1 Introduction The working environment of the material packaging and conveying system is usually quite harsh. The equipment is located in an environment with a lot of dust and high relative humidity. The operation is scattered, so the safety, reliability and ease of maintenance of the conveying and packaging control system are required. In the past, most electrical control systems used discrete relays, contactors and other electrical components as control elements. The control system was complex, difficult to operate, and had a large amount of installation and wiring work, difficult to modify control strategies, and large maintenance work, which seriously affected normal production. Therefore, the material conveying control system became a bottleneck restricting production. The control system composed of a PLC and its WINCC monitoring software[1] with high reliability is used as the main equipment for data acquisition, control loop, automatic sequential operation and calculation. The system realizes the real-time monitoring, automatic control and system operation diagnosis of the belt process control and conveying process of the packaging system, which meets the requirements of system reliability, stability and real-time performance. 2 System Introduction The packaging and conveying control system is divided into two groups: bulk warehouse and packaging warehouse. The bulk warehouse mainly stores bulk materials that do not need to be packaged, and the packaging warehouse is used for finished product packaging. The system mainly includes: lines 1# to 8#, line A (9#, 15#, 16#), line B (10#, 19#, 20#), line C (11#, 17#), line D (12#, 18#), line E (13#, 21#), and line F (14#, 22#). The process flow for each line is shown in Figure 1. In this process flow, in addition to considering the sequential start-up and stop of each belt and issues such as belt slippage and misalignment, related supporting equipment must also be considered. The system mainly includes 67 digital inputs, 16 analog inputs, and 52 digital outputs; the processes that need to be controlled include the start-up, stop, and safe operation of each belt, the selection of each hopper, and the handling of equipment failures. 2 Hardware design of PLC control system 2.1 Hardware configuration According to the equipment and process requirements, the packaging conveying system is composed of upper computer and lower computer. The upper computer uses two PCs: one is used as an operator station to realize the monitoring and data detection of the entire system; the other is used as an engineer station to complete the design and development of configuration software, the development of PLC program and the transmission of software to the CPU unit of PLC through PROFIBUS[2] bus. The lower computer uses Siemens S7-300 series PLC, which is powerful, reliable, easy to maintain and has strong anti-interference ability, to complete the control function of the equipment. The lower computer is divided into two racks and placed in the packaging warehouse and the bulk warehouse respectively. The S7-300 of the bulk warehouse rack and the packaging room rack form a PROFIBUS-DP network structure. The hardware structure configuration of the system is shown in Figure 2. Its specific composition is as follows. (1) Central control unit The central control unit uses CPU315-2DP[3] as the core component of PLC to perform logic and digital operations and coordinate the work of each part of the entire control system. (2) Power Supply Unit The power supply unit uses a 1:1 isolation transformer to power the 220V AC switch input cards of the PLC, and uses a SITOP power supply to power the 24V switch output cards of the PLC. The built-in PS-307/5A DC power supply powers the CPU and some cards. (3) Input/Output Unit The system uses two 8-point analog input units AI8×12Bit, two 32-point output units DO32×DC24V/0.5A, one 16-point output unit DO16×DC24V/0.5A, and five 16-point digital input units DI16×AC120/230V. (4) Communication Module In order to ensure normal communication between the packaging warehouse operation station and the bulk warehouse operation station (distance is about 300 meters), the CP 342-5 communication module was selected in this system. It is configured and programmed through PROFIBUS. 2.2 Variable Allocation The PLC variable allocation of the controlled object is shown in Table 1. 3 Packaging Conveying Control Program Design Ideas 3.1 System Control Mode The control mode of the packaging conveying control system is divided into three types: automatic control, single machine control and on-site manual control. The single machine start mode refers to the start and stop buttons in the interlock diagram of the host computer. When not in the interlock state, the belt can start/stop independently. 