Abstract: This paper introduces the design concept of the electrical control system for the belt conveyor in Weishan Cuizhuang Coal Mine, focusing on the important role of PLC in solving control problems such as automatic control and soft start of the belt conveyor. It analyzes the control objectives required by the electrical control system under the special environmental conditions of underground coal mines, and has good promotional value for the safe use of this type of belt conveyor in the future. Keywords: Belt conveyor, Electrical control system, PLC, Design With the development of mining technology, the application range of belt conveyors in underground coal mines has greatly increased. Not only is the transport volume large, but the transport distance is also long. Its safety, stability, and economy are also very important, requiring the selected control system to be powerful, stable, sensitive, and reliable. The PLC programmable controller, with its simple structure, superior performance, high reliability, flexibility, ease of programming, and convenient maintenance, ensures the stable operation of the entire system. 1 System Control Requirements and Hardware Design Weishan Cuizhuang Coal Mine Co., Ltd. plans to use PLC to control the belt conveyor. The conveyor system has a transport capacity of 650 t/h, a belt speed of 3.15 m/s, a transport distance of 1156.5 m, and an inclination angle of 13°～17°. The conveyor is equipped with three 1140V 250kW motors, with centralized drive at the head, a double-drum, three-motor configuration, and a YOTCS560 speed-regulating hydraulic coupling starting method. The PLC control system should achieve the following functions: ① soft start of the conveyor through control of the speed-regulating hydraulic coupling; ② power balance of the three motors; ③ real-time display of various operating status parameters; ④ fault self-diagnosis and audible and visual alarms; ⑤ manual/automatic switching. The hardware structure diagram of the control system is shown in Figure 1. Due to the unique environment of underground coal mines, the BXWⅡ-120/115 mine-use explosion-proof and intrinsically safe microcomputer control box (hereinafter referred to as the microcomputer box), independently developed and manufactured by Weishan Cuizhuang Coal Mine Co., Ltd., was selected as the main control device. Combined with peripheral switching equipment, signal and protection sensor detection, a control system was formed to closely monitor and control the coal conveying process of the belt conveyor. The electrical control system centered on the microcomputer box is a programmable monitoring and control system specifically designed for underground coal mines. It is used for the electrical control, monitoring, and comprehensive protection of belt conveyors and other equipment in underground coal mines. The system features flexible design, rich display information, complete protection functions, expandable input/output points, voice communication, self-diagnosis, and a quick-opening door structure. This system is now widely used in underground coal mine transportation equipment such as: main shaft surface conveyor belts, underground conveyor belts, centralized transport belts, roadway belts, tunneling belts, transfer belts, and other continuous underground transportation equipment. Based on the requirements of control functions and the number of input/output points, the Mitsubishi MELSEC-Q series PLC was selected as the control core. This series of PLCs adopts a modular structure, including one Q00JCPU module, two QX40 DC input modules, two QY40P output modules, and one Q68ADI analog expansion module. The Q00JCPU is an integrated module combining power supply, baseboard, and CPU. The QY40P output modules are 16-point sinking transistor output modules, whose output points control the start and stop of various devices via intermediate relays. This system uses both a touchscreen and a PC as the human-machine interface. The touchscreen is a Mitsubishi F940GOT color graphic operation terminal, allowing simultaneous viewing of the programmable controller's various software components and data changes. The PLC communicates with the touchscreen via an RS-232C interface with a default data transfer rate of 19.2 kbps. The PLC also communicates with the host PC via an RS-232C interface. Since the PLC is a non-safe product, while the coupling limit switches, oil pressure gauges, and oil temperature gauges are intrinsically safe products, a conversion and isolation circuit is required: 16 intrinsically safe switching signals (without potential contacts) are converted into non-safe outputs with no potential contacts before entering the PLC input. If an intrinsically safe signal is directly connected to the PLC input without an intrinsically safe conversion and isolation circuit, it is equivalent to inserting a non-safe signal into an intrinsically safe circuit, which does not comply with coal mine safety regulations. By configuring an isolation conversion board, signal isolation, conversion, and amplification are completed, greatly extending the PLC control distance and increasing the PLC output capacity. Using the PLC to directly drive the controlled equipment through the isolation circuit reduces intermediate links and improves reliability. Furthermore, with the converter and isolation board configured, when peripheral equipment malfunctions, causing short circuits or high voltage, the PLC will not be directly damaged, ensuring the safety of the host machine and improving the reliability and service life of the entire system. 2. Software Design The system software design is divided into sequential control program design and touch screen human-machine interface design. 2.1 Sequential Control Program Design The sequential control program for the MELSEC-Q series PLC is developed using GX Developer software. GX Developer allows for sequential control programming using a circuit (ladder diagram) list, monitoring of the program's running status, forced data changes, and ON/OFF of input/output signals. The program executes immediately after the PLC starts, first performing internal initialization, and then detecting the current signals of each device and motor. The entire sequential control program includes six main parts: normal start, normal operation, normal stop, emergency stop, fault protection, and power balance. Access to the human-machine interface can be initiated at any time during execution. The control module flowchart is shown in Figure 2. The PLC can also achieve power balance for three motors, as follows: a 4-20mA current signal is taken from the output of the current transformer in the magnetic starter of the three motors and sent to the PLC's Q68ADI analog expansion module. After analog-to-digital conversion, the 4-20mA current signal is converted into a digital signal. Since the two main motors are on the same drum, the power of these two motors is first compared to see if they are balanced. When the current signals of the two motors differ significantly, exceeding the specified range, the PLC adjusts the system. If the conveyor belt speed is within the normal range, it indicates that the motor with the lower current is performing less work. The PLC then starts the servo motor of the hydraulic coupling (jogging) to apply load and compares the current values ​​of the two motors again until the current difference is within a reasonable range. If the current difference between the two motors exceeds the normal range, and the conveyor belt speed exceeds the normal speed, it indicates that the motor with the higher current is performing more work. The PLC then starts the servo motor of the hydraulic coupling (jogging) to reduce the load and compares the current values ​​again until the current difference is within a reasonable range. Once the power of the two motors on the same roller is balanced, the servo motors of the two hydraulic couplings are started to rotate simultaneously. Then, one of them is used as a reference and compared with the third motor using the same method until the power of all three motors is balanced. 2.2 Touchscreen Human-Machine Interface Design The operation display screen utilizes Mitsubishi's Windows 98-based HMI design software FX-PCS-DU-WIN-C to create the screen, displaying the equipment's start/stop and the operation status of various protection sensors in graphical, animated, and Chinese character formats, distinguished by different colors. In case of a fault, the system automatically jumps to the fault alarm screen. After the fault is cleared, pressing the normal stop button resets the system and returns to the status screen. Pressing the display page-turning button allows viewing of various screens. To provide a user-friendly interface, a startup screen, status screen, data sampling charts, and fault alarm records were created, providing users with an excellent operating environment and enabling the display and modification of operating procedures. 3 Conclusion The adoption of PLC control for underground belt conveyors in coal mines has achieved automatic control and protection, greatly improving the system's safety and reliability. After more than a year of use, it has achieved good results, ensuring the transportation of raw coal and reducing maintenance workload. The use of PLC-controlled speed-regulating hydraulic coupling to achieve soft start of belt conveyor reduces the impact force on the high-strength belt, motor, hydraulic coupling, gearbox and drive drum during start-up and stop, which can extend its service life, reduce the impact on the underground power grid during start-up, and improve the reliability and safety of power supply. References: [1] Guo Zongren, et al. Programmable Logic Controller and its Communication Network Technology [M]. Beijing: People's Posts and Telecommunications Press, 1999.