Abstract: This paper introduces the design concept, design method, and design content of a cold storage refrigeration monitoring system composed of PROFIBUS bus. The application of PROFIBUS bus and KingSCADA software in the cold storage monitoring system greatly improves the reliability and automation level of cold storage operation. Keywords: Cold storage, PROFIBUS, PLC, Monitoring 1. Introduction Cold storage is widely used equipment in the food industry and plays a key role in food production. Currently, the refrigeration monitoring systems of large cold storage facilities in China mainly adopt manual or centralized control systems. Due to the large number of refrigeration equipment and their complex structure (up to 150 types of equipment control variables in the system), there is a lot of wiring, cumbersome operation, and information transmission is prone to bottlenecks. The operating status of each device cannot be reflected in a timely manner, causing many inconveniences to production management. To address the above problems, a refrigeration monitoring system is composed of PROFIBUS-DP bus. PROFIBUS is an international, open, and manufacturer-independent fieldbus standard. It includes three compatible versions: PROFIBUS-DP, PROFIBUS-PA, and PROFIBUS-FMS, and is widely used in process automation, manufacturing automation, building automation, and other fields. PROFIBUS-DP is a fieldbus technology used for factory automation workshop-level monitoring and field device-level data communication and control. It enables distributed digital control and field communication from the field device level to the workshop level, thus providing a technical guarantee for the realization of industrial field local area network systems. Using PROFIBUS fieldbus to construct a cold storage refrigeration monitoring system significantly reduces wiring workload and cable investment, avoids signal interference, makes the system more reliable and easier to operate, realizes automatic control of the refrigeration system, and simultaneously completes dynamic monitoring of the refrigeration production process, thereby achieving the goal of automatic monitoring and management of the entire cold storage production process. 2. Composition and Main Functions of the Monitoring System The Shuanghui Industrial Park's 10,000-ton cold storage refrigeration system has four independent open-type screw compressors, supplying cooling to six warehouses. The warehouses are circulated with fans to blow cold air. Warehouses 1-5 are frozen warehouses with 30 temperature measuring points at -18ºC, and warehouse 6 is a refrigerated warehouse with 6 temperature measuring points at 0-4ºC. The system controls 12 sets of fans, totaling 60 fan motors. The control system requires automatic control of the defrosting process, the start and stop of equipment such as compressors, ammonia pumps, water pumps, and air coolers, as well as the increase and decrease of compressor energy. The central control room monitors the operating status of each refrigeration unit in the cold storage, displays the storage temperature, return gas main pressure, condensate main pressure, low-pressure circulation tank level, and the operating parameters and curves of each compressor. The storage temperature and main pressure can be set in the central control room according to actual needs. 2.1 System Composition Based on the above refrigeration process requirements, this monitoring system adopts a PROFIBUS-DP single master-slave working mode to form a distributed real-time monitoring network. The master station is a SIEMENS S7-300 PLC (CPU315-2DP), and the slave stations are three S7-200 PLCs (CPU226) and one remote I/O (ET200M). PROFIBUS-DP communication between the master station and the S7-200 slave stations is achieved through an EM277 module using the CPU226 as a DP slave station. The network structure of the control system is shown in Figure 1. The host computer is an Advantech industrial PC with a built-in dedicated MPI communication network card, CP5611. An S7-300 programmable controller serves as the master station for data processing and refrigeration equipment control, while the industrial PC acts as a human-machine interface (HMI) station, handling only programming and monitoring. Using Siemens' STEP7 V5.1 programming software, the system is first configured with network settings, such as communication port settings, MPI address settings, and data transmission rate settings. Then, the S7-300 master station is configured with hardware via the MPI port, consisting of a PS307 power supply module, a CPU315-2DP, an SM331 analog input module, an SM321 digital input module, and an SM322 digital output module. The host computer uses KingSCADA to configure various screens, enabling data browsing, parameter setting, manual/automatic operation, fault alarms, historical data recording, and comprehensive report management functions. Due to the large number of screens in the system, two industrial PCs are used as HMI stations. The master station has a PROFIBUS-DP interface for data communication with three slave stations. The master station can also remotely control the start and stop of the compressor, ammonia pump, and water pump via I/O OEM277, and can increase or decrease the number of compressors in operation according to the amount of cooling demand. It also collects the pressure of the intake and exhaust manifolds, compressor operating parameters, etc. Since the six cold storage rooms are far from the central monitoring room, three S7-200 PLCs are installed at a strategically central location near the entrances of the six cold storage rooms as remote slave stations for data acquisition. Two of these are temperature acquisition boxes, with temperature sensor signals connected locally. The S7-200s inside the boxes receive I/O configurations from the master station, collect temperature values ​​in a time-sharing manner, and send and receive data from the master station. A TD200 text display is installed in the field box, allowing local checking of temperature values ​​and other information. The third S7-200 PLC collects the current of the 60 fan motors in the six cold storage rooms and sends it to the host computer via PROFIBUS bus, displaying the current values ​​and curves. 