I. Overall Structure of AGV
The four-steering wheel structure is a commonly used wheel system structure for omnidirectional AGVs. Its overall structure adopts a four-steering wheel structure, and the AGV's straight-line movement, translation, turning and obstacle avoidance are achieved by the rotation angle and speed of the four steering wheels. The overall structure of the four-steering wheel AGV is shown in Figure 1.
Figure 1 Overall structure of the four-steering wheel AGV
Four-steering wheel distribution: In this case, steering wheels are distributed at all four corners of the AGV body, enabling the AGV to move in all directions with any radius, directly improving the AGV's carrying capacity. However, its motion model is complex, and the motion control algorithm is difficult, making it a current research hotspot for four-steering wheel robots. The four-steering wheel omnidirectional AGV actuator has strong wear resistance, good passability, and superior load-bearing capacity, making it more suitable for heavy-duty applications, mainly used in factory workshops, logistics warehouses, etc.
II. Four-steering wheel chassis structure
As shown in Figure 2, the four steering wheels of the AGV mobile chassis system serve as both drive and steering wheels. Each steering wheel is controlled by two motors: one motor drives the vehicle, and the other controls its steering. This means four drive motors generate traction, and four steering motors control the wheel angles. This not only optimizes the AGV's design layout but also significantly improves its driving capability. The chassis system exhibits high mobility while ensuring good traction and stability. However, due to its complex mechanical structure and its perceived strong coupling, nonlinearity, and redundant dynamic characteristics, the motion control of this chassis system presents significant challenges. The four-steering wheel chassis layout is relatively complex, and improper design can affect the AGV's ability to navigate uneven surfaces. For example, when traversing uneven surfaces, the drive wheels may become suspended, leading to insufficient driving force and slippage; when traversing raised surfaces, the drive wheels may experience a sharp increase in load, causing the current to exceed the alarm current and resulting in operational malfunctions.
Figure 2 Four-steering wheel mobile chassis
The four-steering-wheel omnidirectional AGV benefits from its unique chassis structure, enabling omnidirectional movement and maximizing its mobility. However, the complex wheel system presents challenges for motion control. The four-steering-wheel AGV can be configured with multiple motion modes, allowing it to move longitudinally, laterally, diagonally, rotate in place, and turn forward and backward, as shown in Figure 3. In practical applications, a switching controller can be designed to allow switching between these motion modes, thereby improving maneuverability and work efficiency.
Figure 3. Movement mode of four-steering wheel AGV
III. System Composition and Navigation
The four-steering wheel AGV consists of a mechanical body, an electrical control system, and a sensor detection system. This paper selects magnetic navigation as the AGV's navigation method. The selected magnetic sensors are located at the center of the front and rear of the chassis, as shown in Figure 1. The electrical control system uses a dedicated AGV controller as the main controller. It controls the driver via a bus protocol to achieve steering and speed control of the steering wheels, enabling the AGV to follow the magnetic strip. Two magnetic navigation sensors are installed at the front and rear. These sensors read the deviation of the magnetic strip relative to the sensor. The main controller obtains data from the sensors, performs calculations and analysis, and ultimately controls the AGV's direction and speed, allowing the AGV to automatically and accurately follow the magnetic strip. When the AGV is moving straight, the front and rear magnetic sensors are activated, with the sensors facing the center of the magnetic track, as shown in Figure 4. When the AGV deviates from the magnetic strip track, it feeds back the relative offset to the main controller. The main controller corrects the trajectory deviation based on the relative offset, achieving navigation control of the moving AGV.
Figure 4 Schematic diagram of magnetic navigation sensor operation
