# Modeling of Walking-Gait Parameters and Walking Strategy for Quadruped Robots
## 2 Materials and Methods
### Analysis of the Movement Law of the Biological Dog  
This article used an adult Labrador Retriever as the object to capture the motion of a biological dog. The experiment was conducted indoors. To ensure sufficient time for motion capture, two LED lights were used indoors for lighting, as shown in Figure 1, for the experimental images of the biological-dog motion capture. This experiment selected a Labrador dog weighing 25 kg. When standing, the length from head to coccyx was 660 mm. The height from the middle of the body to the foot was 460 mm. The body width was 210 mm. For ease of labeling, we chose a pure black Labrador dog. The experimental steps were as follows:  
1. Two weeks before the experiment, the dog trainer trained the Labrador dog on a treadmill until it could move freely on the treadmill. 2. White circular paper pieces with a diameter of 40 mm were used as marking points and pasted on the Labrador dog, marking 14 points. There were four points in the front limb, namely, the foot end, wrist joint, elbow joint, and shoulder joint. There were four points on the hind limb, namely, the foot end, ankle joint, knee joint, and hip joint. The back was divided into 4 sections and labeled with 5 points: S1, S2, S3, S4, and S5. There was a point in the center of the side of the body. 3. The treadmill speed was set to 2 k/h, and the dog trainer directed the movement of the Labrador dog. After the Labrador dog was adjusted, the treadmill accelerated to 3.5 km/h. 4. After the Labrador dog stabilized at a speed of 3.5 km/h, a high-speed camera, model FASTCAM Mini UX100, was used to capture a motion video at 500 Hz in this state. 5. Experimental steps (3) to (4) were repeated and sports videos were shot at four speeds: 4.0 km/h, 6 km/h, 6.5 km/h, and 7 km/h. During the experiment, it was found that the range of change in each joint angle was significantly greater when the movement speed was high than when the speed was low. In order to determine the joint-angle range of the bionic-robot dog, the change in the joint angle of the biological dog when the speed was high was analyzed. Figure 2 shows the change in each joint angle at 7.0 km/h. From Figure 2, it can be concluded that:  
1. The change trend of the three joints of the hind limb was similar to that of the three joints of the front limb. 2. The change trend and angle range of the knee joint and ankle joint of the hind limb were basically the same. 3. The back angle did not change much during the exercise. 4. The variation range of the hip-joint angle was [150\({}^{\circ}\), 160\({}^{\circ}\)], and the variation ranges of the knee-joint and ankle-joint angles were [90\({}^{\circ}\), 130\({}^{\circ}\)]. Figure 1: Experimental image of biological-dog motion capture. The biggest difference between the motion of a quadruped robot and that of a wheeled robot is that the forward and backward movements of a quadruped robot follow a regular gait. The four limbs of the robot need to be coordinated to ensure smooth movement. Without regular movement, it is difficult for the body of a quadruped robot to maintain balance. However, the motion of wheeled robots does not require this consideration of maintaining balance. As long as the ground is level, wheeled robots can maintain balance. Therefore, gait pattern is a unique requirement of quadruped robots. Gait refers to the regular movement process of a robot's limbs continuously switching from state A to state B within a fixed motion cycle. Through the coordinated movement of four limbs, the robot's body can move smoothly forward or backward. The focus of this article is on the walking and jogging states of robots. The walk gait is a static step, which is the most common walking step in the movement of quadrupeds. The movement is characterized by a certain regularity in the movement of the quadrupeds.