2.2 Temperature Inspection Circuit Since there are many temperature monitoring points in the warehouse, the temperature of each monitoring point needs to be converted from the resistance signal of the PT100 thermal resistance temperature sensor into a standard 4-20mA current signal and sent to the S7-200 PLC via a temperature transmitter. Thus, each temperature acquisition station requires 18 temperature transmitters and 5 analog input modules. To reduce costs, a time-sharing drive sampling circuit as shown in Figure 2 is adopted. Utilizing the powerful control function of the PLC, through software programming, the on/off state of 18 intermediate relays KA is controlled within a 60S-120S (adjustable) cycle. In this way, each group only needs one temperature transmitter and one analog input module to collect all temperature values, thereby realizing the reduction of hardware configuration through software processing, reducing the cost of the monitoring system, and reducing the equipment failure rate. 2.3 Main Functions of the System (1) Realize the full-process control of refrigeration equipment such as compressors, ammonia pumps, fans, and defrosting. (2) Display the main process parameters of the refrigeration system in real time (such as temperature, pressure, fan current, etc.). (3) Dynamic simulation display of the operation status of each device (such as compressor start-up and shutdown, low pressure circulation tank liquid level, etc.). (4) Parameters such as temperature and pressure can be set and modified according to actual needs via keyboard. (5) Online detection and alarm of parameters exceeding limits. (6) Automatic generation, data query and printing of various production management reports. 3. Software Design 3.1 PLC Program Design. The software design adopts a modular structure design method, and the program structure is clear, compact and easy to modify. According to the process requirements of the refrigeration control system, the program is divided into two parts: main program and subroutine. The main program mainly completes system initialization, initial parameter setting, loop detection, calling subroutine, output control signal and real-time display of parameters. The subroutine is divided into function blocks FC such as compressor control, air cooler control, low pressure circulation tank liquid level control, defrosting control, fault handling, data acquisition and processing. The connection and processing of related events by each function subroutine is called by the main program OB1, and its program structure is shown in Figure 3. During the programming process, the following issues need to be addressed: (1) Temperature acquisition and processing: The actual temperature value of the warehouse is acquired by the temperature sensor and sent to the analog input module of the PLC. After A/D conversion, the temperature value becomes a dimensionless digital value. In order to display the digital value as a dimensional actual temperature value in the host computer, it must be calculated and converted by the program to become a dimensional value for display. At the same time, the temperature signal acquired by the sensor may be affected by interference signals in a very short time, which may lead to errors in the operation of the air cooler and coolant valve. In order to avoid interference, 6 actual temperature values ​​are acquired within 10 seconds, the average value is calculated by the program, and then the average temperature value is used to control the start and stop of the fan and cooling valve, thereby effectively avoiding interference. (2) Design of DB block for communication: In the refrigeration monitoring system, the communication between the host computer and the slave computer is mainly achieved by reading and changing the DB block of the slave computer. The system consists of nine database blocks (DBs), representing actual temperature data, set temperature data, current data, liquid level data, actual pressure data, set pressure data, defrosting data, alarm data, and various valve opening data. All variables in these data blocks are binary variables. By reading the DBs from the lower-level machine, the corresponding actual temperature value and the on/off status of the evaporative air cooler are displayed on the upper-level machine. By changing the corresponding DBs on the lower-level machine, such as changing the set temperature of a certain warehouse, the values ​​in the DBs can be changed, thereby controlling the PLC's output load—the evaporative air cooler—to start and stop. 3.2 Configuration Software Design The configuration software used is KingSCADA from Beijing Yacon. This software collects various process parameters during the refrigeration process via S7-300, and monitors the status of the fan motor (running, stopped, faulty), the liquid level in the low-pressure circulating tank, and the operation changes of various solenoid valves on the upper-level machine in real time, allowing operators to intuitively understand the system's working status. Dynamic parameters such as temperature, pressure, current, given adjustment value, and fault status can also be displayed using data, charts, bar graphs, and trend graphs. According to the requirements of the monitoring system, a total of 30 screens in 8 categories were configured, including simulation screens of the working status of each unit in the cold storage and refrigeration station, screens of the working status of each field monitoring unit in the entire system, screens for querying, monitoring, and manual operation permissions, screens for saving and printing current and historical values ​​and parameter values, historical curve screens, alarm window screens, and report management screens. Operators can easily switch between various interfaces using a mouse or keyboard, following the prompts on the menus and dialog boxes. 4. Conclusion In the process of automating the cold storage design, a high-performance industrial field LAN was built using a PLC via a PROFIBUS-DP fieldbus, realizing networked monitoring of the cold storage production process, reducing a large amount of wiring, and improving the accuracy and reliability of the system. Due to the intelligence and digitalization of fieldbus devices, compared with analog signals, it fundamentally improves the accuracy of measurement and control and reduces transmission errors. Meanwhile, due to the simplified system structure, reduced equipment and wiring, and enhanced internal functions of field instruments, signal round-trip transmission is reduced, improving system reliability. Furthermore, its standardized equipment and modular functions offer advantages such as simple design and ease of reconfiguration. The PROFIBUS-DP-based fully automatic cold storage monitoring system leverages the advantages of PLC and PROFIBUS-DP, successfully achieving centralized management and decentralized control of cold storage refrigeration. This significantly reduces the labor intensity of workers, improves their working environment, and lays the foundation for realizing "unmanned cold storage" or "information-enabled cold storage." References: 1. SIMATIC S7-300 Programmable Controller Manual, SIEMENS. 2. Xia Dehai. Fieldbus Technology. Beijing: China Electric Power Press, 2003. 3. Yang Xianhui. Fieldbus Technology and Its Applications. Beijing: Tsinghua University Press, 2000. 4. Liao Changchu. PLC Programming and Applications. Beijing: Machinery Industry Press, 2004.