Introduction: Material packaging and conveying systems typically operate in harsh environments, often characterized by high dust levels, high relative humidity, and dispersed operations. Therefore, the safety, reliability, and ease of maintenance of the conveying and packaging control system are paramount. Previously, electrical control systems mostly used discrete relays, contactors, and other electrical components as control elements. These systems were complex, difficult to operate, and involved significant installation and wiring work, making control strategy modifications challenging and maintenance burdensome, severely impacting normal production. Consequently, the material conveying control system became a bottleneck restricting production. However, by adopting a highly reliable PLC control system as the main equipment for data acquisition, control loops, automatic sequential operation, and computation, real-time monitoring, automatic control, and system operation diagnostics of the packaging system are achieved, meeting the requirements for system reliability, stability, and real-time performance.
I. System Structure
The weighing system is controlled by a robotic arm consisting of a PLC, electronic quantitative scale, servo driver, frequency converter, AC contactor, and solenoid directional valve, enabling the entire system to operate automatically. The PLC receives signals from the weighing controller, such as coarse feeding, fine feeding, unloading, and weighing completion, and controls the servo motor via the servo driver, driving the feeding mechanism to achieve automatic feeding control. The control coils of the AC contactor and solenoid directional valve are connected to the PLC output points. When the AC contactor control coil is energized, the normally open contacts of the contactor close, supplying power to the motor. The frequency converter controls the motor speed by changing the output frequency, and the servo driver can achieve closed-loop control of the servo motor's speed, position, and torque.
II. Process Flow
This system primarily uses on/off control. All movements of the material gate are driven by cylinders, which in turn are controlled by corresponding solenoid valves. The unloading process from the weighing box to the packaging machine is achieved by the cylinders driving the opening and closing of the unloading gate. Each cylinder is equipped with a magnetic proximity switch for the unloading gate's closed position. The control process is as follows:
1. When weighing begins, the proximity switch for closing the unloading gate is ON, and the servo motor drives the feeding gate to open for feeding.
2. When the weighing controller sends a weighing completion signal, the servo motor drives the feeding gate to close;
3. If the operator has given a signal to allow unloading, the unloading valve is energized, the unloading cylinder rod retracts, driving the unloading gate to open and unload. When the unloading gate closes, the proximity switch is OFF. After the unloading delay, the unloading valve is de-energized, the unloading cylinder rod extends, driving the unloading gate to close. When the unloading gate closes, the proximity switch turns ON, the servo motor starts, and the next feeding control process begins.
4. After unloading is completed, the control system finishes one packaging cycle, and the weighing system starts working again. This cycle continues until the operator presses the stop button.
III. Hardware Configuration
Based on the operation and control requirements of the weighing-type automatic quantitative packaging machine, the Siemens SIMATIC S7-200 series PLC was selected for the control system. This series of PLCs features a compact structure, modularity, strong scalability, and a rich instruction set. The selected CPU model is CPU226AC/DC/REL, which provides 24 digital inputs and 16 digital outputs. Both input/output interface circuits utilize optocouplers, providing strong adaptability to external interfaces. It also features two RS485 communication/programming ports with PPI, MPI, and free-mode communication capabilities. Since the analog input signals from the sensors need to be processed, an EM235 analog signal processing module with four analog inputs was added. The GD17-BST provides process monitoring, display, and parameter setting functions for the entire system and can communicate directly with the S7-200 series PLC's programming port or extended communication port via the PPI protocol. The rapid development of computer and network technologies has opened up vast development space for industrial automation. This control system adopts a modular design with a compact structure and centralized installation in a cabinet. The functional modules are connected through a parallel backplane bus, dividing the control system into independent but interconnected subsystems to suit control programs with distributed I/O installation.
The system utilizes a CPU with fast I/O processing speed, powerful I/O expansion capabilities, and easy installation. The PLC automatically and cyclically scans the current status of each input/output point and refreshes the output point status according to the logical relationships defined by the program. It controls the start/stop of corresponding motors and the movement of cylinders through frequency converters, AC contactors, and solenoid valves, thereby completing the automatic control of the process flow. The operating interface is equipped with a touchscreen with a built-in universal port, allowing direct interconnection with computers and other devices containing RS485 communication/programming ports via a serial communication cable. The PLC provides a rich instruction set, greatly facilitating design and maintenance.
IV. Software Design
1. System Structure
The block diagram of the quantitative scale and PLC system is shown below. The PLC is the core of the entire electrical control system. Through its CPU's built-in high-speed counter and digital I/O channels, it connects to the detection element, operation element, control element, and weighing terminal. It automatically and cyclically scans the current status of each input and output point, refreshes the status of the output point according to the logical relationship determined by the program, sends start and end weighing commands to the weighing terminal, receives signals from the weighing terminal, controls the alarm to sound and the frequency converter to drive the motor.
The PLC terminal software is written in ladder logic. To improve the terminal's anti-interference capability, the software design incorporates digital filtering, fault self-checking, and control passwords to ensure the correctness and reliability of control operations. The program design adopts a modular and functional structure for easy maintenance and expansion.
2 Data Transmission
The most significant feature of the S7-200 series UPLC is its freeport communication mode. Freeport communication allows user-programmed control of the S7-200 CPU's communication port, enabling user-defined communication protocols and connecting various intelligent devices. In this system, the S7-200 is set as the master station, and the instruments and GD17-BST are set as slave stations, thus forming a master-slave system. This facilitates digital communication and interlocking with other devices. The user interface includes menus for system calibration, weighing parameters, statistical information, system testing, panel parameter settings, and automatic operation. This provides convenience for modifying process parameters and troubleshooting during maintenance.
V. Conclusion
This control system fully leverages the superior performance of the PLC, significantly improving work efficiency. Since the entire material handling process is controlled by a unified system, the coordination and interlocking between various sub-processes are rigorously designed. Furthermore, alarm messages are provided for every controlled parameter in the PLC control program and the human-machine interface, enabling operators to quickly locate and resolve faults. Confirmation prompts are also provided for each critical operation command, eliminating the possibility of misoperation. This control method enhances the automation level of the existing system and reduces the labor intensity of workers.
PLCs have won the favor of many engineering and technical personnel due to their unique advantages. The emergence of Fieldbus Control Systems heralds a new era in industrial automation. PLCs will develop towards more specialized applications to meet the needs of development. Possessing fieldbus communication capabilities will give PLCs a broad space for development.
