Abstract: The landfill leakage detection system uses a PC and a microcontroller as the control core and adopts a distributed monitoring and management form based on the CAN bus. The CAN intelligent measurement and control node collects detection signals in real time and uploads them to the monitoring station after CAN-R232 conversion. The VB-based monitoring interface performs analysis, plotting, saving, printing and other processing on the real-time data. Keywords: landfill leakage, distributed data acquisition, CAN intelligent measurement and control node 1. Introduction Conventional waste disposal often mixes large amounts of toxic substances with domestic waste in landfills, which not only seriously affects the quality of urban environmental sanitation but also damages the living environment. The currently promoted solid waste landfill method can effectively isolate pollutants and effectively manage the solid waste after landfilling. In order to prevent groundwater pollution, the landfill anti-seepage lining system is an essential facility in landfill construction. However, its fatal drawback is that the anti-seepage construction is relatively difficult to meet the requirements. If the industrial detection and control concept is applied to the detection of anti-seepage lining systems, the problem of detecting lining leakage can be easily solved. Based on a thorough understanding of domestic and international landfill leakage detection technologies, China University of Mining and Technology and the Chinese Research Academy of Environmental Sciences have successfully developed a low-cost, high-reliability landfill leakage detection and control system using a master-slave monitoring and management approach and leveraging the advantages of CAN bus networked data transmission. This system is currently in use and has achieved satisfactory results. 2. Overall Structure of the Landfill Leakage Detection System The overall structure of the CAN fieldbus data acquisition and control system meets the requirements of modern control systems for both centralized management and distributed real-time control, offering advantages such as long transmission distances and simple output lines. Based on this, the landfill leakage detection system employs a fully microcomputer-based DC sampling, digital calculation, judgment, and display method to improve leakage detection accuracy. The system consists of a monitoring station, multiple CAN intelligent measurement and control nodes, detection electrodes, power supply electrodes, reference electrodes, and other actuators. The monitoring station comprises a PC and a main control box, while the CAN intelligent measurement and control nodes are data acquisition boxes. [align=center]Figure 1 Leakage Detection System Structure Diagram[/align] 3. System Principle and Function First, each CAN monitoring and control node on site sequentially collects 128 channels of detection analog signals and one channel of power supply analog signal in real time within each zone. The analog signals are converted from analog signals to digital signals by the CAN controller, which then converts them into data conforming to the CAN communication protocol and uploads them to the main control box via the CAN communication interface. Second, the main control box exchanges data with each data acquisition box via the CAN bus, analyzes and processes the acquired data, and then transmits it to the PC via the R232 bus. Finally, the PC's user-friendly interface allows for real-time data analysis, graphical display, storage and retrieval of historical data. 3.1 On-site CAN Intelligent Monitoring and Control Node (i.e., Data Acquisition Box) The on-site CAN intelligent monitoring and control node is based on the 89C52 microcontroller and the SJA1000 independent CAN controller. The ADG406 analog switch selects each of the 128 analog channels for detection analog signals, which are then converted into digital data by the AD976 A/D converter. The SJA1000 controller frames the communication data. The PAC82C250 transceiver converts the TX signal sent by the SJA1000 into a differential signal and sends it to the CAN bus. Simultaneously, it converts the differential signal on the bus into an RX signal for the SJA1000 to receive. 3.2 The main control chassis implements the CAN-RS232 interface conversion function. On one hand, it connects to the field CAN intelligent measurement and control nodes (i.e., data acquisition boxes) via the CAN bus, sending data acquisition commands to the data acquisition boxes and receiving the acquisition results; on the other hand, it connects to the PC at the monitoring station via RS232, processing the data on the microcontroller and uploading it to the PC via the RS232 bus. 3.3 The monitoring station's user-friendly interface allows for configuration of the entire system, setting control parameters for each measurement and control node (i.e., data acquisition box), and reading data uploaded by the main control box. It analyzes the real-time acquired data and provides system management functions such as image display, storage and retrieval of historical data, display printing, and remote monitoring. 