# Design of a Bionic Spider Robot with a Two-Degrees-of-Freedom Leg Structure and Body Frame
## 2 Analysis of Spider Locomotion Behavior
### Gait Analysis
#### 2.2.2 Triangle Gait  
The spider-like robot design process offers two options: a six-foot robot and an eight-foot robot. In comparison, the former is arranged in a uniform and surrounding manner, the load-bearing uniformity of each leg is better, the movement flexibility is better, and the control difficulty and cost are reduced. The latter has a more complex structure and more support points but is heavier. Therefore, we chose the six-legged bionic spider robot as the design object.  
By imitating the triangular gait of insects, the six-legged robot can complete movements such as walking and crossing obstacles on complex terrain, while having high mobility and stability. This kind of robot with triangular gait has good symmetry and simplicity, which can reduce the complexity of control requirements and improve operation  
Figure 1: (**a**) The structure of a spider body; (**b**) the structure of a spider leg.  
efficiency. [PERSON]. [38] and [PERSON] [39] divide the six legs into two groups to form two triangular brackets. When all the legs of one triangular bracket are lifted at the same time, the three legs of the other triangular bracket remain stationary to support the body. In this process, muscle contraction drives the body forward and causes the body to rotate slightly with the fulcrum as the center of rotation. Then, the other triangular bracket repeats the previous action, and the two groups of brackets rotate with each other to achieve forward movement. Its walking trajectory is a zigzag curve. The specific process of walking is shown in Figure 2. In Figure 2a, 2-4-6 legs are lifted and moved forward, completing the specified movement as indicated in Figure 2b, with the 1-3-5 legs lifted up and moved forward, completing the movement as shown in Figure 2c.  
Placing the center of gravity within the triangle ensures the stability of a support triangle with three different legs. However, the shape of the triangle, like isosceles triangles with a vertex angle of \(65^{\circ}\) or \(60^{\circ}\), or irregular triangles, also influences the stability of the biomimetic robot. Three equilateral triangular features that can achieve center coincidence represent the highest level of stability. As long as the robot maintains this triangular state while walking, its overall stability will be optimal. However, due to the irregular terrain characteristics of disaster areas, jungles, cliffs and other actual terrain, the robot is difficult to control when landing, and cannot form the above equilateral triangle state. In this paper, MATLAB software is used to simulate the center of gravity of the triangle, and the change in center of gravity with the structure parameters is analyzed, finding that the change occurred due to the different position of the foot. We use this information to communicate with the control program to adjust the stability of its overall center of gravity, ensuring that the bionic spider robot has enough stability during the process of moving forward, and can adjust various gestures for different situations. The following chapters will deeply analyze the leg structure and body structure of the legs and body of the bionic spider robot.