## 4 Motion Simulation and Experiment of the Lunar Exploration Hexapod Robot
### Kinematic Simulation Analysis of the Lunar Exploration Hexapod Robot
#### 4.2.2 ADAMS Kinematic Simulation Analysis  
Only adding the drive and contact cannot make the robot walk; it also needs to use the STEP function in ADAMS as the drive function. After adding all the drive functions as required, set the simulation parameters: End Time = 20, Steps = 50. After the simulation, the displacement curves of the robot's centroid in the X, Y and Z directions are obtained, as shown in Figure 12. The center of the robot coincides with the displacement of its center of mass in the y-axis direction. In the same simulation time, the displacement of the robot along the y-axis direction in the traditional gait is 2235 mm, and in the inverted gait mode, the displacement is 2895 mm. The displacement distance is increased by 29.53%, indicating that the robot moves faster in the inverted gait. Figure 12b shows that the shift of the robot in the \(x\)-axis direction is less fluctuating than that of the traditional gait curve during the \"inverted gait\" movement. Figure 12c shows that the displacement of the robot in the z-axis direction in the inverted gait mode shows periodic up-and-down fluctuations compared with the traditional gait, and the fluctuation amplitude is smaller. According to the comparison of the two gait fuselages and their displacement curves in all directions, it can be found that the movement speed of the robot is increased by 29.53% in the inverted gait mode, the offset fluctuations in the left and right directions and up and down directions are smaller and the robot achieves faster and more stable movement in the same amount of time. Figure 11: Inverted gait timing diagram: The numbers indicate leg numbers. Figure 10: The “inverted gait” diagram of the lunar exploration hexapod robot: (**a**) initial state of the robot; (**b**) robot morphology at 0.5 s; (**c**) robot morphology at 1 s; (**d**) robot morphology at 2 s; (**e**) robot morphology at 3 s; (**f**) robot morphology at 4 s.  
When the hexapod robot is moving, the angular velocity of the rotation of each joint of the robotic legs reflects the rate of the robot's movement. Figures 13 and 14 show the comparison curves of the angular velocities of the hip and knee joints of the two groups of legs in the two gait states. As can be seen from Figures 13 and 14, the angular velocity of the hip and knee joints of the two groups of legs changes periodically. The maximum angular velocities of the hip and knee joints of the two groups of legs are, respectively, 60 rad/s and 120 rad/s in the traditional gait mode of the robot. In the inverted gait mode, the second group of legs swung backward within 0-0.5 s, causing the robot to assume an inverted starting posture, providing more space for the subsequent swinging motion of the robot's hip joints. Consequently, the robot's hip joints exhibited an increased swing amplitude during the swinging motion, resulting in a faster forward speed for the robot. The maximum angular velocity of the hip joints for both the first and second groups of mechanical legs was 75 rad/s. Compared to the traditional gait mode, the angular velocity increased by 25%. The maximum angular velocity of the knee joints for both the first and second groups of mechanical legs was 120 rad/s. It can be observed that in the \"inverted gait\" mode, by expanding the swing range of the robot's legs and increasing the angular velocity of hip joint movement, the robot's gait frequency and effective step length per step were improved, consequently enhancing the robot's movement speed. Figure 12: Displacement curves of the center of mass of the lunar exploration hexapod robot: (**a**) robot’s center of mass displacement along the \(y\)-axis; (**b**) robot’s center of mass displacement along the \(x\)-axis; (**c**) robot’s center of mass displacement along the \(z\)-axis. Figure 13: Angular velocity curves of robot’s hip joint rotation: (**a**) first group of robotic legs; (**b**) second group of robotic legs.