Abstract: PLC control technology has been widely applied in various fields of automation control. This paper details an automatic monitoring system for conveyor belt systems in coal mines. This system utilizes a Siemens S7-300 PLC control core and can communicate with various intelligent electronic devices to achieve automatic monitoring of the coal mine conveyor belt system. The solution is easy to implement and highly reliable. Compared with the original system, the control system based on this solution has significantly improved in performance and automation level, providing important reference value for similar systems. Keywords: Master computer; PLC; Acquisition control Abstract: PLC control technology has been widely used in the field of automation. A kind of automatic monitoring system for belt conveyors is described in detail. The system uses a Siemens S7-300 PLC as the centralized control center, which can communicate with various intelligent electronic devices. Therefore, automatic monitoring of belt conveyors can be implemented. The scheme has high reliability and is easy to implement. There is a great advance in the performance and automation of the new automation system compared to the former. It has very important reference value for correlative systems. Key Words: Master computer; PLC; Acquisition control 1 Introduction With the rapid development of computer and control technology, the use of master computers and PLCs for centralized monitoring in factories and mines has become increasingly common, and the technology is becoming more and more advanced. This paper takes the centralized control system of the conveyor belt in the Danhou Coal Mine in Hebei Province as an example, and studies and develops a PLC-based monitoring system for conveyor belts in coal mines. It is suitable for the current needs of industrial enterprises for automation. It has been put into actual operation and has achieved good results. 2 System Design 2.1 System Overview The Danhou Coal Mine, belonging to Hebei Kailuan Mining Group, is located near Beifangcheng Village, Yongquanzhuang Township, Weixian County, Zhangjiakou City, Hebei Province. It is situated in the central area of ​​the Zhangjiakou mining area, with a geological reserve of 313.74 Mt, a recoverable reserve of 177.08 Mt, a designed capacity of 1.5 million tons/year, and a service life of 81.4 years. It officially commenced production in October 2006. The Danhou Coal Mine's conveyor belt control system (see Figure 1) can be divided into two parts: 1. Screening workshop system; 2. Coal storage and loading system. The screening workshop system includes the conveyor belt from the main shaft to the screening workshop, the gangue transfer conveyor belt, the gangue conveying conveyor belt, and the six gangue sorting conveyor belts and three scraper conveyors within the screening workshop. The remaining conveyor belts belong to the coal storage and loading system. In terms of design, the system is required to achieve both local and centralized control modes. Centralized control can be divided into various control modes such as interlocking control and single-machine control, which can be flexibly selected by the operator according to the actual situation on site. This ensures flexible operation and easy maintenance during normal system operation, and local control can ensure the normal operation of the belt conveyor in the event of system failure or communication interruption, thereby improving system stability. 2.2 System Hardware The entire system can be divided into two layers from top to bottom: the centralized control management layer and the local control management layer. The centralized control management layer consists of two host computers and one switch. During system operation, the two host computers are redundant and are directly interconnected with the field equipment through the switch, thereby realizing the monitoring of the field equipment. [align=center] Figure 1 System Structure Diagram[/align] The local control management layer consists of a PLC, a switch, and a belt protection device. The PLC is the core of the entire control system. In this system, Siemens S7-300 is used. Since the coal storage subsystem and the screening subsystem have a mutual interlocking relationship in practice, the screening subsystem and the coal storage subsystem can be constructed into a DP network (the communication principle diagram of the system is shown in Figure 2). Therefore, all PLCs in this system use CPU315-2DP. During network setup, the screening subsystem acts as the master station, and the coal storage subsystem acts as the slave station. An Ethernet module CP341 is added to the screening subsystem PLC cabinet, allowing the host computer to interconnect with field devices via a switch, thus achieving centralized control. In addition, various protection devices such as belt misalignment detection, coal pile-up detection, and pull-wire switches are installed along the belt conveyor line to ensure the system can react quickly in case of belt conveyor malfunctions. [align=center] Figure 2 System Communication Principle Diagram[/align] 2.3 System Software The software of this system mainly consists of two parts: the host computer's configuration and monitoring software and the field PLC's programming software. [align=center] Figure 3 System Software Operation Interface Diagram[/align] The host computer's programming software uses Intellution's IFIX 3.5, which has the following characteristics. a. Real-time library display: Displays all real-time points within the system; b. Real-time remote control: Selects a control switch on the screen to issue remote control commands in real time; c. Real-time and historical curves: Can be set to display all recorded telemetry points within the system; d. Real-time and historical alarms: Alarms are divided into severe alarms, medium alarms, and minor alarms; e. Event and alarm record query: Event and alarm logs can be queried by day; f. Real-time event and alarm printing: When an alarm event occurs, the event printer prints it in real time. The host computer monitoring software communicates with the field PLC via OPC, which can conveniently and flexibly obtain telemetry and tele-signaling information of field electromechanical equipment and its protection devices to achieve remote control. The configuration software interface of the host computer is shown in Figure 3. The programming software for the field PLC uses Siemens Step7. When the PLC is in "RUN" working mode, except for power-on initialization, other programs adopt a cyclic scanning method, which is called "PLC scanning working mode". Its execution flow is shown in Figure 4. 