# FSM-HSVM-Based Locomotion Mode Recognition for Exoskeleton Robot
## 2 Materials and Methods  
### Exoskeleton Structure and Data Collection  
The Beijing Institute of Technology developed a new type of lower limb exoskeleton robot powered by motor drive with a maximum load capacity of 60 kg. This exoskeleton robot mechanism closely resembles the structure of human lower extremity. The length of exoskeleton rods such as the thigh and lower leg can be adjusted based on the subject's body shape to accommodate different subjects. Instead of focusing on a ball-and-socket structure, the BIT exoskeleton designs the three degrees of freedom at the hip joint independently to reduce interference between the lower limb exoskeleton and human movement and to improve wearing comfort. Additionally, the knee joint is designed as a hinged structure hinged around the coronal axis, which is simpler and more reliable. After rigorous research and demonstration, the structure of the lower limb exoskeleton robot of BIT can meet the design requirements. The total mass is lighter and more flexible compared to the hydraulic exoskeleton [26]. Key exoskeleton components incorporate elastic units and limit devices for the comfort and safety of human wear. The mechanical system of the exoskeleton consists of a bionic back frame, hip assist mechanism, knee assist mechanism, and lower limb support mechanism. The back frame adopts the split design of human bionics and conforms well to the human body, allowing it to be worn by personnel of a specific height and body shape. The hip assist mechanism uses a direct drive motor to assist the body in sagittal flexion, while the knee assist mechanism uses a rope wheel structure to assist the knee joint. Not only should the lower limb support mechanism connect to other mechanisms as a support, but it should also be responsible for transmitting gravity to the ground.  
As shown in Figure 1, the movement data of lower limbs are collected by plantar pressure detection system and inertial measurement units. After collecting the motion information of each component, it is transmitted via the RS485 bus to the arm main control computer to complete the exoskeleton robot's driving and control. The data transmission frequency is 100 Hz, and the core chip utilizes a Samsung S5P6818 development board (ARM Cortex-A53 architecture), 2 GB RAM, 16 GB EMMC. The plantar pressure detection system records stress data using four force sensitive resistors (LOSON LSH-10) placed in the sole and heel of shoes. Each force sensitive resistor (FSR) has a 200 kg measurement limit and an overall accuracy of 0.5% full scale (FS). The inertial measurement units (IMU JY901) are positioned in the middle of the exoskeleton's thighs and shanks to collect lower limb motion data. IMU YY901 could read the three-axis acceleration, three-axis angular velocity, three-axis magnetic field and other original sensor data, then calculate the three-axis attitude angle in real-time using the Kalman filter algorithm and the dynamic decoupling algorithm. As human walking is parallel to the sagittal plane, only the Z-axis rotation angle is required. The measurement range is \(-\)180-180\({}^{\circ}\), and the dynamic angle resolution is 0.1\({}^{\circ}\). The hip joint is the angle between the vertical and the thigh directions, whereas the angle of the knee joint is the angle between the thigh and the shank.