# Research on the Posture Control Method of Hexapod Robot for Rugged Terrain
## 1 Introduction
[PERSON] [26] analyzed the leg configuration of the foot robot and found that the reasonable distribution of foot end force and joint torque can minimize the energy consumption of the standing posture. [PERSON] et al. [27] transformed the freedom redundancy problem into an LP solution and proposed a force distribution algorithm to reduce the internal force in the tangential direction. [PERSON] [28] used the quadratic programming method to determine the optimal foot force and corresponding joint torque of all legs. The problem of foot force distribution is related to the performance of multi-legged robots, which is of great significance to the improvement of walking performance. As shown in Figure 1, it is a comparison between the stick insect and the robot HITCR-II model. The robot HITCR-II uses stick insects as a bionic prototype. Stick insects have a typical form and gait suitable for crawling. The limbs of stick insects are distributed in an elliptical shape, which not only increases the stability of movement, but also reduces the possibility of collision between legs during walking. Hexapod robot HITCR-II has 18 joints and complex movements. As shown in Figure 2, \(\sum_{O_{0}}\) is the reference coordinate system based on the ground, and \(\sum_{O}\) is the trunk coordinate system, the origin of which coincides with the geometric center of the trunk. The X, Y, and Z coordinate axes meet the right-hand helix law, the X axis is consistent with the forward direction of the robot, and the Z axis is perpendicular to the torso surface and opposite to the direction of gravity. The three left legs of the robot are represented by L\({}_{1}\), L\({}_{2}\), and L\({}_{3}\) respectively, and the three right legs are represented by R\({}_{1}\), R\({}_{2}\), and R\({}_{3}\), respectively. Starting from L\({}_{1}\), the robot is arranged in a counterclockwise order. \(\sum_{O_{m0}}\)\((1\leq m\leq 6,\ m\in Z)\) is the leg coordinate system. In Figure 3, \(\sum_{O_{mm}}(1\leq n\leq 3,\ n\in Z)\) is the coordinate system of the n-th joint of the m-th leg. The coordinate axis is consistent with the direction of the joint axis, and the forward direction and the opposite direction of gravity are positive. The Y\({}_{m}\)-axis is connected to the lower joint of the joint. The axis of one limb coincides and points to the next joint. The foot end coordinate system is represented by \(\sum_{O_{m4}}\), and its coordinate axis direction is consistent with \(\sum_{O_{m3}}\). Figure 1: Insect robot and hexapod robot HITCR-II. Figure 2: The reference coordinate system and the trunk coordinate system. The main parameters of the robot HITCR-II are shown in Table 1. The weight is 3.2 kg, the length and width of the trunk are 320 and 136-196 mm respectively, and the longitudinal and transverse spans under the initial pose are 400 and 380 mm, respectively. Compared with hexapod robots with the same performance at home and abroad, the robot has obvious advantages of miniaturization and lightness. In order to reduce the volume and weight of the joint, a joint design scheme based on brushless DC motor drive, harmonic reduction, and synchronous belt drive was developed. The final structural parameters of the realized joint are 58.5 mm long, 42 mm wide, 28 mm thick and 118 g heavy. The base joint angle range is \(-45^{\circ}\leq\alpha\leq 45^{\circ}\), base-femoral joint angle is \(-90^{\circ}\leq\beta\leq 60^{\circ}\), and femoral-tibial joint angle is \(0\leq\gamma\leq 120^{\circ}\). Figure 4 is the structure of modularized joint. \begin{table}
\begin{tabular}{c c} \hline \hline
**Name** & **Parameter** \\ \hline Total weight & 3.2 kg \\ Length of the torso & 320 mm \\ Width of the torso & 136–196 mm \\ Length of the leg & 310 mm \\ Longitudinal span under initial pose & 400 mm \\ Lateral span under initial pose & 380 mm \\ Height & 180 mm \\ Maximum speed & 0.18 m/s \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The structure parameters of the robot. Figure 4: The structure of modularized joint. Figure 3: Joint coordinates of legs.