3.2 Control Program Design The belt conveying system has a total of twenty-two belts. According to the belt conveying process, it can be divided into two parts: lines 1# to 8# and lines A to F. According to the control room and bulk warehouse control room and the on-site belt operation, the control strategy of the conveying system is as follows: (1) Select the control mode: remote automatic control, on-site manual control or remote manual control mode. (2) Control the operation of the packaging line and bulk warehouse line according to whether packaging is required, and stop them in the required sequence. (3) Control the start and stop of lines A to F according to the material level in the trough. The STEP7 user program in this system is divided into organization block (OB), function block (FC) and data block (DB). The function block is used to establish the user program according to the control task. The entire control process is divided into analog signal processing, A-F line start/stop, and 3-8# line belt start/stop according to the process. Total material level calculation, analog quantity transformation, material level calculation of trough, alarm processing, 1-6# belt speed processing, 1-2# belt start/stop and distributor selection, etc. Program blocks are used. According to the specific situation of the process flow, the statement list (LAD) is used for programming. Figure 4 shows the control flow diagram of 3#-6# belt. The data block is used to store the data of belt speed and material level of trough. 3.3 Control design idea (1) The loop start sequence is from downstream to upstream (the material direction is upstream), and it is started one by one with a certain delay. If there is no fault during the loop start process, it is a normal start; if there is a fault, it is an abnormal start. When the program encounters a fault, it will not continue to start. (2) The loop stop sequence is from upstream to downstream. It includes normal stop and accident stop. Normal stop is sequential stop, that is, during normal operation, the program stops the equipment one by one from upstream to downstream with a certain time delay. Accident stop is when a device in the loop fails during startup or normal operation, the upstream device stops immediately, and the downstream device can run. (3) In the logic ladder diagram, all interlocking loops with branches have memory function. Because the previous device can start the devices of each branch loop as needed, the memory function of the loop ladder logic can ensure that the device with the faulty loop can stop correctly. (4) The host computer can display the running status of the entire belt. It can also display the device running on a single loop. 4 WINCC configuration software structure design Industrial control configuration software can collect data from programmable controllers, various data acquisition cards and other devices in real time, issue control commands and monitor whether the system is running normally. The configuration software can make full use of the powerful graphics editing function of Windows to display the running status of the monitoring device in an animated manner, conveniently construct the monitoring screen and realize the control function, and generate reports [4], historical trends, etc., providing a convenient software development platform for the development of industrial monitoring software, and improving the overall quality of industrial control software. WINCC developed by Siemens is a configuration software that runs on Windows 2000. Its function is to create dynamic display windows, and the provided toolbox allows for easy creation of real-time graphs, historical graphs, and alarm record displays. Within the screen window, various graphic objects can be configured to establish corresponding animation connections, reflecting the industrial control process in a clear and vivid way. Figure 5 shows the structure of the monitoring software according to the requirements of the packaging control system. WINCC and S7-PLC are both Siemens products, seamlessly integrated and with built-in communication protocol connections. This control system and the host computer configuration software fulfill the requirements of the material conveying measurement and control system. It concisely and vividly simulates the entire system's process flow, allowing operators to observe the entire conveying situation, including various alarms, on the computer screen in the control room. Authorized operators can operate any conveyor belt individually or in an interlocking manner in the control room, and switch between manual and automatic modes. 5. Main functions of the monitoring system (1) Display function: display of process flow, measured value, equipment operating status, operation mode, alarm, etc., screen call, etc.; (2) Alarm processing and report generation function: record alarm occurrence time, fault content, etc., and manage alarm information. The system reports include daily, monthly, etc.; (3) Historical trend function: display the belt speed and trough level on site in curve graph. The screen of each trend curve mainly includes screen name, time, trend, etc.; (4) Screen system modifies the system level parameters to realize automatic/manual switching of the system; (5) Management permission: realize different levels of system management permission. The system operator can select the operation mode, view trend curves and reports, etc.; the system engineer can modify the monitoring software and the lower machine software. (6) Operation control function: according to the buttons on the interface, each belt can be operated, such as: start, stop; set and select the level according to the process requirements. 6. Conclusion The automatic material transportation control system described in this article has been running normally in the industrial site for one year. Because the entire material handling process is controlled by a comprehensive control system, the coordination and interlocking between various sub-processes are meticulously 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 to eliminate the possibility of misoperation. This control method improves the automation level of the existing system and reduces the labor intensity of workers.