The AGV hardware system is responsible for information sensing and motion control, and is a key factor affecting the performance of the AGV system. This paper mainly provides a brief introduction to the AGV motion control system, laying the foundation for subsequent theoretical research.
The motion control system is the core component of the AGV system, acting as its brain. The motion controller receives task commands, converts them into speeds for each motor, and then sends them to the driver to drive the motors, thereby controlling the movement of the AGV. Simultaneously, the motion controller receives feedback signals from various sensors on the AGV and analyzes and processes the received information. It makes movement decisions based on pre-stored information, the AGV's operating status, and surrounding environmental information, resulting in higher reliability, accuracy, and efficiency for the AGV. The motion control system primarily ensures the stable operation of the drive system and the AGV, mainly responsible for basic control functions such as starting, stopping, speed adjustment, and emergency braking, thereby controlling the entire movement process of the AGV and achieving its movement and positioning.
Figure 1 shows the structural framework of an AGV.
The function of the motion control section is to control its own movement according to the expected task given by the decision control section. The motion control subsystem can be divided into two parts: velocity trajectory generation and velocity trajectory tracking. The velocity trajectory generation section generates an optimal velocity trajectory from the current point to the target point based on the "task" set by the decision control section, according to the AGV's current position, current speed, target point position, and target point speed. The velocity trajectory tracking section controls the AGV's drive mechanism, controlling the AGV's speed in real time to follow the generated velocity trajectory, enabling the AGV to complete its planned various positions and postures. The AGV motion control system hardware mainly consists of a motion controller, servo driver, reducer, DC motor, etc. In order to realize the communication and transmission functions between the motion controller and each drive, it is necessary to select a suitable communication protocol to ensure the compatibility of communication protocols between devices. For AGV manufacturers, some manufacturers adopt in-house research and development of motion control systems, while others adopt system integration solutions. Generally, in-house research and development uses MPU+DSP or FPGA as the main processor, and the peripheral circuits are generally developed based on ARM, X86 chips or core boards. Manufacturers who adopt system integration solutions typically use commonly available motion control systems on the market, saving a significant amount of R&D time. Below is a brief introduction to commonly used motion control systems for AGVs.
1. Motion controller
Motion controllers are the core of motion control systems and are the primary application devices for motor control in industry. As the "brain" of the control system, they enable servo drives, motion interpolation, and motor speed control. Furthermore, they provide various digital and analog input/output interfaces to process sensor signals. The motion controller is mainly responsible for detecting and converting environmental information, detecting the AGV's position and attitude in the global coordinate system, implementing motion control algorithms and path planning algorithms, and converting the algorithm results into motion commands to send to the driver. The driver responds to the controller's commands, drives the motor, and controls the AGV's speed. Motion controllers come in various vehicle-mounted controller forms, including MPUs, PLCs, and industrial PCs, each with its own advantages and disadvantages in terms of size, functionality, cost-effectiveness, and ease of development. In key motion control technologies, the diverse types, styles, and appearances of AGVs necessitate fitting a sufficient number of electronic components into a limited space. Therefore, a suitable size, reasonable cost-effectiveness, and the ability to meet functional requirements are particularly important.
Figure 2 Vehicle controller
2. Servo driver
If the controller is the brain of the AGV, then the driver is its heart, providing current to the motors that drive the wheels, enabling the AGV to move. Since AGVs are generally powered by batteries, with voltages typically around 12V to 72V, they employ low-voltage DC servo drivers.
Figure 3 Common Servo Driver Brands
The servo drive converts low-level signals from the controller into high-power current and voltage to regulate the motor's motion. After receiving feedback from the sensors, the servo drive also adjusts the AGV based on the payload weight and the AGV's current speed to ensure proper operation and control.
3. DC motor
The motor is the driving source of the entire AGV motion control system. Currently, motors on the market are broadly divided into AC and DC motors. However, AC motors, if using batteries, also require inverters and other equipment, which increases the size, weight, and complexity of the AGV, as well as the control difficulty. In comparison, DC motors have certain advantages. They have strong overload capacity and are more resistant to load disturbances, meeting the requirements from the perspectives of cost and structural simplicity. Currently, brushless DC motors are widely used in wheeled AGVs in industry. Brushless DC motors have advantages such as stable torque output and high starting torque, making them suitable for applications with high torque requirements.
Figure 4 DC low-voltage servo motor