3. Key System Technologies 3.1 System Communication Program Design The main PLC control cabinet (sieve molecular system PLC control cabinet) of this system is installed in the low-voltage power distribution room on site. Due to the long distance from the dispatch center and the severe electromagnetic interference on site, Ethernet and fiber optic transmission technologies are used to realize the communication between the SIMATIC S7-300 PLC and the host computer human-machine interface. The main PLC cabinet is equipped with an Ethernet module CP341 and an Ethernet-to-fiber optic switch. To correctly transmit and receive information, a set of conventions regarding the information transmission order, information format, and information content is necessary; this set of conventions is called a protocol. In designing the communication program, this system adopts a modular programming design concept, dividing the program into several program blocks. Each program block contains program instructions for some devices and tasks. Each functional area is divided into different blocks for programming, which is beneficial for multiple people to program simultaneously and also facilitates program debugging and fault finding. The PLC in the system needs to handle multiple communication protocols; separate processing programs for each protocol are compiled and placed in different functional modules (FCs). In the PLC's main program block OB1, the processing programs of these protocols can be executed sequentially through call statements to achieve communication with these integrated protection devices or intelligent instruments. 3.2 DP Network Configuration Based on the interlocking relationship between the screening subsystem and the coal storage subsystem, the screening subsystem and the coal storage subsystem can be configured into a DP network. This makes the system's logical relationship clearer, while also providing good scalability and saving costs. [align=center] Figure 4 System Flowchart[/align] For DP network configuration, please refer to the relevant Siemens manuals. The following points should be noted in particular. a. When configuring master and slave stations, the slave station should be configured first, followed by the master station; b. When configuring the hardware, the Consistency of both the master and slave stations must be set to All; c. During programming, the master station's OB1 must contain OB1, OB82, OB86, OB100, and OB121; d. Since the host computer will perform read and write operations on the PLC during system operation, SFC14 and SFC15 function blocks must also be added to the program blocks of both the master and slave stations. 3.3 Interlocking Control of Field Equipment To compensate for the deficiencies in the anti-misoperation function of field equipment and ensure safe production, interlocking control should be implemented for all field equipment. This system has both mechanical and logical interlocking, achieving the requirement of "starting equipment sequentially against the coal flow and stopping equipment sequentially with the coal flow". Specific implementation: Mechanical interlocking: The normally closed point of the operation return signal of the logically first started equipment is connected in series into the secondary control loop of the equipment started later; Logical interlocking: As shown in the flowchart, the operation return signal of the equipment is used as a prerequisite for executing the next program. 4 Conclusion (1) By utilizing large and medium-sized PLCs (such as Siemens S7-300), communication with various intelligent electronic devices can be achieved, facilitating the monitoring of field equipment. (2) Since the system was put into operation in October 2006, the maintenance workload has been greatly reduced, and most faults can be directly displayed on the computer monitor, reducing the fault finding process. The system is simple to operate and easy to maintain, improving system security, reducing operating costs, greatly reducing downtime, and improving economic benefits. The innovations of this paper are: 1. This paper systematically describes the method of constructing a centralized control system with PLC as the control core. The control system based on this scheme has been greatly improved in performance and automation compared with the original system, and has important reference value for similar systems. 2. For the processing of subsystems in the centralized control system, the usual practice is to configure each subsystem with an Ethernet communication module and then use each subsystem as a node to form a ring network. In this system, multiple subsystems with logical interlocking relationships are regarded as a node in the network, reducing the Ethernet communication module and saving costs. For multiple subsystems under the same node, a cascading method is used to configure a DP network, which greatly reduces the network burden. 3. This paper gives a detailed method and precautions for configuring a DP network. 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