# Research on the Posture Control Method of Hexapod Robot for Rugged Terrain
## 4 Posture Maintenance Strategy for Heavy Rugged Terrain  
### Analysis of Pose Adjustment Strategy for Heavy Rugged Terrain  
Compared with keeping the torso moving steadily in the initial posture under lightly rugged terrain, when walking on heavy rugged terrain, the robot needs to adjust the posture actively according to the changes in the terrain to adapt to the changing rugged terrain more widely and ensure the leg working space has good movement flexibility. For heavily rugged terrain, the robot adopts a posture adjustment strategy to adjust the height and inclination of the torso. As shown in Figure 11a, the pose of the robot represented red lines is analyzed in coordinate system \(\sum_{O_{0}}\) based on the earth. The coordinates of the origin of the torso coordinate system \(\sum_{O}\) in \(\sum_{O_{0}}\) represent the position of the torso. When walking in complex terrain, the combined effect of posture adjustment and traveling motion makes the origin of the torso coordinate system move from \(O\) to \(O^{\prime}\). In pose adjustment, facing convex terrain, a trunk height change amount is \(\Delta H_{pz1}\), and pitch angle change amount is \(\Delta\theta_{px1}\); facing recessed terrain, b trunk height adjustment amount is \(\Delta H_{pz2}\), and pitch angle adjustment amount is \(\Delta H_{pz2}\). Walking continuously changes the position of the torso coordinate system as the benchmark of motion planning through the adjustment of trunk height \(H_{pz}\) and inclination \(\theta_{px}\), then:  
\[H_{pz}=\sum_{i=0}^{n}\Delta H_{pzi}+\Delta H,\ \theta_{px}=\sum_{i=0}^{n} \Delta\theta_{pxi} \tag{14}\]  
where \(\Delta H_{pzi}\) and \(\Delta\theta_{pxi}\) are the changes in the height and inclination of the torso during posture adjustment, and \(\Delta H\) is the change in the height of the torso caused by the traveling motion during the non-posture adjustment. The movement of the robot is determined by the trajectory of the end of the foot. Traveling movement does not produce the change of the torso inclination because the trajectory of the end of the foot is planned in the torso coordinate system. \(H_{pz}\) and \(\theta_{px}\) can approximately represent the change in altitude and slope of the current terrain from the initial horizontal plane, respectively. Pose adjustment achieves wide adaptability to gradual and rugged terrain through continuous adjustment of trunk height and inclination changes. Pose control has a clear distinction  
Figure 10: Structure of a two-loop sliding mode control system. between light and severely rugged terrain. However, in practical applications, a type of terrain is slightly rugged in a certain walking cycle, while the overall observation during a period of travel is severely rugged. The analysis of this type of terrain is shown in Figure 11b.The blue lines represent the foot track of the robot. When the origin of the leg coordinate system \(\Sigma_{O_{m0}}\) is at point A, the end of the foot swings from position a to position b, and the end point of the foot end is located in the lower guide layer of the trajectory. Thus, the terrain is judged to be slightly rugged, and the posture maintenance strategy is stimulated. From the rugged terrain adaptive foot trajectory planning this article, the end position of the foot end on the trajectory of this cycle is the starting position of the next cycle. When the foot end swings again from position b to position c, the termination point is at the upper adjustment layer, and the terrain is determined to be heavily rugged. The behavior can be expressed as follows:  
\[\Delta H_{1}<H_{LB},\;H_{LB}<\Delta H_{1}+\Delta H_{2}<H_{L} \tag{15}\]  
where \(\Delta H_{1}\) is the height of the end point of the foot end of the first cycle relative to the start point, \(\Delta H_{2}\) is the height of the end point of the foot end of the second cycle relative to the start point, \(H_{LB}\) is the height of the guide layer under the trajectory, and \(H_{L}\) is the height of the track guide layer.