# Design of a Bionic Spider Robot with a Two-Degrees-of-Freedom Leg Structure and Body Frame
## 3 Design and Analysis of Structure
### Design and Analysis of Leg Structure  
#### 3.1.1 Degree of Freedom Analysis  
In the design of biomimetic spider robots, the number of DOFs in the leg mechanism is crucial for their ability to travel smoothly over rough terrain and walk rapidly on flat surfaces. Mechanically, the execution of complex movements increases with the number of DOFs. For biomimetic spider robots, the ability to perfectly replicate spider walking actions allows for adapting to most environments. However, in terms of control, the increase in DOFs necessitates independent control for each leg to meet the needs of different environments, which in turn complicates the robot's control system. It is necessary to consider the coupling relationship between the control systems of the legs and deploy appropriate sensors to assist in identifying the robot's state. Both the cost and the complexity of the  
Figure 2: Schematic diagram of triangular gait operation, (**a**) initial state diagram; (**b**) motion process state diagram; (**c**) final state diagram.  
data will increase. If we combine the analysis from Section 2 with both aspects, the spider's leg movements can be simplified into two movements: swinging and lifting. The two-DOF structure is simple and effectively improves the above problems. [PERSON] et al. [40] developed a two-DOF hand prosthesis with layered control and microprocessor-based fine tuning that simplifies the operator's control requirements and has some usability. [PERSON] et al. [41] introduced a speed control system for DC servomotors that utilizes a two-DOF controller parametrization to enhance closed-loop system performance, including disturbance suppression and robustness to parameter variations, while maintaining the command input response. [PERSON] et al. [42] propose a classification framework for two-DOF mechanical systems that prioritizes the largest feedback linearizable subsystem with internal stability and highlights the significance of generalized conjugate momenta in achieving stable closed-loop systems. These studies have pointed out the advantages of two DOFs.  
According to the analysis of the triangular gait in Section 2.2.2, we call the group that provides support the support phase and the other group the swinging phase.  
The legs are divided into two structural units with 1 degree of freedom. In this configuration, structural unit I performs a lifting action on the vertical plane and structural unit II performs a swinging action on the horizontal plane. The simplified process of the legs in this study is shown in Figure 3.  
In Figure 3, the body, thigh, and shank are, respectively, labeled as 13, while the knee joint and hip joint are labeled as 24. The two-DOF mechanism is equivalent to its actuation components; therefore, actuators are placed at the hip joint and knee joint. Nonetheless, the knee joint will be subjected to forces from other directions, which can affect the motor's working load and stable torque output. In the following section, we will discuss the new mechanism developed in this study to address this issue.