4. System Software Structure The system software includes field CAN intelligent node software, main control chassis control software, and monitoring station software. The field CAN intelligent measurement and control node software is written in the structured programming language C51 and has functions such as real-time data acquisition, real-time data processing, real-time control, and CAN bus communication. The main control chassis control software is also written in C51 and mainly realizes functions such as receiving real-time data from CAN nodes, data analysis, and conversion. The monitoring station software is written in the object-oriented programming language Microsoft Visual B6.0 and performs operations such as real-time data analysis, graphical display, storage and query of historical data. 4.1 The flowchart of the field CAN measurement and control node program (Figure 2) and the flowchart of the main control chassis program (Figure 3) are shown below: [align=center] Figure 2 Flowchart of the control program from the control chassis Figure 3 Flowchart of the control program from the main control chassis[/align] 4.2 Main configuration function modules of the host computer software (1) Image function The graphical display part of the real-time data is used by the system to accurately locate the location of the leaks on the landfill liner. Its accuracy and efficiency should be guaranteed. This system software uses ActiveX components to embed and link other applications, using Excel as an external ActiveX component, so that the VB application has the analysis and calculation functions of Excel. Data sources are extracted in real time through DDE hotlinks to obtain relevant data. By analyzing and calculating the data, the potential distribution map of the landfill liner is drawn to accurately locate the location of the vulnerability. Based on these results, the relevant properties of the control are adjusted to realize the monitoring screen display data update and screen change. Figures 4 and 5 depict the potential distribution map and equipotential line distribution map of a vulnerability on the landfill liner under normal system operation. [align=center] Figure 4 Potential distribution map of single vulnerability detection[/align] [align=center] Figure 5 Equipotential line distribution map of single vulnerability detection[/align] (2) Chart function The Excel spreadsheet software has the ability to extract data sources in real time through DDE hotlinks and has application programmable characteristics, so it can develop intelligent reports that meet the usage habits of on-site operators. VBA is applied to Excel, and through VBA programming, the interface objects of Excel (such as worksheets, cells, menu bars) are controlled to generate Excel tables that meet user requirements. In this project, VBA add-ins are used to hide the built-in menu bar and toolbar of Excel, and a customized user toolbar is created, making it an Excel template containing only three items: print, print preview, and exit (this ensures basic operations while preventing operators from modifying the report and preventing accidental operations). Historical data from queries is written into the cells of the specified Excel template, forming a new report and automatically saving it as an Excel worksheet in the application's current directory. 5. System Anti-interference The anti-interference performance of the master-slave control system is also a very important aspect. In order to ensure the reliable and safe operation of the system, strong anti-interference measures must be taken. This system has taken corresponding anti-interference measures in both software and hardware, which effectively suppressed the impact of external interference on the system operation and achieved good results. 5.1 Software anti-interference measures (1) The method of multiple sampling and then taking the average value is adopted. (2) In the communication program, in order to ensure the reliability of data transmission, a checksum is added at the end of each group of data. If the checksum is wrong, the data is retransmitted, and the number of retransmissions is specified. If the number of retransmissions is exceeded, it indicates that there is a problem with the communication line, that is, an error message is given so that the operator can check the line. 5.2 Hardware anti-interference measures (1) The A/D, communication and other modules in the data acquisition box are all photoelectric isolated. (2) The signal line and communication cable are all shielded. (3) The sampling signals all use standard current signals of 4-20mA. The signal processing part of the data acquisition box uses isolation amplifiers to effectively eliminate the influence of external electromagnetic interference on the signal. (4) The data acquisition box has a built-in watchdog circuit. Once the program encounters interference and enters an infinite loop, the watchdog circuit will take effect, causing the program to jump to the beginning and avoid crashing. 6. Conclusion Practice and application results show that the landfill leakage detection system has the characteristics of strong anti-interference ability, high degree of networking, long communication distance and user-friendly human-machine interface. The system adopts CAN bus technology to form a master-slave data acquisition and control system, which greatly improves the real-time performance, feasibility and remote monitoring function of the system, and opens up a broader prospect for the widespread application of modern control theory in the field of environmental protection. References: 1. Wu Kuanming, CAN Bus Principles and Application System Design, Beijing University of Aeronautics and Astronautics Press, 1995. 2. Ma Zhongmei et al., C Language Application Design for Microcontrollers, Beijing University of Aeronautics and Astronautics Press, 1999. 3. Wang Xingzhi et al., Anti-interference Technology for Microcontrollers and Application Systems, Beijing University of Aeronautics and Astronautics Press, 2001. 4. Yue Jianhua, DC Electrical Exploration in Mines, Xuzhou: China University of Mining and Technology Press, 2000. 5. Liu Changli et al., Theoretical Methods and Engineering Technology for Sanitary Landfill Disposal of Waste, Beijing: Geological Publishing House, 1999.