMMBS can control the patient's level of bodyweight unloading during gait training. The modular architecture of the developed system allows the implementation and validation of the control strategy on the system with ease.