# Modeling of Walking-Gait Parameters and Walking Strategy for Quadruped Robots
## 2 Materials and Methods  
### Analysis of the Movement Characteristics of the Biological Dog  
One of the focuses of quadruped robot research is its gait. Different gaits correspond to different movements of the robot [16]. A good gait does not only improve the stability of the quadruped robot, but also minimizes the fluctuation of its energy consumption, and, therefore, energy can be optimized [17]. As a result of evolution, organisms have achieved a high degree of conformity with the environment in their shape, structure, and movement patterns [18]. The current state of evolution can be said to be the optimal state in the biological environment. The bionic quadruped robot imitates the creature in shape and movement, so that the robot can approximate the movement of the creature in the same biological environment [19]. Since the quadruped robot is moderate in size and easy to control, its movement can be well observed [20].  
The research object of this paper is a bionic-robot dog. Compared with bipeds, quadrupeds are more stable. When subjected to lateral force or moving on uneven and rough roads, the biological dog can quickly adjust the position of its center of gravity to bring the body into a state of balance. Compared to robots with hexapods and above, quadrupeds have simpler locomotion patterns. Furthermore, the biological dog is moderate in size, easy to domesticate, and can carry out various exercise experiments. When designing a bionic-robot dog, imitating the shape and movement of a biological dog can improve the environmental adaptability of the robot and make its movement more reasonable. To study the bionic-robot dog, the structural characteristics and motion laws of the biological dog should be studied first, so as to provide a basis for the mechanism design.  
An important feature of the bionic robot was that the body structure of the robot was designed to imitate the biological structure in nature. In the process of designing the limb structure, we referred to the limb structure of the dog. The quadruped robot needed to move forward and backward like a living creature. For a series of actions such as steering, each limb must have several freely rotatable joints. Each limb of the robot in this paper was designed with 2 active joints and 1 passive joint. The two active joints were located in the thigh limb. One active joint that could rotate 180 degrees was at the connection with the calf, and another active joint that could rotate 180 degrees was at the connection between the thigh and the torso. The passive joint was for passive adjustment when the foot of the robot was in contact with the ground. After completing the design of the joints of the robot's limbs, the design of the posture relationship between the torso and the limbs of the robot was carried out. Referring to the two-symmetrical model of the quadruped limb structure in bionics, usually the two front limbs of a robot have the same size and structure, and the two hind limbs have the same size and structure. Therefore, it was only necessary to determine the structure of the front and hind limbs.  
After the above analysis, the robot limbs in this paper were designed with a total of 12 joints, including 4 active joints at the hip joints and 4 joints at the knee joints, and 4 passive joints at the feet. For each limb, the 2 active joints of the knee joint and the hip joint controlled the forward and backward movements of a limb of the robot and the movements of raising and dropping the limb, respectively. The passive joints were used to simulate the impact of the soles of the feet when the limbs land. The topological structure of the robot suggested a limb structure with better symmetry and more convenient motion analysis. Then, we processed the video captured of the biological dog's motion. Using Fastcam Analysis in the video-processing software PHOTON 1.0 to extract the position coordinates of marker points, we analyzed and processed the extracted coordinate data in MATLAB R2021a.