1. Introduction
Our school's independently developed automated production line is a PLC-based teaching system and a modular production training system. It integrates electrical control, PLC applications, sensors, and pneumatics technologies, and consists of seven workstations: automatic feeding, transmission and detection, vacuum sorting, color sorting, position adjustment, parts supply, and assembly unloading. The automatic feeding station provides workpieces to other workstations. Workpieces are gripped by a pneumatic robot, moved, lifted, and swung, placed on the conveyor belt, and then returned to their original position.
2. System Hardware Design
The automatic feeding module is the starting unit of the entire automated production line system. It mainly consists of a workpiece loading pipe and ejection device, support frame, valve group, terminal block assembly, PLC, buttons, cable tray, and base plate. Its main function is to automatically push workpieces placed in the hopper onto the material tray, so that the robotic arm of the conveying unit can grasp and transport them. It has both manual and automatic operation modes. Before operation, ensure that the feeding tray is filled with material.
2.1 Action Process
The main body of the feeding unit is a rotating tray with two tubular hoppers. A diffuse reflection photoelectric switch is installed at the feeding position of the tray opening to detect whether there is material in the tray opening. When a workpiece arrives at the tray opening, the photoelectric switch detects its presence and sends a signal to the system indicating that there is a workpiece in the unit's tray. In the control program of the conveying unit, this signal can be used to drive the robotic arm to grab the workpiece and send it onto the conveyor belt. Under the influence of gravity, the workpieces in the hoppers automatically move down one hopper, preparing for the next grab. If there is no workpiece in the current tray opening, the tray will rotate to the next opening under program control to continue detection. The tray stops rotating when both tray openings are empty.
2.2 Pneumatic Circuit Design
The pneumatic control circuit is the actuator of this working unit, consisting of a pneumatic manipulator and a loading turntable, completing the loading and unloading operations. The manipulator is structurally composed of two linear cylinders, one swing cylinder, and a gripper. The return state is: gripper closed, translation cylinder retracted, lifting cylinder lowered, swing cylinder 0°. One working cycle is: gripper closed → gripper open → lifting cylinder raised → translation cylinder advanced → gripper closed → translation cylinder retracted → swing cylinder 180° → translation cylinder advanced → lifting cylinder lowered → gripper open → translation cylinder retracted → lifting cylinder raised → swing cylinder 0°. The manipulator can realize the translation, lifting, swinging, and gripping actions of the arm. Magnetic proximity switches installed on the cylinders detect the limit positions of each cylinder, and position control is achieved through PLC. Five 2-position 5-way dual-electro-controlled solenoid valves are centrally mounted on the manifold to control the translation, lifting, and swinging actions of the manipulator arm, as well as the rotation of the loading turntable.
2.3 PLC Control
In the feeding unit, the magnetic switch for detecting the cylinder position, the photoelectric switch for detecting the presence of workpieces at the material tray opening, the conveyor belt congestion detection sensor, and control buttons, along with the PLC, require a total of 14 input terminals to receive external signals. The output terminals for solenoid valve control actions, fault signals, and flag signals require a total of 13 output terminals. A Siemens S7-300 master unit is selected, which can exchange information with the slave unit. Its I/O wiring principle is shown in Figure 1.
3. System Software Design
There are two main methods for designing PLC ladder diagram programs: the empirical method and the sequential control method. The basic idea of the sequential control method is to divide a system's working cycle into several sequentially connected steps, represented by programming elements (such as memory bits M). Within any step, the state of each output remains unchanged. The signal that causes the system to move from the current step to the next is called the transition condition. This can be an external input signal, such as the on/off state of a button or limit switch, or a signal generated internally by the PLC, such as the contact signals of timers and counters.
