# FSM-HSVM-Based Locomotion Mode Recognition for Exoskeleton Robot
## 2 Materials and Methods
### Feature Selection and Analysis of Motion Characteristics  
When people walk with a lower limb exoskeleton robot, a large number of experimental studies [17; 18; 19; 20; 27; 28] indicate that the joint angle of lower limb and plantar pressure undergo very obvious periodic changes. At the same time, the motion characteristics and change laws of various modes and gait phases are distinct, particularly the change amplitude, peak value, and change trend. These distinctions serve to distinguish between various motion modes. As a result, the hip angle, knee angle, and plantar pressure are utilized as input signals, and the resulting information combination is expressed as follows:  
\[Data=[\theta_{LH},\theta_{LK},\theta_{RH},\theta_{RK},F_{LB},F_{LH},F_{RB},F_{ RH}], \tag{1}\]  
where \(F_{LB}\), \(F_{LH}\), \(F_{RB}\), \(F_{RH}\) are the pressure on the ball and heel of left and right legs, and \(\theta_{LH}\), \(\theta_{RH}\), \(\theta_{LK}\), \(\theta_{RK}\) are the angle of the hip and knee joints of the left and right legs. Through a comprehensive and detailed analysis of the obtained motion signals, it is possible to extract feature information with high discrimination, which lays the foundation for the later research of the recognition algorithm. Figure 3 depicts the joint angle and plantar pressure for each of the five locomotion modes.  
Using walking on FW as an example, the toe off time is the beginning of a cycle. At this point, the swinging leg is behind the body, and the hip angle of the swinging leg is close to its minimum value. With an increase in the swing period, the angle of hip flexion gradually increases. The maximum angle of flexion of the hip joint was reached when the swing foot touched the ground during the support period. As the duration of support increased, the angle of hip flexion began to gradually decrease. At the conclusion of the support period, the hip angle reaches its minimum value. The knee joint's change law of knee joint is comparable to that of the hip joint, with the exception of the time and range of extreme values. The plantar pressure clearly demonstrates that the heel contacts the bottom first, followed by the sole and the pressure change curve is very smooth without multiple extreme points.  
Figure 2: Experiments of different locomotion modes. (**a**) Flat Walking; (**b**) Up the Stairs; (**c**) Down the Stairs; (**d**) Up the Ramp; (**e**) Down the Ramp.  
When ascending and descending stairs and ramps, the knee angle starts to decrease from its peak at the end of the swing period, and rebounds for a time during early and late support, and then decreases. The change range and peak value of hip angle are greater than those of flat walking, and the change curve of hip angle is more sinusoidal like.  
In the initial phase of descent on stairs or a ramp, the knee angle reaches the maximum and then gradually decreases. At the conclusion of the swing, the angle reaches its minimum and begins to gradually increase as the support begins. In the initial phase of support, there is a period of decline, followed by an increase to its maximum level. The hip joint's change range and peak value are the smallest. The hip angle varies from 10\({}^{\circ}\) to 20\({}^{\circ}\), especially when descending the ramp. This is because the hip joint is primarily responsible for maintaining body balance. Upon contact with the ground, the pressure on the heel and sole began. Specifically, there is a noticeable oscillation at the top of the stairs.  
Figure 3: The lower limb information under different locomotion modes. (**a**) Flat Walking; (**b**) Up the Stairs; (**c**) Down the Stairs; (**d**) Up the Ramp; (**e**) Down the Ramp.